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Abstract and Figures

Most running studies have considered level running (LR), yet the regulation of locomotor behaviour during uphill (UR) and downhill (DR) running is fundamental to increase our understanding of human locomotion. The purpose of this article was to review the existing literature regarding biomechanical, neuromuscular and physiological adaptations during graded running. Relative to LR, UR is characterized by a higher step frequency, increased internal mechanical work, shorter swing/aerial phase duration, and greater duty factor, while DR is characterized by increased aerial time, reduced step frequency and decreased duty factor. Grade also modifies foot strike patterns, with a progressive adoption of a mid- to fore-foot strike pattern during UR, and rear-foot strike patterns during DR. In UR, lower limb muscles perform a higher net mechanical work compared to LR and DR to increase the body’s potential energy. In DR, energy dissipation is generally prevalent compared to energy generation. The increased demands for work as running incline increases are met by an increase in power output at all joints, particularly the hip. This implies that UR requires greater muscular activity compared to LR and DR. Energy cost of running (C r) linearly increases with positive slope but C r of DR decreases until a minimum slope is reached at −20 %, after which C r increases again. The effects of slope on biomechanics, muscle contraction patterns and physiological responses have important implications for injury prevention and success of athletes engaged in graded running competitions.
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REVIEW ARTICLE
Biomechanics and Physiology of Uphill and Downhill Running
Gianluca Vernillo
1,2,3
Marle
`ne Giandolini
4,5
W. Brent Edwards
1
Jean-Benoı
ˆt Morin
6
Pierre Samozino
5
Nicolas Horvais
4,5
Guillaume Y. Millet
1
ÓSpringer International Publishing Switzerland 2016
Abstract Most running studies have considered level
running (LR), yet the regulation of locomotor behaviour
during uphill (UR) and downhill (DR) running is funda-
mental to increase our understanding of human locomotion.
The purpose of this article was to review the existing lit-
erature regarding biomechanical, neuromuscular and
physiological adaptations during graded running. Relative
to LR, UR is characterized by a higher step frequency,
increased internal mechanical work, shorter swing/aerial
phase duration, and greater duty factor, while DR is char-
acterized by increased aerial time, reduced step frequency
and decreased duty factor. Grade also modifies foot strike
patterns, with a progressive adoption of a mid- to fore-foot
strike pattern during UR, and rear-foot strike patterns
during DR. In UR, lower limb muscles perform a higher
net mechanical work compared to LR and DR to increase
the body’s potential energy. In DR, energy dissipation is
generally prevalent compared to energy generation. The
increased demands for work as running incline increases
are met by an increase in power output at all joints, par-
ticularly the hip. This implies that UR requires greater
muscular activity compared to LR and DR. Energy cost of
running (C
r
) linearly increases with positive slope but C
r
of
DR decreases until a minimum slope is reached at -20 %,
after which C
r
increases again. The effects of slope on
biomechanics, muscle contraction patterns and physiolog-
ical responses have important implications for injury pre-
vention and success of athletes engaged in graded running
competitions.
Key Points
Grade-specific biomechanical adaptations occur in
uphill and downhill running. These include changes
in foot strike pattern and ground reaction forces,
joint kinematics and kinetics, and impact shock.
The observed adaptations directly impact
neuromuscular activation, as demonstrated by
changes in lower limb electromyographic activity
with grade, these changes being specific to the
considered muscles.
Energy cost of running is also affected by slope,
which linearly increases as slope increases and
linearly decreases as slope decreases until a
minimum value is observed at -20 % grade.
&Guillaume Y. Millet
gmillet@ucalgary.ca
1
Human Performance Laboratory, Faculty of Kinesiology,
University of Calgary, 2500 University Dr NW, Calgary,
AB T2N 1N4, Canada
2
Department of Biomedical Sciences for Health, Universita
`
degli Studi di Milano, Milan, Italy
3
CeRiSM, Research Center for Sport, Mountain and Health,
University of Verona, Rovereto, TN, Italy
4
Salomon SAS, Innovation and Sport Science Lab,
74996 Annecy, France
5
Laboratory of Exercise Physiology, University Savoie Mont
Blanc, 73376 Le Bourget-du-Lac, France
6
Universite
´Co
ˆte d’Azur, LAMHESS, Nice, France
123
Sports Med
DOI 10.1007/s40279-016-0605-y
1 Introduction
Running performance derives from a combination of
anatomical, physiological and behavioural characteristics
that are uniquely evolved in humans [1,2]. Accordingly,
the study of human running has always been of great
interest for exercise physiologists and biomechanists in
order to increase our understanding of the physiological
and biomechanical bases of running capabilities in humans.
However, the vast majority of studies have considered only
level running (LR). This is not surprising because until
recently, the major running events have been road races
such as 10 km, half-marathon and marathon, which are
usually run on flat courses [3]. This, however, is not always
the case. South Africa, for instance, has held the world’s
most famous ultra-marathon race since 1921 [3], the
Comrades Marathon, which consists of a *90-km long
course that varies each year between the ‘up’ run [87 km,
1167 m of positive elevation change (D?)] starting from
Durban, and the ‘down’ run [89 km, 1810 m of negative
elevation change (D-)] starting from Pietermaritzburg.
More importantly, ultra-trail running has become increas-
ingly popular [4]. Indeed, in the last 40 years, there has
been an exponential growth of participation in ultra-trail
races (Fig. 1), likely due to greater appeal of these com-
petitions compared to road and track events [4]. There are
countless races worldwide over a broad range of distances
and variations in slope. In Europe, Ultra-Trail du Mont
Blanc
Ò
(UTMB) and Tor des Geants
Ò
(TdG) are famous
mountain ultra-marathon races, characterized not only by
an extreme distance/duration (166 and 330 km, respec-
tively) but also a high elevation (±9500 and ±24000 m,
respectively) [58]. The total number of participants is now
close to 10000 over five different distances for the UTMB
and 800 for the TdG, with 6500 and 5000 runners refused
in the 2015 edition, respectively. In North America, the
161 km Western States Endurance Run
Ò
(with 5500 m of
D?and 7000 m of D-) has increased exponentially in the
number of finishers from 1977 to 2008 [4].
Ultra-trail and shorter trail running races typically
involve running over (extremely) long distances on rough
terrain with large positive/negative elevation changes
throughout [58]. Thus, the study of the physiological and
biomechanical changes associated with graded running is
important because the ability to appropriately regulate
locomotor behaviour in response to changes in grade is
fundamental to increase our understanding of the control of
human locomotion [915]. Running speed was reported to
decrease by 0.1 to 0.3 kmh
-1
for every 1 % change in
gradient [16,17], leading to important biomechanical,
neuromuscular and physiological adaptations. Events such
as ultra-trail running are likely to be at the extremes of
human tolerance [18], and understanding these adaptations
is of paramount importance for health, training and com-
petition reasons.
The purpose of this review is to provide a comprehen-
sive overview of the literature related to the biomechanical,
neuromuscular and physiological responses to graded
running. In this review, manuscripts were acquired by
searching the electronic databases of MEDLINE, PubMed,
ScienceDirect, SPORTDiscus and Web of Science using
the following keywords in various combinations: ‘level’,
‘uphill’, ‘downhill’, ‘incline’, ‘decline’, ‘grade’, ‘gradient’,
‘slope’, ‘hilly’, ‘running’, ‘physiology’, ‘biomechanics’.
Due to the narrow focus on graded running, a relatively
limited number of articles were retrieved, and conse-
quently, no limit to the search period was applied. Elec-
tronic database searching was supplemented by examining
the bibliographies of relevant articles. In the literature, the
terms ‘slope’ and ‘gradient’ are interchangeably used, and
are employed in this paper to indicate running with varia-
tions in slope unless specified otherwise. The important
effects of graded running on fatigue and tissue damage
warrant their own critical evaluation, and are therefore not
considered in the current review.
2 Biomechanical Adaptations
2.1 Spatiotemporal Parameters and Centre of Mass
Kinematics
Several studies have systematically examined the influence
of uphill running (UR) on kinematic variables
[10,12,14,1923] (Fig. 2) and observed differences when
compared to level running (LR). In UR, higher gradients
have been reported to be associated with a higher step
frequency [10,12,19,24] and consequently increased
internal mechanical work [12]. On the other hand,
Fig. 1 Number of 161-km ultra-trail races in North America from
1977 through 2008 (dashed line) and number of [100-km ultra-trail
races in France (solid line). Based on data from Hoffman et al. [4] and
from personal data
G. Vernillo et al.
123
Gottschall and Kram [10] did not observe any variation in
contact and aerial times at a given running speed
(10.8 kmh
-1
) between LR, UR and downhill running (DR)
(±5.2, 10.5 and 15.8 %). Thus, no changes in duty factor,
step frequency, and stride length were observed. Similarly,
Telhan et al. [24] reported no changes in step frequency or
length when comparing –7 % DR, LR, and 7 % UR at
11 kmh
-1
. UR is characterized by a shorter swing/aerial
phase duration and a greater proportion of the stride cycle
spent in stance, i.e. duty factor (e.g. *33 vs. *29 % for
UR and LR, respectively) [23].
Contact time was found to be constant in DR at
10 kmh
-1
, while aerial time increased at –8 and –5 %
grade compared to LR and UR (2, 5 and 8 %) at the same
speed, resulting in a lower step frequency in DR [19,25].
Similarly, a greater stride length was reported during DR at
12 kmh
-1
and –17.4 % compared to 17.4 % UR at a
similar running speed [26].
