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Effects of Foot Strike and Step Frequency on Achilles Tendon Stress During Running

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Achilles tendon (AT) injuries are common in runners. The AT withstands high magnitudes of stress during running which may contribute to injury. Our purpose was to examine the effects of foot strike pattern and step frequency on AT stress and strain during running utilizing muscle forces based on a musculoskeletal model and subject specific ultrasound-derived AT cross sectional area. Nineteen female runners performed running trials under six conditions including rearfoot strike and forefoot strike patterns at their preferred cadence, +5%, and -5% preferred cadence. Rearfoot strike patterns had less peak AT stress (P<.001), strain (P<.001), and strain rate (P<.001) compared to the forefoot strike pattern. A reduction in peak AT stress and strain were exhibited with a +5% preferred step frequency relative to the preferred condition using a rearfoot (P<.001) and forefoot (P=.005) strike pattern. Strain rate was not different (P>.05) between step frequencies within each foot strike condition. Our results suggest that a rearfoot pattern may reduce AT stress, strain, and strain rate. Increases in step frequency of 5% above preferred, regardless of foot strike pattern, may also lower peak AT stress and strain.
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365
ORIGINAL RESEARCH
Journal of Applied Biomechanics, 2016, 32, 365 -372
http://dx.doi.org/10.1123/jab.2015-0183
© 2016 Human Kinetics, Inc.
Michael Lyght, Matthew Nockerts, and Thomas W. Kernozek are with the
La Crosse Institute for Movement Science, Department of Health Profes-
sions—Physical Therapy Program, University of Wisconsin-La Crosse,
La Crosse, WI, USA. Robert Ragan is with the Department of Physics,
University of Wisconsin-La Crosse, La Crosse, WI, USA. Address author
correspondence to Thomas W. Kernozek at tkernozek@uwlax.edu.
Effects of Foot Strike and Step Frequency on Achilles
Tendon Stress During Running
Michael Lyght, Matthew Nockerts, Thomas W. Kernozek, and Robert Ragan
University of Wisconsin-La Crosse
Achilles tendon (AT) injuries are common in runners. The AT withstands high magnitudes of stress during running which may
contribute to injury. Our purpose was to examine the effects of foot strike pattern and step frequency on AT stress and strain
during running utilizing muscle forces based on a musculoskeletal model and subject-specic ultrasound-derived AT cross-
sectional area. Nineteen female runners performed running trials under 6 conditions, including rearfoot strike and forefoot strike
patterns at their preferred cadence, +5%, and –5% preferred cadence. Rearfoot strike patterns had less peak AT stress (P < .001),
strain (P < .001), and strain rate (P < .001) compared with the forefoot strike pattern. A reduction in peak AT stress and strain
were exhibited with a +5% preferred step frequency relative to the preferred condition using a rearfoot (P < .001) and forefoot
(P=.005) strike pattern. Strain rate was not different (P > .05) between step frequencies within each foot strike condition. Our
results suggest that a rearfoot pattern may reduce AT stress, strain, and strain rate. Increases in step frequency of 5% above
preferred frequency, regardless of foot strike pattern, may also lower peak AT stress and strain.
Keywords: kinematics, kinetics, modeling, force
Distance running is a common exercise modality. Approxi-
mately 56% of recreational runners sustain an overuse injury, with
Achilles tendon (AT) injuries accounting for 5% to 18%.1,2 During
running, forces are transmitted through the AT with loads of up to 8
times body weight.3 The large, repetitive loading may result in col-
lagenous micro trauma and tissue degeneration.4 The degeneration
may progress if loading occurs before repair is complete, resulting in
weakened and dysfunctional tissue with accompanied morphologi-
cal changes, including fatty inltration, capillary proliferation, and
a loss of collagen structure and ber integrity.5 Research has shown
that the incidence of AT injuries is higher in men; however, AT inju-
ries in female athletes have increased over the past 2 decades.6,7 It
has been suggested that estrogen may have an inhibitory effect on
the rate of collagen deposition and tissue repair after exercise.8 The
mechanical properties (ie, stress, strain, and strain rate) of tendons
have been shown to be associated with the orientation and deposi-
tion of collagen bers within the tendon.9 As such, understanding
the behavior of the AT mechanical properties in female runners may
provide guidance to the treatment of AT-related running injuries and
minimize the risk of injury.