Changes in external mechanical energy both during UR
and DR are concomitant with changes in running kine-
matics (Fig. 2; Table 1). Lower vertical displacement of
the centre of mass (CoM) during the stance phase and
greater vertical displacement during the aerial phase were
illustrated in DR compared to UR (e.g. *–20 and ?44 %
at 12 kmh
-1
and ±17.4 % grade, respectively) [26]. In
this study [26], the vertical displacement during the braking
(downward displacement) and propulsive (upward dis-
placement) phases was not separated. Lussiana et al. [25]
specified that running at 10 kmh
-1
and –8 and –5 %
grades induced greater downward displacement of the CoM
during the stance phase compared to running at shallow or
positive grades.
The use of laboratory settings to simulate UR and DR
implies significant technical challenges, limiting the sub-
ject’s ability to regulate running speed [17]. Accordingly,
Townshed et al. [17] investigated speed regulation and
kinematics parameters during a 9525-m overground UR
and DR. They illustrated that, when compared with LR,
stride length was 20.5 % shorter and 16.2 % longer during
UR and DR, respectively, suggesting that running speed
during overground graded running is predominantly regu-
lated by stride length.
2.2 Foot strike and Ground Reaction Forces
Grade has been shown to modify foot strike pattern during
running. Gottschall and Kram [10] and Lussiana et al. [19]
reported that runners progressively adopted a mid-foot or
forefoot strike pattern as slope changed from LR to UR.
Lussiana et al. [19] also reported a less pronounced heel
strike angle during running on inclined versus declined
gradients for a constant running speed. During downhill
trail running conditions, it was observed that high-level
trail runners (based on their skill in DR) adopted a midfoot
strike pattern, whereas medium-level trail runners adopted
a rearfoot strike pattern, irrespective of the technical profile
Fig. 2 Changes in contact time (CT) and step frequency (SF) as a function of speed and grade. Circles denote SF and CT when minimalistic
shoes were wore by the subjects
Biomechanics and Physiology of Graded Running
123
of the run (e.g. steep or shallow slope, with or without
rocks) [27]. In real trail running, however, runners may not
adopt a single foot strike pattern because of the need to
adapt to uneven surfaces, as was observed in a high-level
trail runner [28]. It has been recently observed that the foot
strike pattern adopted during a 6.5-km downhill trail run
(with 1264-m of negative elevation change and run as fast
as possible) differently influences the components of tibial
shock [29]. Although a forefoot strike reduces impact
severity and impact frequency content along the tibial
transverse axis, a rearfoot strike decreases them in the tibial
axial direction.
Gottschall and Kram [10] investigated the ground
reaction forces in running at 10.8 kmh
-1
at grades from
–15.8 to ?15.8 %. Normal (i.e. perpendicular to running
surface) impact force peaks decreased with slope, but
normal active force peaks remained the same. On the
contrary, the parallel propulsive force peaks and impulses
increased with grade [10]. That is, the parallel propulsive
impulse was 57 % lower in DR and 68 % higher in UR at
–15.8 and ?15.8 % slope, respectively, when compared to
LR. These changes likely reflect the need for developing
greater propulsive forces to generate the required upward
acceleration imposed by grade [22]. Recently, Padulo et al.
Table 1 Summary of studies investigating the effect of uphill and downhill running on spatio-temporal variables
Study N Running speed
(km·h-1)a
Slope
(%) CT AT DF SF SL
DeVita et al. [26] 13 12.1 +17.4 - - - -
12.1 -17.4 - - - -
Gottschall and Kram [10] 10 10.8 +15.8
10.8 +10.5
10.8 +5.2
10.8 -5.2
10.8 -10.5
10.8 -15.8
Lussiana et al. [19] 14 10.0 +8.0 -8/5 - -
10.0 +5.0 -8/5 - -
10.0 +2.0 -8/5 --8/5 -
10.0 -2.0 --8/5 -
10.0 -5.0 - -
10.0 -8.0 - -
Padulo et al. [20] 16 14.0 +2.0 -
15.0 +2.0 -
16.0 +2.0 -
17.0 +2.0 -
18.0 +2.0 -
14.0 +7.0 -
15.0 +7.0 -
16.0 +7.0 -
17.0 +7.0 -
18.0 +7.0 -
Padulo et al. [22] 18 15.0 +2.0 -
15.0 +7.0 -
Padulo et al. [21] 65 70% VO2max +2.0
+7.0
Snyder and Farley [14] 9 10.1 +5.2 - - - -
10.1 -5.2 - - - -
Swanson and Caldwell [23] 12 16.2 +30.0 - - -
Telhan et al. [24] 19 11.0 +7.0 - - -
11.0 -7.0 - - -
Nindicates the number of subjects
Changes from level and/or uphill (downhill) running are indicated by black and/or grey arrows, respectively. Ascendant or descendant arrows
indicate significant increases or decreases, respectively, compared to level running and/or uphill running. Horizontal arrows indicate no change
CT contact time, AT aerial time, DF duty factor, SF step frequency, SL step length
a
Except Padulo et al. [21] where the running speed is expressed in percentage of velocity associated with maximum oxygen uptake (vVO
2
max)
G. Vernillo et al.
123
[22] investigated ground reaction forces during running at
15 kmh
-1
on a 50-m asphalt road with different grades.
These authors observed increases in forces and impulses of
12 and 14.7 %, respectively, as the slope increased from 0
to 7 %.
The impact force at foot contact is determined by the
effective lower extremity mass, landing velocity and pas-
sive shoe stiffness [30,31]. Effective mass changes as a
function of joint angle, therefore if kinematics do not
drastically change across gradients, for a given shoe, only
the speed at which the foot strikes the ground would
influence the impact force. As such, one would expect
higher impact forces in DR than in LR or UR. Indeed, a
greater impact peak force was observed during –7 % DR
compared to LR or 7 % UR at 11 kmh
-1
[24]. Similarly,
Gottschall and Kram [10] reported greater normal impact
forces at –15.8, –10.5 and –5.2 % DR than in LR for a
similar running speed (Fig. 3a, b), as well as higher loading
rates at the –15.8 and –10.5 % gradients compared to LR.
Regarding the antero-posterior component of force, greater
braking peak and impulse were observed by Gottschall and
Kram [10] in the three tested DR conditions compared to
LR. For instance, during –15.8 % DR, the braking impulse
increased by 108 % compared to LR [10].
2.3 Mechanics and Joint Kinetics
Knowledge of the changes in joint mechanics and kinetics
during UR and DR running may be important to understand
the adaptive mechanisms induced by sloped running, and
can provide additional information on the risk factors for
lower limb injuries. In the following sections, we discuss
how graded running may impact joint mechanics and
kinetics. A summary of the studies investigating the effect
of UR and DR on the main kinematics and kinetics
parameters at ankle, knee and hip joints is presented in
Table 2.
2.3.1 Mechanical Energy Fluctuations of the CoM
During LR, runners’ CoM behaviour oscillates cyclically
during each stride [32], and both the potential and kinetic
energy fluctuations are in-phase [15,33]. External work,
defined as the sum of potential, and horizontal and vertical
kinetic work associated with the displacement of the CoM,
was measured at different grades (-15 to 15 %) and speeds
(7.9 to 11.8 kmh
-1
)[12]. It was found that negative
external work (i.e. the work done to decelerate the body’s
CoM with respect to the environment [9]) linearly
increased as the slope decreased [12]. For instance, nega-
tive external work was *25 % of the total external work
during UR at 15 % grade, but was *75 % during DR at
–15 % grade. Therefore, more net mechanical energy
generation is required in UR, i.e. energy generation is
greater than absorption. Minetti et al. [12] measured
internal and external mechanical work during UR at dif-
ferent speeds. They reported that both positive external
mechanical work (i.e. the work done to move the body’s
CoM with respect to the environment [9]) and internal
mechanical work (i.e. the work done to move the lower
limbs with respect to the CoM [9]) per unit of distance
travelled increase linearly with slope. Thus, UR requires a
greater net mechanical work with each step to increase the
body’s potential energy [13], and the muscles must perform
greater net positive work both to raise the body’s CoM to
its height at toe-off and to give it sufficient kinetic energy
to reach its highest point during the aerial phase [14].
In DR, albeit positive external work is necessary at
shallow slopes (-5 %), energy dissipation, rather than
energy generation, is generally observed [14,15]. Snyder
et al. [15] measured the mechanical energy fluctuations of
the CoM during LR, UR and DR at 10.8 kmh
-1
. They
observed that some positive mechanical energy is still
required at shallow grades (-5.6 and –10.5 %), but almost
all mechanical energy is negative at –15.6 % grade.
Accordingly, at 12 kmh
-1
, both the total negative and
positive work of the joints were 38 % greater and 265 %
lower in DR (-17.4 %) than in UR (?17.4 %), respec-
tively [26].
2.3.2 Ankle Joint Kinematics and Kinetics
Combined with video analysis, ground reaction forces can
provide useful information about the joint torques of the
support leg during running. Data on the effects of grade
on ankle kinematics are rather heterogeneous, but there is
basically no major or consistent change between LR, UR,
and DR both in terms of ankle kinematics and negative
work. Ankle positive work does not change at shallow
slopes but increases in UR at steep slopes compared to
DR.
Over 30 min of DR (-7 %) and LR (5 % faster than the
individual anaerobic threshold: 12.7 ±0.7 kmh
-1
), ankle
range of motion during the braking phase (dorsiflexion)
only increased after 15 min of DR [34]. Comparing UR
(30 % grade) and LR at 16.0 kmh
-1
, Swanson and Cald-
well [23] observed that the ankle was more dorsiflexed at
foot strike and exhibited a lower dorsiflexion range of
motion in UR. However, the plantar flexion range of
motion was greater during the propulsive phase in UR.