Altered mechanical properties may predispose the AT to injury
as adequate tendon integrity is necessary to withstand the high
magnitude forces involved in running. Tendon stress is a function
of the force applied to the tendon divided by its cross-sectional
area and may be an important factor for injury risk.10 Most tendons
experience peak stress values below 30 MPa; however, the AT has
been shown to withstand peak stresses of 70 MPa to 80 MPa.10–12
The magnitude of AT stress developed during sporting activities
such as running may signicantly contribute to the pathomechanics
of AT injuries.13
Likewise, a reduction in AT stress may be benecial in the
management and prevention of running-related AT injuries. One
would surmise that having a larger cross-sectional area would be
benecial toward reducing average stress for the given force. It has
been suggested that the AT of runners may undergo an adaptive
hypertrophic response to modulate the stress imposed on the tendon
due to the repetitive, high-loading nature of running.14 However, a
recent study found no difference in the weight-normalized AT cross-
sectional area between female habitual runners and those who did
not regularly engage in sporting activities.15 The results suggest that
the female AT may not undergo a similar hypertrophic response to
running as shown in their male counterparts.16 Furthermore, recent
ndings have illustrated that the AT mechanical properties’ response
to exercise may differ between sexes, with the female AT displaying
different strain and tendon elongation behavior.17
General running form may have an effect on AT loading.
Almonroeder et al reported that altering foot strike pattern can
inuence AT loading in running.18 Heiderscheit et al19 and Clarke
et al20 reported reductions in ankle and shank kinetics, respectively,
with increases in running cadence. These alterations largely have
been suggested to reduce the risk or reoccurrence of running-related
injury.18–20 Foot strike pattern and step frequency alter the transmis-
sion of forces to the AT as they are inuenced by the muscle forces
of the triceps surae and motions of the ankle. As such, running
mechanics may have signicant implications on AT mechanical
properties by inuencing kinematic, kinetic, and spatiotemporal
factors involved in running.18,19
To better understand how running mechanics may inuence AT
mechanical properties, it is important to control for the variations in
AT cross-sectional areas among individuals. To our knowledge, there
are presently no studies that have examined the effects of foot strike
pattern and step frequency using subject-specic, AT cross-sectional
366 Lyght et al
JAB Vol. 32, No. 4, 2016
area data. Therefore, the purpose of the study was to investigate
the inuence of foot strike pattern and step frequency on peak AT
stress, strain, and strain rate. We also compared differences in spa-
tiotemporal and joint kinematic and kinetic biomechanical factors
between running conditions during the manipulation of foot strike
pattern and step frequency. Our primary hypothesis was that peak
AT stress, strain, and strain rate will be lower during the rearfoot
strike (RFS) pattern compared with the forefoot strike (FFS) pat-
tern. We also hypothesized that there will be an inverse relationship
between peak AT stress, strain, and strain rate with step frequency
within foot strike conditions.
Methods
Participants
Nineteen healthy, female subjects (age: 21.5 ± 1.3 y; height: 166.4
± 5.6 cm; mass: 59.5 ± 8.7 kg; weekly mileage: 31.9 ± 18.8 km/
wk), who were running a minimum of 10 miles per week, partici-
pated in the study. Fourteen (74%) of the participants subjectively
reported habitually running with a RFS pattern, while the remaining
5 (26%) reported a FFS pattern. Exclusion criteria included any
reported pregnancy, cardiovascular pathology, current lower extrem-
ity pain, surgery within the last year, and less than 5 on the Tegner
activity scale—a measure for regular participation in recreational
activities that require running. Participants were informed of the
procedures, benets, and potential risks involved in the study, and
signed a letter of informed consent before participation; all methods
were approved by the institutional review board at the University
of Wisconsin-La Crosse.