Ankle angular velocity was also considerably lower during
the stance phase in UR than in LR. During the swing phase,
ankle range of motion was greater in UR than in LR, with
angular velocity being similar between the two conditions.
Although Telhan et al. [24] observed no changes in ankle
kinematics between LR, UR and DR (±7%)at11kmh
-1
,
Biomechanics and Physiology of Graded Running
123
Buczek and Cavanagh [35] showed that the instant of
maximum dorsiflexion, ankle peak power absorption, ankle
negative work and the relative duration of the ankle neg-
ative work period over the stance phase increased by 10,
36, 52 and 34 %, respectively, during –8.3 % DR com-
pared to LR at a given speed (15.9 kmh
-1
). Furthermore,
the power generation at the ankle joint was decreased by
49 % in DR (-17.4 % grade) compared to UR (?17.4 %)
at 12 kmh
-1
and no difference in ankle negative work was
observed between DR and UR [26]. Roberts and Belliveau
[13] observed no variation in the net work done at the ankle
joint in UR compared to LR at 10.8 to 12.6 kmh
-1
and 0,
10 and 21.2 % grades.
2.3.3 Knee Joint Kinematics and Kinetics
Knee flexion at contact increases in UR compared to LR
and DR [23,24]. During the braking phase, knee power
absorption increases in DR as a result of a greater knee
range of motion; while during the propulsion phase, knee
power generation increases at steep UR gradients but is
constant across grades at shallow slopes [26].
DR was associated with substantially more knee exten-
sion at initial contact than LR [34,35] and UR [19]ata
given speed [range: 10–16 kmh
-1
]. DR was also associ-
ated with greater knee range of motion in the sagittal plane
during the braking phase [34,35], as well as a delayed time
Fig. 3 Ground reaction forces
(GRFs) expressed as percentage
of body weight (BW) in
different gradient conditions
during treadmill running at
10.8 kmh
-1
:anormal;
bparallel. Adapted from
Gottschall and Kram [10], with
permission. cTypical signals of
tibial accelerations for the
vertical (solid line) and
transversal (dashed line)
components measured along the
tibial anteromedial aspect in
various conditions of slope
during a trail running race.
From authors’ personal data
G. Vernillo et al.
123
Table 2 Summary of studies investigating the effect of uphill and downhill running on the main kinematics and kinetics (angle of flexion at foot strike, sagittal range of motion, negative work
or power absorption and positive work or power generation) at ankle, knee and hip joints
Ankle kinematics and kinetics Knee kinematics and kinetics Hip kinematics and kinetics
Study n
Running
speed
(km·h-1)
Slope
(%)
Dorsiflexion
at FS
Sagittal
RoM
Negative
work/
power
absorption
Positive
work/
power
generation
Flexion
at FS
Sagittal
RoM
Negative
work/
power
absorption
Positive
work/
power
generation
Flexion
at FS
Sagittal
RoM
Negative
work/
power
absorption
Positive
work/
power
generation
Buczek and Cavanagh [35] 716.0 -8.3 -- - -
DeVita et al. [26] 13 12.1 +17.4 -- - - --
12.1 -17.4 -- - - --
Padulo et al. [20] 14 10 .0 +8.0 -8/5 --- - - --- - -
10.0 +5.0 -8/5 --- - - ---- -
10.0 +2.0 --- - - ---- -
10.0 -2.0 --- - - --- - -
10.0 -5.0 -8/5 --- - - ---- -
10.0 -8.0 -8/5 --- - - ---- -
Mizrahi et al. [34] 14 12.7 -7.0 -- --- -
Swanson and Caldwell [23] 12 16.2 +30.0 ---- - - --- - -
Telhan et al. [24] 19 1 1.0 +7.0 -- - - -- -
11.0 -7.0 -- - - -- -
nindicates the number of subjects
Changes from level and/or uphill (downhill) running were indicated by black and/or grey arrows, respectively. Ascendant or descendant arrows indicated significant increases or decreases,
respectively, compared to level running and/or uphill running. Horizontal arrows indicate no change
FS foot strike, RoM range of motion
Biomechanics and Physiology of Graded Running
123
to peak knee flexion [35] at a constant running speed (*13
or 16 kmh
-1
) compared to LR. These kinematic changes
lead to greater negative work (?21 %) and relative dura-
tion of the negative work at the knee over the stance phase
(?7 %) compared to LR [35], as well as increased power
absorption at the knee compared to LR and UR [24].
DeVita et al. [26] observed that 54 % more power was
developed at the knee joint during DR at 12 kmh
-1
and
–17.4 % compared to UR at ?17.4 % and the same speed,
which may be attributed to greater energy absorption at the
knee joint during the braking phase of running. When
comparing UR (30 % grade) and LR at 16.0 kmh
-1
, it was
observed that the knee was more flexed at foot strike in UR
than in LR (59.7°vs. 21.0°, respectively) [23]. These
authors also reported a lower knee range of motion during
the braking phase (flexion), but a greater knee range of
motion during the propulsion phase (extension) in UR.
When comparing UR and LR at 10.8 and 12.6 kmh
-1
at 0,
10 and 21.2 % grades, Roberts and Belliveau [13] observed
no effect of slope on the net work done at the knee joint.
2.3.4 Hip Joint Kinematics and Kinetics
In DR, the hip range of motion increases, inducing greater
hip power absorption compared to LR [24,26,34]. Con-
versely in UR, hip power generation increases compared to
DR [13].
More specifically, greater hip range of motion was
reported during the braking phase at –7 % DR compared to
LR at 12.7 ±0.7 kmh
-1
[34]. Swanson and Caldwell [23]
observed that the hip was more flexed at foot strike in UR
(30 % grade) than in LR at the same speed (16 kmh
-1
).
Although in LR the hip flexed during the braking phase and
then extended during the propulsion phase, the authors
stated that in UR the hip was extending rapidly at foot
strike, more slowly during the braking phase, and rapidly
again during the propulsion phase. The authors also
reported a greater hip range of motion during the propul-
sion phase (extension) in UR than in LR. Swanson and
Caldwell [23] investigated the kinetics of the lower limb
muscles during LR and UR at 16.2 kmh
-1
and 30 %
grade. They observed that average hip power during the
swing phase was *200 % higher during UR. Roberts and
Belliveau [13] extended this work by measuring joint
kinematics during running between 10.8 and 12.6 kmh
-1
at 0, 10 and 21.2 % grades. They found that the net work
done at the hip increased with running incline (*?140 %
at 21.2 % grade). Using a musculoskeletal model, Yoko-
zawa et al. [36] confirmed these previous findings, illus-
trating that the estimated muscle torque of the hip extensors
and flexors were greater during UR at different speeds and
9.1 % grade.
Hip power during the braking phase was also found to
be higher at –7 % DR compared to LR at the same running
speed (11 kmh
-1
)[24]. Hip joint power was lower at
–17.4 % DR compared to 17.4 % UR, for a similar speed
[26]. It is worth mentioning that DeVita et al. [26] noticed
that the ground reaction force vector was directed farther
from the hip joint centre in UR than in DR, lengthening the
lever arm. The authors proposed that the shorter moment
arm at the hip reduced the work demand on the hip flexor
and/or extensor muscles.
2.3.5 Summary
In summary, the overall joint work on the lower limbs
seems to be 28 % higher in UR than in DR [26]. However,
it also seems that graded running does not dramatically
affect the distribution of negative work between the joints.
Both during DR and UR, the knee joint performed the
highest negative work (DR: 63 %, UR: 58 %), followed by
the ankle (DR: 23 %, UR: 30 %) and the hip (DR: 15 %,
UR: 12 %) [26]. This distribution changes when positive
work is considered. Hip seems to be the most stressed joint
(DR: 48 %, UR: 55 %), followed by the ankle (DR and
UR: 32 %) and knee (DR: 20 %, UR: 13 %) [26]. Thus,
there are differences in joint stabilization between UR and
DR that may directly influence, for example, the risk of
developing graded running-injuries.
2.4 Impact Shock Attenuation in Downhill
As mentioned in Sect. 2.2, initial foot contact kinetics
change as a function of slope. Impact accelerations are also
largely affected by DR. DeVita et al. [26] postulated that
the greater impact force in DR would likely cause larger
accelerations of musculo-skeletal tissues, requiring greater
energy dissipation by muscles as well as the heel pad,
bones and spinal discs. In laboratory experiments (i.e.
standardized running speed) and in real practice (i.e. run-
ning speed increases as the gradient decreases), negative
correlations were observed between slope and axial,
transverse (i.e. along the axis of the anteromedial tibial
aspect) and resultant peak tibial accelerations [28,37,38]
(Fig. 3c), as well as with high-frequency vibration content,
i.e. median frequency, along these three acceleration
components [28]. Hamill et al. [38] reported that tibial
shock increased by 30 % during DR on a –8.7 % slope
compared to LR at the same speed. Chu and Caldwell [37]
observed an average of 23 % and 48 % increase in peak
tibial and head acceleration, respectively, at –12 % DR
compared to LR (15 kmh
-1
). Increases of 51 and 125 % in
impact-related frequencies (i.e. power spectral densities
within the 12–20 Hz bandwidth) were also observed at the
G. Vernillo et al.
123
tibia and head, respectively [39]. According to these find-
ings, DR induces a decrease in shock attenuation [37].
However, Mizrahi et al. [34] observed similar peak tibial
acceleration but larger peak sacral acceleration in –7 % DR
compared to LR at 12.7 ±0.7 kmh
-1
. These authors also
reported a lower amplitude within the impact frequency
range at the tibia during DR compared to LR, but no dif-
ferences at the sacrum were observed. Interestingly, Chu
and Caldwell [37] found a bimodal response in peak shock
attenuation in DR, i.e. half of the subjects illustrated
increased shock attenuation during DR compared to LR
while the other half illustrated decreased shock attenuation.
Analysing these two subgroups, the authors found several
differences in their respective kinematic adaptations to DR:
(i) those with reduced shock attenuation in DR displayed a
5°greater dorsiflexion and 4.3°lower hip flexion at heel
strike than those with increased shock attenuation; (ii) at
mid-stance, the subgroup with reduced shock attenuation
exhibited greater dorsiflexion, knee flexion, and lower hip
flexion compared to the subgroup with increased shock
attenuation; (iii) the subgroup with reduced shock attenu-
ation increased the stance and stride duration compared to
the subgroup with increased shock attenuation. Therefore,
the results of Chu and Caldwell [37] suggest that shock
attenuation can be increased during DR by adopting a less
pronounced heel strike and a forward leaning trunk. In this
sense, it has been recently observed during a downhill trail
run that the more anterior the foot strike pattern, the greater
the axial and resultant impact-related vibrations (i.e.
12–20 Hz) between tibia and sacrum. Since knee flexion at
initial contact increases when forefoot striking (e.g. Shih
et al. [40]), one could assume that the improved shock
attenuation with anterior foot strike patterns could be
related to a greater knee flexion at initial contact. Indeed,
Gottschall and Kram [10] proposed that impact forces can
be moderated by increasing knee flexion at initial contact
and reducing stride length during DR. This is in line with
previous observations that increased knee flexion improves
shock attenuation during various dynamic and static tasks
[4144].
3 Neuromuscular Adaptations
Table 3shows differences in the electromyographic
(EMG) activity of different muscle groups investigated as
a function of running grade. Abe et al. [45] compared
vastus lateralis activity among 0 and ±5 % grades at
11.9 kmh
-1
and observed lower activity during the con-
centric phase (i.e. propulsion) of DR compared to UR but
not LR. These authors did not observe any differences
between slopes in the intensity and duration of vastus
lateralis activity during the eccentric phase (i.e. braking).
One would expect more vastus lateralis activity during the
eccentric phase in DR. Indeed, as previously mentioned,
the absolute and relative negative works as well as the
percentage of stance time in negative work were signifi-
cantly higher for knee extensor muscles in DR than in LR
[35]. A possible explanation for the lack of significant
difference in the study of Abe et al. [45] could be the
rather low gradient examined (±5 %) which may have
induced a minimal change in the power absorption per-
formed by the knee extensors, suggesting also that a
minimal gradient of –7 % is necessary to significantly
increase knee power absorption [24,35]. The ratio between
vastus lateralis muscle activity during the eccentric phase
to activity during the concentric phase was significantly
greater during DR compared to UR [45]. Mizrahi et al.
[34] investigated the consequences of 30-min of DR
(slope: –7 %) compared to LR at a speed slightly higher
than anaerobic threshold (12.7 ±0.7 kmh
-1
). They found
no difference in EMG activity of the rectus femoris muscle
between LR and DR in the first 15 min of running. While
the effects of fatigue are beyond the scope of the present
review, it can be noted that differences between LR and
DR running appeared after 15 min [34].
The slope-related changes in UR biomechanics as dis-
cussed above would be expected to require higher activa-
tion patterns of lower limb muscles, i.e. greater motor unit
recruitment in UR. However, the effects of slope on EMG
activity are not the same for all lower limb muscles. This is
likely due to the varying roles of different lower limb
muscles in producing force at various phases of the gait
cycle [46] and to the fact that slope alters joint mechanics
non-uniformly [26]. EMG studies have provided consid-
erable information on the timing of individual muscle
activity throughout the gait cycle of UR. Globally, a greater
activation compared to LR is usually found in the iliopsoas,
gluteus maximus, adductor muscles, hamstrings and vastii
muscles, tibialis anterior, and gastrocnemius (Table 3).
More specifically, the hip flexors have been found to
generate more energy and higher moments during the
swing phase [13,23]. During the braking/absorption phase
(i.e. from foot strike to mid-stance), higher activation has
been measured for the gluteus maximus, vastii muscles,
gastrocnemius and soleus whereas during the propulsion
phase (i.e. from mid-stance to toe-off), higher activation
has been measured for the gluteus maximus, hamstrings
and vastii muscles, gastrocnemius and soleus [22,23,47].
Collectively, these studies illustrated that greater EMG
activity in the lower limb muscles (e.g. up to 83 and 100 %
for gluteus maximus and vastus lateralis, respectively)
exists in UR at a given speed. This greater EMG activity is
likely associated with a greater force production [48] pri-
marily for concentric muscle contractions during the sec-
ond phase of stance [12].
Biomechanics and Physiology of Graded Running
123
Exercise-induced contrast shifts in magnetic resonance
images before and after (time between termination of
exercise and completion of the post-exercise image being
11–12 min) high-intensity running at *115 % of the peak
oxygen uptake (VO
2peak
) were examined by Sloniger et al.
[49]. These authors showed that the EMG activity of the
lower limb muscle was 6 % greater during UR at 10 %
grade compared to LR. Using the same technique, a sub-
sequent study demonstrated that, compared to LR, UR
required a greater activation of the vastus group (?23 %)
and soleus (?14 %) paralleled by less activation of the
rectus femoris (-29 %), gracilis (-18 %) and semitendi-
nosus (-17 %) [50].
It is important to note that most of the aforementioned
studies assessed EMG activity/muscle activation at a given
absolute speed, limiting their applicability to real world
settings where speed is naturally reduced during UR. The
behaviour of muscle activity when the exercise intensity
(or the energy expenditure) is kept nearly constant is cur-
rently unknown.
4 Physiological Consequences of Biomechanical
and Neuromuscular Changes
4.1 Energy Cost of Running
In LR, the energy cost of running (C
r
), defined as the
amount of energy spent to transport the subject’s body a
given distance [51], does not change with speed when
expressed as oxygen uptake [52]. However, when C
r
is
expressed in terms of caloric unit cost it seems to be more
sensitive to changes in speed, even when normalized per
distance travelled [53,54]. Changes in the kinetic and
potential energy in one stride are almost in-phase [9],
implying that the energy storage accomplished by a mus-
cle-tendon unit and passive muscle elasticity (the so-called
stretch-shortening cycle [55,56]) contribute to one of the
energy-saving mechanisms during running [45,5759].
This feature is recognized as one of the major determinants
of C
r
[9]. Indeed, in running, the storage and release of
elastic energy contributes to accelerate the body upwards
during the propulsive phase and reduce energy production
needed during the concentric phase, since the advantage of
elastic energy is how much muscle work it can replace
[14,15,60]. During LR, it has been estimated that the
elastic energy stored in the Achilles’ tendon and the foot
arch aponeurosis accounted for approximately 43 % of the
total positive mechanical energy at each step [15]. How-
ever, during UR and DR, Snyder et al. [14] hypothesized
that the use of elastic energy may be compromised due to a
mismatch between the possibility to store the elastic energy
during landing and to use that elastic energy during take-
off. Indeed, at 10 kmh
-1
the maximum possible elastic
energy use was 20.4 and 11.7 % lower when UR (?5.2 %)
and DR (-5.2 %) were compared to LR [14], reflecting
more a decrease in the maximum possible elastic energy
storage and return rather than a change in the anatomically
estimated elastic energy storage [15]. Yet, even if energy
released from the stretch-shortening cycle is low and the
ability of the muscle tendon units to store elastic energy
during landing and to release that energy during take-off is
reduced [12], the main explanation for the higher C
r
in UR
is the increased net mechanical energy generation required
to overcome the potential energy associated with slope.
Thus, greater muscle activity (see above) is required to
generate a relatively high amount of positive (concentric)
work during the push-off phase in order to both raise the
CoM and offset the diminished maximum possible elastic
energy storage and return [14]. This ultimately results in a
C
r
increase. In the last decade, several studies have
examined the effects of increased gradient on C
r
. Despite
some methodological differences in the way C
r
was
expressed [i.e. oxygen cost (mlkg
-1
m
-1
) or caloric cost
(Jkg
-1
m
-1
)], all studies report a linear increase in C
r
with
each increment in the slope gradient (see an example in
Fig. 4)[12,14,19,22,45,61,62].
When measured on a treadmill at different moderate
slopes, a reduction of C
r
, as well as heart rate, ventilation or
total EMG, has been consistently observed in DR com-
pared to LR and UR [45,6366], with the effects of grade
not being significantly different between males and females
[65]. For moderate slopes, the following equation has been
proposed [64]:
VO2¼6:8192 þ0:1313 vþ1:2367 % grade
where vis the running velocity in m min
-1
. This would
suggest that C
r
(when expressed as oxygen uptake) is 22 %
lower at –5 % DR compared to LR. However, when
considering steeper slopes (from –45 to ?45 %), the
following fifth-order polynomial regression has been
proposed [61]:
Cr¼155:4i530:4i443:3i3þ46:3i2þ19:5iþ3:6
where iis the gradient in %. According to this equation, the
decrease in C
r
is no longer linear after –20 % grade, where
the relationship inverts and C
r
increases with further
decreases in downhill slope (Fig. 4)[61]. Note that this
optimal slope was found to be lower (-10 %) by the same
authors in a previous paper [12] yet C
r
seems to plateau
between –10 and –20 %. This is in line with the downhill
slope at which mechanical energy must no longer be gen-
erated (-16 %) according to Snyder et al. [15]. In other
words, this optimal negative grade can be explained by the
fact that on steeper downhill grades, mechanical energy
dissipation must occur, whereas on less steep downhill
G. Vernillo et al.
123
grades, though more mechanical energy is dissipated than
generated, some positive mechanical energy must be gen-
erated [15]. Interestingly, the slope allowing for the best C
r
(-20 %) is steeper than the slope at which the best energy
cost of walking is observed [61]. Gottschall and Kram [10]
argued that at progressively steeper declines, the parallel
propulsive impulse decreased exponentially, while the
parallel braking impulse increased linearly. This different
change in the propulsive and braking impulses could
explain the increase in the metabolic cost below –20 %
[61] due to higher concentric muscle contractions at steeper
versus shallow declines [10].