Protocol
Before running trials, a GE LOGIQ Ultrasound P6 (Waukesha, WI,
USA) was used to image the AT with a ML6-15 probe. Participants
rested prone on a treatment table with their right ankle measured
to 90° in a neutral position with a goniometer. This position
was chosen to avoid the anisotropy effect by facilitating contact
between the probe and the tendon.21 Ultrasound gel (Aquasonic
Clear, Faireld, NJ, USA) was applied to the head of the probe.
The probe was placed 10 cm proximal to the calcaneal insertion
on the posterior aspect of the shank between the lateral and medial
malleoli perpendicular to the AT to collect transverse images of
the AT. Ultrasound AT cross-sectional areas were measured using
ImageJ (Wayne Rasbrand, National Institutes of Health, USA)
software. After images were obtained, participants were tted with
a standard model of footwear (Model 625SB, New Balance, Boston,
MA, USA). Preferred step frequency (172.8 ± 8.5 steps/min) was
obtained by instructing participants to run on a treadmill set at 3.5
m/s at their preferred step frequency. Step frequency was visually
determined by counting the number of foot strikes over a 30-second
period. The number obtained was multiplied by 2 and preferred step
frequency was calculated as steps per minute.
After instruction, participants ran down a 20-m runway under
6 conditions, including RFS and FFS patterns at their preferred
cadence, at +5% preferred cadence, and at –5% preferred cadence.
A random number generator was used to determine order of testing.
Participants were permitted a period of time to practice between
each condition; the period consisted of completing a minimum of 3
practice running trials. A successful trial was dened as a running
performance in which the foot strike pattern met the predened
criteria, correct adherence to step frequency was maintained, speed
was in accordance to the allocated range, and the desired foot con-
tacted the force platform. Speed was controlled to a range of 3.33
m/s to 3.68 m/s, calculated by the elapsed time (seconds) it took
the runner to pass through 2 photoelectric timing gates placed 2 m
apart at the midpoint of the runway. This standardized speed was
chosen based on the average training speed of our participants.
Cadence was visually determined by the experimenters, and the
runners were instructed to match the sound of the metronome. Foot
strike pattern, step frequency, and targeting of the force platform
were closely monitored during all trials by 3 assessors. Agreement
from all 3 assessors regarding step frequency and lack of targeting
the force platform was required for a successful trial. Targeting
was determined through subjective assessment whether the sub-
ject increased or decreased their step length to make contact with
the force platform. Running cadence was monitored visually with
assistance from a metronome set to the desired step frequency. An
in-shoe pressure system (Novel GMBH, Munich, Germany) was
used to monitor and verify foot strike pattern. Similar to Cavanagh
and Lafortune,22 a subject’s center of pressure (COP) located on the
rear third part of the foot at initial contact was indicative of a RFS
pattern. A FFS pattern was dened as the COP occurring on the
anterior third part of the foot at initial contact. The in-shoe pressure
data from the steps within our motion capture space were checked to
determine if either a rearfoot or forefoot loading pattern was used.
If this was not clear, the trial was repeated. Foot strike pattern was
later conrmed during post processing by calculating the location
of the COP relative to the foot as described by Cavanagh and Lafor-
tune.22 If the subject targeted the force platform and the foot strike
pattern and/or step frequency did not match the testing condition,
the trial was discarded and repeated. A total of 5 successful trials
were completed for each of the 6 conditions.
Instrumentation
Before running trials, participants were prepared for 3D motion
analysis where participants were equipped with 47 reective mark-
ers for data collection.23 A static, neutral, standing calibration was
collected before all dynamic running trials. Kinematic data were
recorded at 180 Hz with 15 Motion Analysis cameras (Motion
Analysis Corporation, Santa Rosa, CA, USA) that surrounded the
runway. Kinetic data were simultaneously collected using a force
platform (Model 4080, Bertec Corporation, Columbus, OH, USA)
sampling at 1800 Hz.