Vertical speed directly measured during UR is very
close to that predicted by Minetti et al. [61] whereas the
predicted speed for DR overestimates the measured speed
[61]. Minetti et al. [61] identified methodological issues
that hampered accurate reproduction of outdoor conditions
in the laboratory (e.g. differences between the rough terrain
and smooth treadmill surface). We are not aware of any
study measuring C
r
in UR or DR in the field at a constant
speed. The key point is that C
r
measured during running
does not reflect the reality in the field. In particular,
assuming that C
r
is independent of speed in DR is probably
wrong. Minetti et al. [61] also introduced the notion of
vertical cost of running (C
r-vert
), defined as the energy
expenditure to run a distance that corresponds to a vertical
displacement of 1 m. Contrary to classic C
r
,C
r-vert
was
found to be stable below –20 % and above ?20 %.
Recently, Giovannelli et al. [67] extended this knowledge,
observing that at a fixed vertical speed of 1.26 kmh
-1
there is a range of angles for which C
r-vert
is minimized
(between 37.2 % and 70 %), with a minimum value at
50.9 %.
4.2 Other Physiological Specificities of Graded
Running
It is known that hilly races (even though not characterized
by a net change in elevation) are not as fast as level ones.
Staab et al. [68] suggested that although running pace
changed inversely with percentage grade on hilly courses,
subjects were not able to maintain a constant energy
expenditure during the race. Indeed, the increase in DR
pace was inadequate to maintain a level VO
2
[68], i.e. the
change from LR to UR and from LR to DR resulted in a
40 % increase and a 27 % decrease in VO
2
, respectively.
This indicates that the greater metabolic demands of UR
are not compensated for by the lower metabolic demands
of DR [69]. In an attempt to investigate speed regulation
during overground running, Townshend et al. [17] showed
that while natural pace was reduced in UR and increased in
DR compared to LR, these pace changes were not enough
to keep VO
2
stable: VO
2
was found to be 100 % of venti-
latory thresholds in UR, 89 % in LR and 79 % in DR.
Interestingly, these authors also reported that the velocity
in LR was systematically influenced by the preceding
slope, i.e. UR or DR [17]. It has also been reported that in
simulated competition conditions, lactate increased in UR
compared to LR even though running pace decreased [68].
In line with this finding, for a given blood lactate con-
centration, which also corresponded to the same VO
2
but
obviously different velocities (i.e. *8.5, 11 and
Table 3 Summary of studies examining the effects of uphill and downhill running on the electromyography (EMG) activity of different lower
limb muscles
Study nRunning speed
(kmh
-1
)
Slope
(%)
ILP GMed GMax HA RF MH BF VL VM TA MG SOL
Abe et al. [45] 8 11.9 0 vs. ?5?
0 vs. -5?
-5 vs. ?5%
Padulo et al. [22] 18 15 0 vs. 2 %!?!!?
0 vs. 7 %!?!!?
Swanson et al. [23] 12 16.2 0 vs. 30 %%??%?%%
Wall-Scheffler et al. [47] 34 6.5, 9.7 and 13 0 vs. 10 %%%%%%% %
0 vs. 15 %%%%%%% %
0 vs. 20 %%%%%%% %
Yokozawa et al. [36]
a
6 18 0 vs. 9.1 %%%%?%?? ?
nindicates the number of subjects
%,!,?indicate increase, decrease, no change in EMG activity, respectively, as a function of the slope change
ILP iliopsoas, GMed gluteus medius, GMax gluteus maximus, HA hip adductors, RF rectus femoris, MH medial hamstring, BF biceps femoris,
VL vastus lateralis, VM vastus medialis, TA tibialis anterior, MG medial gastrocnemius, SOL soleus
a
Indicates that muscle activities of the lower limbs were assessed by using a musculoskeletal model
Biomechanics and Physiology of Graded Running
123
13.5 km h
-1
for UR, LR and DR, respectively), Kolkhorst
et al. [70] reported that rating of perceived exertion tended
to be higher in DR than LR and UR, and LR tended to be
higher than UR. Graded running is associated with modi-
fied breathing patterns, i.e. the locomotor-respiratory cou-
pling [71]. It is worth reporting that increasing or
decreasing the stride frequency away from preferred values
alters the metabolic cost similarly during LR, UR and DR
[14] and this is true when the stride frequency is manipu-
lated or the optimal stride frequency applied [14].
Costill et al. [72] demonstrated that glycogen depletion
(as assessed by muscle biopsy) was higher in the vastus
lateralis, gastrocnemius and soleus muscle fibres after 2-h of
UR at 10 % grade compared to 2-h of LR at the same relative
intensity (i.e., *75 % VO
2max
). Given that glycogen uti-
lization by human skeletal muscle varies as a function of both
work done and intensity [73], this finding confirms that a
higher percentage of muscle mass is recruited during UR in
these muscles. Furthermore, the greater muscle activity
observed during UR (see Sect. 3) seems also to be respon-
sible for a higher peak oxygen deficit [49].
A few studies systematically examined the influence of
slope on maximal accumulated oxygen deficit, an indicator
of the anaerobic capacity [74]. Olesen [62] determined this
during treadmill running at 1, 10, 15 and 20 % grade.
Compared with running at 1 % grade, maximal accumu-
lated oxygen deficit increased by 37 % at 10.5 % grade and
*80 % at 15 % grade, without any further increase at
20 % grade. Walker et al. [75] and Sloniger et al. [49]
reported similar findings, with a 26 and 21 % increase in
the maximal accumulated oxygen deficit as treadmill grade
was increased from 0 to 10 %, respectively. Together,
these findings may indicate that the maximal anaerobic
energy production is greater during UR due to increased
skeletal muscle mass activation in the lower limbs
[49,62,75], even if differences in running efficiency as
well as testing and calculations procedures (i.e. a linear
relationship between work rate and energy demand) cannot
be ruled out [76].
In summary, graded running induces specific adapta-
tions related to modified physiological strain proportional
to the slope gradients from –20 to ?45 %, and natural
reductions in pace may not necessarily allow for a reduc-
tion in total strain. Furthermore, given the higher muscle
activation and the increase in the anaerobic energy pro-
duction that leads to lower C
r
, runners exert a higher
physiological strain during UR compared to LR. Con-
versely, due to the mechanical consequences discussed in
Sect. 2, DR may result in more mechanical stress, partic-
ularly in non-familiarised runners, leading to muscle
damage and lower limb injuries.
5 Conclusion and Future Directions
The present review shows that several grade-specific dif-
ferences exist between LR, UR and DR regarding biome-
chanics, neuromuscular adaptations and physiological
responses. Higher step frequency and increased internal
mechanical work, shorter swing/aerial phase duration, and
greater duty factor are the main kinematic features of UR.
Compared to LR, DR is characterized by a similar contact
time and a tendency toward higher aerial time and lower
step frequency. A progressive adoption of a mid- to fore-
foot and rear-foot strike pattern has been observed during
UR and DR, respectively. During UR, lower limb muscles
perform more net mechanical work compared to LR and
DR to increase the body’s potential energy. The increased
demand for work during UR is met by an increase in power
output at all joints, particularly at the hip which induces
Fig. 4 Metabolic energy cost of
running (C
r
) as a function of
grade. Asterisk indicates
significantly different from level
gradient (P\0.0001). Based on
data from Minetti et al. [61]
G. Vernillo et al.
123
greater muscular activity compared to LR and DR, and in
turn a linear increase in the energy cost of running. In DR,
energy dissipation is higher than energy generation and this
decreases the energy cost during DR until a minimum is
reached at –20 % and increases again at steeper negative
slopes. Thus, the metabolic cost associated with various
types of muscle contractions remains a valid explanation
for the high and low cost of UR and DR, respectively, from
–20 to ?45 % grades.
DR increases tibial shock and impact force, which have
been associated with overuse injuries. Additionally, the
muscle activity required for the increased power and
eccentric energy absorption during DR would place addi-
tional stress on musculoskeletal tissues. However, since in
graded (e.g. trail) running, the locomotion pattern changes
more often than in LR, overuse injuries related to repetitive
movement may be attenuated in graded running compared
to LR.
In conclusion, the present review represents a useful
synthesis of all research describing the relevant biome-
chanical and (neuro)physiological changes associated with
graded running. However, as we have highlighted
throughout this review, important gaps in our biomechan-
ical and physiological understanding of graded running still
exist. In particular, controlled training studies or well-de-
signed interventional experiments are needed to investigate
the effect of manipulating both running speed and positive/
negative slope at the same relative intensity on muscle
activity.
Compliance with Ethical Standards
Funding No sources of funding were used to assist in the preparation
of this article.
Conflict of interest Gianluca Vernillo, Marle
`ne Giandolini, W. Brent
Edwards, Jean-Benoı
ˆt Morin, Pierre Samozino, Nicolas Horvais and
Guillaume Y. Millet declare that they have no conflicts of interest
relevant to the content of this review.
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Biomechanics and Physiology of Graded Running
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... External mechanical work is performed mainly to elevate and lower the body center of mass when moving uphill and downhill, respectively [11]. Although stride time is shorter during uphill running than during level-ground running, the duration for which positive mechanical work is generated is longer due to the longer contact time [14,15]. Therefore, the bouncing mechanism gradually disappears to contain the increase in muscular power (push in uphill vs. brake in downhill) by decreasing the downward (in uphill) versus the upward (in downhill) displacement of the body center of mass. ...
... Specifically, uphill running is characterized by a higher step frequency, increased internal mechanical work, shorter swing/aerial phase duration, and greater duty factor in comparison to downhill running. Furthermore, the downhill is characterized by reduced step frequency, decreased duty factor and increased aerial time [15]. ...
... The ability to predict performance in hilly distance running may lead to a better understanding of the characteristics of training and improved performance among distance runners. Previous findings have found better relation between CoT and performance when slope-specific analyzed [15]. Furthermore, performance models applied to trial running have shown that VO 2 max and fat mass explained 84% of the performance in short trial running [16]. ...
Article
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Incline and level running on treadmills have been extensively studied due to their different cardiorespiratory and biomechanical acute responses. However, there are no studies examining the performance determinants of outdoor running on hilly terrains. We aimed to investigate the influence of anthropometrics, muscle strength, and cardiorespiratory and gait spatiotemporal parameters during level (0%) and inclined (+7%) running on performance in level and hilly 5-km races. Twenty male recreational runners completed two 5-km outdoor running tests (0% vs. +7% and −7%), and two submaximal (10 km·h−1) and incremental treadmill tests at 0 and 7% slopes, after complete laboratory evaluations. The velocity at maximal oxygen consumption (VO2max) evaluated at 7% incline and level treadmill running were the best performance predictors under both hilly (R2 = 0.72; p < 0.05) and level (R2 = 0.85; p < 0.01) conditions, respectively. Inclusion of ventilatory and submaximal heart rate data improved the predictive models up to 100%. Conversely, none of the parameters evaluated in one condition contributed to the other condition. The spatiotemporal parameters and the runners’ strength levels were not associated to outdoor performances. These results indicate that the vVO2max evaluated at similar slopes in the lab can be used to predict 5-km running performances on both level and hilly terrains.
... The difficulty in studying trail running performance is related to the diversity of terrain, incline and race distance. In particular, large uphill and downhill sections of a race provoke biomechanical changes that are related to different energy demands (Balducci et al., 2017;Vernillo et al., 2017). For this reason, the application of the "classical prediction model" is questionable (Ehrstrom et al., 2018). ...
... Ehrstrom et al. (2018) estimated a muscle fatigue index determined by averaging maximal concentric torque values and found that together with VO2 max and running economy at 10% incline could predict the performance of a 27-km race to 98%. The importance of neuromuscular characteristics in trail racing is warranted because there are findings that uneven terrain of steep up hills and down hills increases the recruitment of vastus lateralis (Vernillo et al., 2017) and provokes fatigue similar to resistance and eccentric training (Balsalobre-Fernandez, Santos-Concejero & Grivas, 2015). ...
... Furthermore, when % of body fat was combined with VO2 max the predictive power of the model was increased from 59 to 83%. We suppose that these findings are related to the increased positive external mechanical work during up hills associated with the replacement of centre of mass and the advantage that low adiposity athletes have in the propulsion of the body (Vernillo et al., 2017). ...
Article
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p>The recent worldwide popularity of trail running has raised the necessity of studying the physiological profile of this sport. Although trail running races are long distance endurance events, the variety of their terrain, incline and duration prevents the application of the classical predictive model of level running. Thus, the aim of the present study was to investigate the physiological and anthropometric parameters that determine short trail race performance. Twenty-five moderately trained trail runners participated in a 15 km trail running race, consisting of 9 km positive and 6 km negative incline. Four days after the race they followed a laboratory protocol for the measurement and estimation of anthropometric and physiological parameters (maximal oxygen uptake, velocity at maximal oxygen uptake, ventilatory threshold, velocity at ventilatory threshold, running economy, flexibility, muscle power, aerobic capacity). The results revealed high correlations between the 15 km race performance and velocity at maximal oxygen uptake (r = 0.81), ventilatory threshold (r = 0.88), muscle power of knee extensor (r = 0.50 – 0.53), anaerobic capacity (r = 0.65) and body fat percentage (r = 0.7). Another two parameters that were highly correlated with the 15 km mountain trail race performance were both the positive and negative incline time (r = 0.95 and r = 0.96, respectively). Our conclusions confirmed previous findings that performance in trail running cannot be predicted with the same variable model as level running. Article visualizations: </p
... And it might be due to the increased fibre work done of the Gas muscle, as the mice suffered from EHS via running on a rotating running wheel in the hot and humid environment which considered as the uphill running, mice performed uphill running mainly with the fore foot strike pattern, which is characterized by a higher step frequency, increased internal mechanical work, shorter swing/aerial phase duration, and greater duty factor. 28 Moreover, the heat production of the Gas muscle is more as the murine fast-twitch muscle than that of the slow-twitch soleus muscles. 2 Collectively, the types of cell death in Gas muscle tissue were still not addressed. Our bioinformatics analysis showed that the activation of ferroptosis potentially play an important role in the RM development following EHS. ...
... 35 In addition, the Hippo signalling pathway in skeletal muscle is involved in the regulation of exercise adaptation. 28 However, both the exercise (physiological limitation) and heat stress (environmental factor) are the most importantly predisposing factors for EHS. 36 In current study, genetic inhibition of Yap was able to suppress the primary myoblasts ferroptosis following EHS in vitro, and pharmacological inhibition of YAP with verteporfin blocks the RM development following EHS in vivo; overexpression of Yap mRNA can upregulate the expression of ACSL4, while genetic inhibition of YAP can downregulate the expression of ACSL4 in vitro. ...
Article
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Background: Rhabdomyolysis (RM) is a common complication of exertional heat stroke (EHS) and constitutes a direct cause of death. However, the mechanism underlying RM following EHS remains unclear. Methods: The murine EHS model was prepared by our previous protocol. RNA sequencing is applied to identify the pathological pathways that contribute to RM following EHS. Inhibition of the acyl-CoA synthetase long-chain family member 4 (ACSL4) was achieved by RNA silencing in vitro prior to ionomycin plus heat stress exposure or pharmacological inhibitors in vivo prior to heat and exertion exposure. The histological changes, the iron accumulation, oxidized phosphatidylethanolamines species, as well as histological evaluation and levels of lipid metabolites in skeletal muscle tissues were measured. Results: We demonstrated that ferroptosis contributes to RM development following EHS. Ferroptosis inhibitor ferrostatin-1 administration once EHS onset significantly ameliorated the survival rate of EHS mice from 35.357% to 52.288% within 24 h after EHS (P = 0.0028 compared with control) and markedly inhibited RM development induced by EHS. By comparing gene expression of between sham heat rest (SHR) (n = 3) and EHS (n = 3) mice in the gastrocnemius (Gas) muscle tissue, we identified that Acsl4 mRNA expression is elevated in Gas muscle tissue of EHS mice (P = 0.0038 compared with SHR), so as to its protein levels (P = 0.0001 compared with SHR). Followed by increase in creatine kinase (CK) and myoglobin (MB) levels, the labile iron accumulation, decrease in glutathione peroxidase 4 (GPX4) expression, and elevation of lipid peroxidation products. From in vivo and in vitro experiments, inhibition of Acsl4 significantly improves muscle cell death caused by EHS, thereby ameliorating RM development, followed by reduction in CK and MB levels by 30-40% (P < 0.0001; n = 8-10) and 40% (P < 0.0001; n = 8-10), restoration of GPX4 expression, and decrease in lipid peroxidation products. Mechanistically, ACSL4-mediated RM seems to be Yes-associated protein (YAP) dependent via TEA domain transcription factor1/TEA domain transcription factor4. Conclusions: These findings demonstrate an important role of ACSL4 in mediating ferroptosis activation in the development of RM following EHS and suggest that targeting ACSL4 may represent a novel therapeutic strategy to limit the skeletal muscle cell death and prevent RM after EHS.
... Several studies have confirmed the benefits of running, but also a high injury risk [11][12][13], defined as tissue damage or other derangement of normal physical function due to participation in sports, resulting from the rapid or repetitive transfer of kinetic energy [14]. In the case of trail running, the combination of high mileage, together with changing terrain and large uphill and downhill slopes forces the athlete to a continuous change in terms of technique and fatigue management [15]. These characteristics have led many authors to generally accept that mountain races, and more specifically ultra-distance races, induce serious and harmful alterations in the neuromuscular [16], energetic [17], biomechanical [18], and musculoskeletal oxygenation fields [19]. ...
... On the other hand, the continuous change of the movement pattern caused by downhill reduces the number of repetitive gradual onset injuries related to the repetition of movements because, as mentioned above, the variation of the terrain modifies the support pattern continuously, lengthening or shortening the stride and adapting the footprint to the characteristics of the ground. This results in the non-repetition of a footprint pattern [15] unlike what happens in races on flat terrain and without abrupt changes in elevation (either asphalt or tartan). ...