Data Processing
Using the Human Body Model (Motek ForceLink, Amsterdam,
Netherlands), muscle forces were calculated based on a 44 degree
of freedom (DOF) musculoskeletal model with 16 rigid segments,
where the head was modeled as a single segment with 3-DOF rela-
tive to the thorax. The trunk was modeled as 3 segments (pelvis,
midtrunk, and thorax), where 3-DOF were coupled as based on kine-
matics to allow equal amounts of exion, twist, and side-bending
in each. The upper arms had 6-DOF relative to the thorax and were
tracked as separate linkages, elbow with 2-DOF, and wrist with
2-DOF to enable the model to be tracked in a real-time environ-
ment.24 The pelvis segment was able to rotate and translate in all 3
dimensions with respect to the ground and had 6-DOF. The method
by Bell et al was used to model the hip joint as a ball-in-socket with
3 rotational DOF.25 The knee joint was constrained as a function of
knee exion and modeled as a single DOF hinge joint, the subtalar
joint was modeled with 1-DOF, and the ankle joint was modeled
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Achilles Tendon Stress During Running 367
JAB Vol. 32, No. 4, 2016
with 1-DOF. Inertial characteristics of these segments were based
on participants’ total body mass and segment lengths.26 In all, 300
muscle tendon units were represented: 86 in the legs, 204 in the
arms, and 10 in the trunk. Muscle insertion points and wrapping
points were determined by Delp et al.27
The Human Body Model used global optimization as a kine-
matic solver to determine skeletal model kinematics using the
Levenberg-Marquardt algorithm, and then the equations of motion
were used to determine the joint moments.28,29 The residual loads,
3 force and 3 moments on the pelvis were minimized in the inverse
dynamics processing within the Human Body Model. Muscle
force estimates were based on the joint moments by minimizing
a static cost function such that the sum of squared muscle activa-
tions was related to maximum muscle strengths at each time step
of the model.30 The static optimization was solved with a recurrent
neural network.31
The specic moment arms from the Delp’s data are based on
ankle angle (a), and were as follows:
Lateral Gastroc (LG) = 0.0421895–2*0.0056403*(a)–
3*0.0059199*((a)^2))
Medial Gastroc (MG) = 0.0411549–2*0.0052579*(a)–
3*0.0057784*((a)^2))
Soleus (S) = 0.0407103–2*0.0070189*(a)–
3*0.0055918*((a)^2)))
The muscle forces from the Human Body Model were then used
to quantify the AT force by summing the muscle force output from
the medial and lateral gastrocnemius and soleus. The AT stresses
and strains were calculated for the 10 cm of the tendon closest to
the calcaneus insertion point, where most AT injuries occur.32 Since
this segment is distal to all the muscle attachments, the AT tension
was calculated by simply adding the soleus and gastrocnemius
forces. The stress was calculated by dividing the AT force by the
cross-sectional area, which was measured 10 cm from the calca-
neus insertion point. The strain (again, for the lowest 10 cm of the
AT) was calculated using the average Young’s modulus of 819 cm
reported by Wren et al,32 whose in vitro stress–strain experiments
were performed on the lowest 10 cm of the AT. The strain rate was
then determined from the strain versus time curve.
Statistical Analysis
Two-way (foot strike and step frequency) multivariate analysis
of variance (MANOVA) statistics with repeated measures were
used to examine the main effects of foot strike pattern and step
frequency. Alpha was set to .05. Follow-up univariate analyses were
performed to examine differences in joint kinematic and kinetic,
spatiotemporal, and step frequency within foot strike pattern data.
In the presence of signicant main effects, Bonferroni post hoc
analyses were used to examine pairwise comparisons. Effect sizes
were also used to quantify the practical importance of the observed
effects between foot strike patterns and step frequencies.33 Statisti-
cal calculations were completed using SPSS 22.0 software (IBM,
Armonk, NY, USA).
Results
All 19 participants were included in the nal analyses. Average
foot strike indices were 75.3% ± 9.1% for the FFS condition and
24.7% ± 8.9% for the RFS condition. Multivariate analysis gener-
ated a Wilks’s λ of 0.015 (P = .001), indicating that the variables of
interest could be investigated independently. Follow-up univariate
tests revealed signicant foot strike effects for all measured vari-
ables (Table 1). There were signicant main effects for both foot
strike pattern (F(3,16) = 69.57, P < .001) and step frequency (F(6,13)
= 10.94, P < .001), but no interaction between foot strike and step
frequency (F(6,13) = .309, P = .921). Signicant step frequency
effects were observed for all measured variables except the distance
from the center of mass (COM) to heel and ankle angle at initial
contact (Table 2).