Article
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The aim of this study was to analyze the injury incidence in young trail runners according to the body region, type, mode of onset, and moment of occurrence, both in total and detailed by sex. Thirty-five male and sixteen female young elite trail runners, aged between 15 and 22 years, completed a questionnaire regarding the injury incidence in the last 2 years. Comparison of the proportions of the injury incidence within groups (all, male, and female runners) and between groups (male vs. female runners) was computed using z and Fisher’s exact tests. Results showed that most of the injuries in male runners occurred in ankle (54.3%; p < 0.001; ES = 0.520). New injuries were the most common type in male (60.0%; p < 0.001; ES = 0.829) and female runners (52.0%; p = 0.005; ES = 0.585). Acute sudden onset (55.7%; p = 0.002; ES = 0.722) and repetitive sudden onset injuries (48.0%; p = 0.002; ES = 0.141) were the most frequent in male and female athletes, respectively. Joint sprains (48.6%; p < 0.001; 0.464) were the most reported injuries in male runners. Comparative analysis between sexes showed that exacerbation injuries were higher in females (24.0%) than in male runners (8.6%), with p = 0.046 (ES = 0.205). However, female runners reported less incidence by acute sudden onset injuries (32.0%) than male runners (55.7%), with p = 0.042 (ES = 0.209). Young trail runners showed a specific injury profile due to the distinctive characteristics of the mountain terrain compared to the athletic modalities
... On the other hand, as in other endurance disciplines [17][18][19][20] , an analysis has also been made of the physical performance and physiological characteristics of the runners taking part in trail running 3,6,8,15,16,[21][22][23][24] . It is common to use laboratory tests to measure the cardiovascular capacity of runners 3,8,15,16 analysing maximal and submaximal aerobic performance markers that make it possible to determine the physiological characteristics of the athletes and to subsequently prescribe, monitor and assess training. ...
... Although a number of studies conducted with asphalt runners reported greater mass in the lower limbs (generally measuring the calf circumference) is related to poorer running economy [56][57][58] , leading to poorer performance on level or slightly sloping terrain, the results obtained herein show that this trend may be the opposite for races on sloping terrain. Vernillo et al. 24 reported that the power required for uphill terrain is greater than for level running given that, with the increase in the elevation profile, runners tend to run with a forefoot strike pattern 24 , probably demanding more work from the ankle extensor muscles (gastrocnemius and soleus) 55 . It is possible that these differences between level running and graded running can explain the contradictory results obtained when compared to previous studies. ...
Article
Purpose: The aim of this study was to describe the anthropometrical and cardiovascular characteristics of short course trail runners and analyze the associations, if any, between both anthropometric and cardiovascular features of amateur trail runners. Material and method: Anthropometrical evaluation and an incremental maximum test with 10% of grade on a treadmill were performed on a group of 10 short distance amateur trail runners. Results: Significant negative correlations were found between the body max index (BMI) and the speed at VT1 (Vel VT1) (r = -0,95, p < 0,001), or the time to reach VT1 (r = -0,91, p = 0,002) and between the body fat percentage and the respiratory exchange ratio at VT2 (r = -0,80, p = 0,016) or the time to reach VT2 (r = -0,83, p = 0,01). Calf circumference was also found to be positively associated with oxygen consumption at VT1 (r = 0,74, p = 0,037), at VT2 (r = 0,90, p = 0,002) and with the maximal oxygen uptake (r = 0,85, p = 0,007). Conclusions: Results indicate that both body fat percentage and calf circumference could be related to the performance on an incremental test protocol with inclination in amateur trail runners.
... deficits in voluntary activation) alterations that affect muscle force production and, ultimately, performance (Easthope et al., 2010;Giandolini et al., 2016c;Millet et al., 2011). Furthermore, the modifications in the step mechanics and spring-mass behaviour during and after TR events (Degache et al., 2013;Vernillo et al., 2017) suggested that those alterations in the neuromuscular function, together with muscular pain, could induce changes in running mechanics to minimize further impairments. For instance, the increase on step frequency during prolonged TR exercise could be the consequence of adopting a safer and smoother running style to minimize the displacement of the centre of mass and limit pain and/or mechanical stress (Degache et al., 2016;Morin et al., 2011b). ...
Article
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The aim of this study was to investigate if acute impairments of the footwear midsole materials could interact with the modifications of the running patterns after a fatiguing trail running race. We hypothesized that introducing a control pair of shoes after the race, would modify running mechanics by partially correcting the alterations generated by the deteriorated properties of the shoes worn during the race. Eighteen participants ran on a treadmill at 10km/h pre- and post-race in three conditions varying the step frequency (free or fixed) or the footwear (control or personal shoes). Spatiotemporal parameters, tibial accelerations across the running conditions and shoe mechanical properties were measured. Significant changes in shoe characteristics (i.e. reduced thickness and increased stiffness) were measured after the race in the personal footwear. At free step frequency, significant changes in contact (+1.3 ± 3.5%) and flight (-19.4 ± 23.7%) times, step frequency (+2.9 ± 2.9%) and peak-to-peak amplitude acceleration at the tibia level (-3.6 ± 18%) were found. The peak-to-peak amplitude of the mediolateral axis showed a significant shoe x time interaction meaning that the control footwear could have had an impact in the running pattern. However, contrary to our hypothesis, no interaction effects were found between the changes in footwear and the changes in spatiotemporal parameters. In conclusion, impairments of shoes characteristics affect impact but not running mechanics.
... The added longitudinal bending stiffness was of interest in this study especially for the uphill running condition. During uphill running, foot strike patterns tend to drift towards mid-foot (Gottschall and Kram, 2005;Vernillo et al., 2017). With this shift to mid-foot strike, we anticipated the mechanical advantage about the ankle to be modified due to the location of the resistive force from the center of pressure. ...
Article
Road-racing shoes recently experienced major changes. In the recent past, lightweight, thin midsole shoes were thought to help runners maximize their performance. But, in 2017, Nike released the Vaporfly shoe which transformed the thinking about racing shoe design. Incorporating a curved carbon fiber plate embedded in a thick, compliant and resilient midsole resulted in a reduced metabolic cost across a range of running speeds. We hypothesized the new style of shoes would be less effective uphill than downhill due to the larger ground reaction forces and hence greater elastic energy storage in the shoe during downhill running. Eighteen runners completed two days of testing, each comprising two trials of two shoe models (Saucony Endorphin Pro (EP) and Type A) and three grade conditions (uphill, level and downhill), i.e. 12 trials per day. Oxygen uptake, ground reaction forces, and lower-body kinematics were captured during each condition. Comparisons of the percent metabolic benefit were made between shoes for each grade. Stride rate, ground time, peak vertical force, and flight time were regressed with the percent metabolic benefit of the EP over the Type A shoe across grades. Metabolic benefits of the Endorphin Pro were similar across the three grade conditions (p = 0.778). No significant correlations were observed between how much benefit one runner got over another specific to grade. The new style of road-racing shoes effectively decreases metabolic cost equally across grades. Differences in running mechanics between runners did not explain greater individual metabolic benefits between shoe conditions during uphill or downhill running.
Chapter
Walking and running are the basic means of influencing an individual’s condition, his or her health and fitness. Due to the fact that various forms of physical load are used in movement training, the cause must be described by a single number, which reflects the volume, intensity, and form of physical load. One of the possibilities is to determine the energy cost (EC) of the applied physical activities. Possibilities of evaluation of EC in laboratory and field conditions using the speed of movement allow to streamline movement training. To achieve the desired lasting effect, it is necessary that the total EC exceeds the so-called stimulus threshold, that is, the subject of physical training must reach a certain minimum level of total EC of applied physical training. The total energy content of exercise allows you to design individual exercise programs. In the paper, we present the relationships between energy and speed of movement for the most commonly used physical activities to increase fitness in people without regular physical training–walking and running in different age groups and for men and women and the principles of design of movement interventions using this parameter, as well as the implemented programs and their effect.
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Strike index is a measurement of the center of pressure position relative to the foot length, and it is regarded as a gold standard in classifying strike pattern in runners. However, strike index requires sophisticated laboratory equipment, e.g., force plates and optical motion capture. We present a method of estimating strike index using data from a shoe-mounted inertial measurement unit (IMU) analyzed by a participant-independent convolutional neural network (CNN), which consists of convolutional, max-pooling, and fully-connected layers. To promote data variability, 16 participants were required to land with three strike patterns (rearfoot, midfoot, and forefoot strike) while running on an instrumented treadmill in four conditions i.e., two footwear types and two running speeds. Using the proposed approach, strike index was estimated with a root mean square error of 6.9% and a R² of 0.89. Training and testing the model with different variations of the data collected showed that the model was robust to changes in speed. The proposed approach enables accurate estimation of strike index outside of traditional gait laboratories. This solution potentially improves running performance and reduces injury risk in distance runners.
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Objectives Hip/groin running-related injuries (RRI) are relatively uncommon. It is unclear if runners of either sex are disproportionately affected. Our objective was to systematically review differences in hip/groin RRIs between males and females. Data Sources A structured and comprehensive search of four medical literature databases was performed (PubMed, Embase, Ovid Medline, and CINAHL). Terms searched were: risk, epidemiology, hip injury, groin injury, overuse injury, running, sprinting, and track and field. Study Selection Studies reporting sex-specific data on hip/groin RRIs in adult runners were included. Data was extracted and reviewed independently by two authors. Study Appraisal and Data Synthesis Sex-specific injury rates, risk factors, and return to sport (RTS) following hip/groin RRI were extracted. Risk of bias was assessed using the Joanna-Briggs Institute Critical Appraisal Tool. Results 10 studies with 7,353 total runners were included: 2,315 (47%) males and 2,559 (53%) females. The mean age of included runners was 37.3 ± 8.9 years and mean weekly running distance was 10.4 ± 8.4 km. Hip/groin injuries comprised 10.1% (491/4,874) of total RRIs, including 6.3% of RRIs sustained by males and 11.0% by females. Three studies reported significantly higher rates of hip/groin RRIs in female runners. One study reported significantly higher rates of gluteus medius and adductor RRIs for females and males, respectively. One study identified female sex as an independent risk factor for hip/groin RRIs. Three studies reported on RTS after hip/groin RRIs: the pooled RTS rate was 81.4% (57/70) at 1 to 368 days after injury. Limitations Data was pooled when possible; however, there was considerable clinical, methodological, and statistical heterogeneity across studies. Conclusions Hip/groin RRIs comprise a greater percentage of total injuries among injured female runners relative to males. Females may be at a higher risk for sustaining hip/groin RRIs, though more research on risk factors and RTS is needed.
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Vertical kilometer foot races consist of a 1,000 m elevation gain in less than 5,000 m of overall distance and the inclines of the fastest courses are ~30°. Previous uphill locomotion studies have focused on much shallower angles. We aimed to quantify the metabolic costs of walking and running on very steep angles and to biomechanically distinguish walking from running. Fifteen runners (10 M, 5 F, 32.9±7.5 years, 1.75±0.09 m, 64.3±9.1 kg) walked and ran for 5 minutes at 7 different angles (9.4°, 15.8°, 20.4°, 24.8°, 30.0°, 35.0° and 39.2°) all at a fixed vertical velocity (0.35 m/s). We measured the metabolic rates and calculated the vertical costs of walking (Cwvert) and running (Crvert). Using video analysis, we determined stride frequency, stride length and duty factor (fraction of stride that each foot is in ground contact). At all angles other than 9.4°, Cwvert was cheaper than Crvert (average -8.45%±1.05%; p<0.001). Further, broad minima for both Cwvert and Crvert existed between 20.4° and 35° (average Cwvert 44.17±0.41 J・kg(-1)・m(-1) and average Crvert 48.46±0.35 J・kg(-1)・m(-1)). At all angles and speeds tested, both walking and running involved having at least one foot on the ground at all times. But, in walking, stride frequency and stride length were ~28% slower and longer, respectively than in running. In conclusion, we found that there is a range of angles for which energy expenditure is minimized. At the vertical velocity tested, on inclines steeper than 15.8°, athletes can reduce their energy expenditure by walking rather than running.
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Neuromuscular fatigue has traditionally been examined using isolated forms of either isometric, concentric or eccentric actions. However, none of these actions are naturally occurring in human (or animal) ground locomotion. The basic muscle function is defined as the stretch-shortening cycle (SSC), where the preactivated muscle is first stretched (eccentric action) and then followed by the shortening (concentric) action. As the SSC taxes the skeletal muscles very strongly mechanically, its influence on the reflex activation becomes apparent and very different from the isolated forms of muscle actions mentioned above. The ground contact phases of running, jumping and hopping etc. are examples of the SSC for leg extensor muscles; similar phases can also be found for the upper-body activities. Consequently, it is normal and expected that the fatigue phenomena should be explored during SSC activities. The fatigue responses of repeated SSC actions are very versatile and complex because the fatigue does not depend only on the metabolic loading, which is reportedly different among muscle actions. The complexity of SSC fatigue is well reflected by the recovery patterns of many neuromechanical parameters. The basic pattern of SSC fatigue response (e.g. when using the complete exhaustion model of hopping or jumping) is the bimodality showing an immediate reduction in performance during exercise, quick recovery within 1–2 hours, followed by a secondary reduction, which may often show the lowest values on the second day post-exercise when the symptoms of muscle soreness/damage are also greatest. The full recovery may take 4–8 days depending on the parameter and on the severity of exercise. Each subject may have their own time-dependent bimodality curve. Based on the reviewed literature, it is recommended that the fatigue protocol is ‘completely’ exhaustive to reduce the important influence of inter-subject variability in the fatigue responses. The bimodality concept is especially apparent for stretch reflex responses, measured either in passive or active conditions. Interestingly, the reflex responses follow parallel changes with some of the pure mechanical parameters, such as yielding of the braking force during an initial ground contact of running or hopping. The mechanism of SSC fatigue and especially the bimodal response of performance deterioration and its recovery are often difficult to explain. The immediate post-exercise reduction in most of the measured parameters and their partial recovery 1–2 hours post-exercise can be explained primarily to be due to metabolic fatigue induced by exercise. The secondary reduction in these parameters takes place when the muscle soreness is highest. The literature gives several suggestions including the possible structural damage of not only the extrafusal muscle fibres, but also the intrafusal ones. Temporary changes in structural proteins and muscle-tendon interaction may be related to the fatigue-induced force reduction. Neural adjustments in the supraspinal level could naturally be operative, although many studies quoted in this article emphasise more the influences of exhaustive SSC fatigue on the fusimotor-muscle spindle system. It is, however, still puzzling why the functional recovery lasts several days after the disappearance of muscle soreness. Unfortunately, this and many other possible mechanisms need more thorough testing in animal models provided that the SSC actions can be truly performed as they appear in normal human locomotion.
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Full-text available
Foot strike identification has become an important topic since it may be related to injury risk and performance. Due to step variability and the influence of environmental features on running biomechanics, it is relevant to assess as many steps as possible in field conditions. Our purpose was to apply a novel simple method to assess foot strike and impact from continuous acceleration measurements over a 45 km trail running race. Three wireless tridimensional accelerometers were set on the left tibia and shoe (at the heel and metatarsals) of the current best ultratrail runner. Vertical, antero-posterior and resultant peak tibial accelerations and median frequencies were measured. Step frequency (SF) was calculated from tibial acceleration. Foot strike was quantified from the time between heel and metatarsal peak accelerations (THM). Foot strike classification was performed according to THM criteria and expressed in percentages of rearfoot, midfoot and forefoot steps. Multiple linear regressions were computed to assess relationships between the impact magnitude and slope, SF and THM. Over the first 20 km, 5530 steps were analysed. The pattern classification revealed on average 18.5% of rearfoot strike, 32.6% of midfoot strike and 48.9% of forefoot strike over the »82 min analysed in the runner studied. The impact magnitude for him may be related to slope, also taking into account speed, SF and landing technique. The main findings of this study were that (1) portable accelerometers make possible the assessment of foot strike and shock acceleration in situ, (2) the antero-posterior and resultant components of tibial acceleration should not be neglected in the measurement of stress severity, and (3) the trail running world champion presents an atypical foot strike profile.
Technical Report
Full-text available
Foot strike identification has become an important topic since it may be related to injury risk and performance. Due to step variability and the influence of environmental features on running biomechanics, it is relevant to assess as many steps as possible in field conditions. Our purpose was to apply a novel simple method to assess foot strike and impact from continuous acceleration measurements over a 45 km trail running race. Three wireless tridimensional accelerometers were set on the left tibia and shoe (at the heel and metatarsals) of the current best ultratrail runner. Vertical, antero-posterior and resultant peak tibial accelerations and median frequencies were measured. Step frequency (SF) was calculated from tibial acceleration. Foot strike was quantified from the time between heel and metatarsal peak accelerations (THM). Foot strike classification was performed according to THM criteria and expressed in percentages of rearfoot, midfoot and forefoot steps. Multiple linear regressions were computed to assess relationships between the impact magnitude and slope, SF and THM. Over the first 20 km, 5530 steps were analysed. The pattern classification revealed on average 18.5% of rearfoot strike, 32.6% of midfoot strike and 48.9% of forefoot strike over the »82 min analysed in the runner studied. The impact magnitude for him may be related to slope, also taking into account speed, SF and landing technique. The main findings of this study were that (1) portable accelerometers make possible the assessment of foot strike and shock acceleration in situ, (2) the antero-posterior and resultant components of tibial acceleration should not be neglected in the measurement of stress severity, and (3) the trail running world champion presents an atypical foot strike profile.
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Trail runners are exposed to a high number of shocks, including high-intensity shocks on downhill sections leading to greater risk of osseous overuse injury. The type of foot strike pattern (FSP) is known to influence impact severity and lower-limb kinematics. Our purpose was to investigate the influence of FSP on axial and transverse components of shock acceleration and attenuation during an intense downhill trail run (DTR). Twenty-three trail runners performed a 6.5-km DTR (1264 m of negative elevation change) as fast as possible. Four tri-axial accelerometers were attached to the heel, metatarsals, tibia and sacrum. Accelerations were continuously recorded at 1344 Hz and analyzed over six sections (~400 steps per subject). Heel and metatarsal accelerations were used to identify the FSP. Axial, transverse and resultant peak accelerations, median frequencies and shock attenuation within the impact-related frequency range (12–20 Hz) were assessed between tibia and sacrum. Multiple linear regressions showed that anterior (i.e. forefoot) FSPs were associated with higher peak axial acceleration and median frequency at the tibia, lower transverse median frequencies at the tibia and sacrum, and lower transverse peak acceleration at the sacrum. For resultant acceleration, higher tibial median frequency but lower sacral peak acceleration were reported with forefoot striking. FSP therefore differently affects the components of impact shock acceleration. Although a forefoot strike reduces impact severity and impact frequency content along the transverse axis, a rearfoot strike decreases them in the axial direction. Globally, the attenuation of axial and resultant impact-related vibrations was improved using anterior FSPs.
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