Peak AT stress decreased by 24.0% when running with a RFS
pattern versus a FFS pattern (Figure 1). In addition, the RFS pattern
exhibited a reduction in AT force, strain, and strain rate relative to
the FFS pattern. Kinematic, spatiotemporal, and kinetic changes
between foot strike patterns were also observed. The FFS pattern
resulted in greater knee exion and ankle plantar exion angles
at initial contact; however, the distance from the COM to heel at
initial contact was greater while running with a RFS pattern. A FFS
resulted in a greater peak plantar exion moment.
Running with a step frequency of +5% above preferred fre-
quency yielded reduced AT stress compared with the preferred
condition in both the RFS and FFS patterns (Figures 2–3). Similarly,
compared with the preferred step frequency, the +5% condition
exhibited a reduction in strain in both RFS and FFS patterns, with
differences of 3.7% and 2.8%, respectively (Table 1). However,
peak AT stress and strain were not different (P > .05) between the
preferred condition and the –5% condition within either foot strike
pattern. Strain rate was not different (P > .05) between any of the
step frequency conditions within foot strike patterns.
Observed knee and ankle kinematics were not different between
step frequencies in the RFS pattern; however, +5% step frequency
within the FFS pattern resulted in greater knee exion angle at initial
contact (P = .005). In addition, the distance from the COM to heel
at initial contact did not change between conditions within either
foot strike pattern. A step frequency of 5% above preferred resulted
in a 2.5% and 4.0% decrease in peak plantar exion moment for
both FFS and RFS patterns, respectively.
Discussion
The purpose of this study was to compare the inuence of foot
strike pattern and step frequency on the AT mechanical properties
in female runners. Our hypothesis that peak AT stress, strain, and
strain rate would be lower while running with a RFS pattern was
supported. In addition, the magnitude of the effect sizes and percent-
age differences indicated a clinically relevant reduction compared
with running with a FFS pattern. We further hypothesized an inverse
relationship between step frequency and peak AT stress, strain,
and strain rate would be observed within foot strike conditions. In
partial support of this hypothesis, we observed a reduction in peak
AT stress and strain when step frequency was increased above
preferred; however, a difference in strain rate was not supported. In
addition, these changes observed with increases in step frequency
showed a small effect size.
Our ndings demonstrate that a substantial reduction in peak
AT stress and strain occurred while running with a RFS pattern
versus the FFS pattern. Estimated peak AT stress in this study ranged
from 57 MPa to 75 MPa at 3.5 m/s (±5%) using subject-specic AT
measurements. In comparison, a running trial performed by Ker et al
estimated AT stress at 53 MPa while running at 4.5 m/s.34 Komi et al
estimated peak stresses of 110 MPa in barefoot running at 3.8 m/s in
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368 Lyght et al
JAB Vol. 32, No. 4, 2016
Table 1 Mean (± SD) for AT stress, strain, and strain rate, muscle force and kinematic data, as well as means
SD), P-values, and effect sizes (ES), and mean differences (MD) for foot strike main effects
Measure Step
Frequency
Foot Strike Foot Strike Main Effect
P
-value ES MDFFS RFS FFS RFS
Peak AT stress (MPa) –5% 76.4 (17.1) 58.3 (16.0)
Preferred 74.7 (16.5) 56.9 (15.7) 74.5 (16.4) 56.6 (15.5) < .001 1.12 17.9
+5% 72.5 (16.3)* 54.5 (15.2)*
Strain (%) –5% 7.5 (1.7) 5.7 (1.6)
Preferred 7.3 (1.6) 5.6 (1.5) 7.3 (1.6) 5.6 (1.5) < .001 1.10 1.7
+5% 7.1 (1.6)* 5.4 (1.5)*
Strain rate (%·s–1) –5% 104.1 (21.6) 88.7 (14.3)
Preferred 104.1 (18.1) 86.9 (14.1) 103.6 (19.6) 87.3 (14.3) < .001 0.95 16.3
+5% 102.5 (20.0) 86.2 (15.2)
Peak force (BW) –5% 7.5 (1.8) 5.7 (1.7)
Preferred 7.4 (1.7) 5.6 (1.7) 7.4 (1.7) 5.6 (1.6) < .001 1.10 1.8
+5% 7.2 (1.7)* 5.4 (1.6)*
COM heel distance at
initial contact (m) –5% 0.36 (0.03) 0.42 (0.02)
Preferred 0.36 (0.03) 0.41(0.03) .35 (0.04) .41 (0.03) < .001 1.70 -0.06
+5% 0.35 (.04) 0.41 (0.03)
Knee angle at initial
contact (°) –5% 16.9 (7.8) 15.7 (7.9)
Preferred 17.4 (7.9) 15.9 (9.1) 18.1 (7.8) 16.5 (8.7) = .007 0.19 1.6
+5% 19.9 (7.8)* 17.9 (9.2)
Ankle angle at initial
contact (°) –5% 12.7 (3.7) –18.0 (5.0)
Preferred 12.1 (5.5) –17.2 (6.0) 11.8 (4.8) –17.5 (5.2) < .001 5.86 29.3
+5% 10.6 (5.1) –17.3 (4.7)
Peak plantar exor
moment (N·m) –5% 135.6 (30.7) 103.7 (29.5)
Preferred 132.8 (30.7) 101.3 (29.9) 132.7 (29.9) 100.7 (29.0) < .001 1.09 32.0
+5% 129.5 (29.7)* 97.2 (28.9)*
Abbreviations: AT = Achilles tendon; BW = body weight; COM = center of mass; FFS = forefoot strike; RFS = rear foot strike.
Note. Sign convention for ankle angle: negative = dorsiexion, positive = plantar exion.
* Signicantly different than preferred (P < .05).
2 individuals using buckle transducer data.35 These authors reported
strain values of up to 5% per stride while our study estimated strain
values were between 6% and 7%. In addition, Lichtwark and Wilson
reported an average maximum strain of 5.8% while running at 2.78
m/s utilizing direct measurements via ultrasound.36 These values are
well below the reported failure strain range of 12.8% to 49.2% that
has been previously reported.32 Furthermore, Perl et al performed
a similar study with subjects running on a treadmill at 3.0 m/s with
their preferred step frequency.37 The authors reported an 8.13%
difference in AT strain between foot strike patterns while the cur-
rent study showed a 24.0% difference between FFS and RFS. The
difference in values obtained may be attributed to methodological
differences, including running speed.
Peak AT stress and strain showed an inverse relationship
with step frequency during RFS and FFS and with a higher step
frequency. Our changes due to alterations in step frequency were
systematic but showed a small effect size. For example, AT stress
changed about 6% between the +5% and –5% cadence conditions.
These overall changes in step frequency are generally supported
by Heiderscheit et al, where they observed lower ankle energy
generated during +5% and +10% increases in cadence.19 Since the
subject’s running speed was controlled, any change in step frequency
inuenced step length. Typically, with a reduction in step length,
the COM vertical excursion is decreased.19 Less AT force, stress,
and strain, despite having a small effect size, may be related to such
a decrease in COM vertical excursion. More knee exion at initial
contact with an increase in step frequency has been shown to assist
with shock absorption in running.39
Similarly, our ankle joint kinematics and kinetics between
conditions may partially explain the observed difference in peak
AT stress, strain, and strain rate. During the FFS pattern, the ankle
was positioned in greater plantar exion at initial contact. Similar
to Clarke et al,20 the current study found no difference in ankle
joint angles at initial contact between step frequency conditions
during running. In addition, consistent with a previous report, the
FFS pattern exhibited a greater plantar exion moment (percentage
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369JAB Vol. 32, No. 4, 2016
Table 2 Means (± SD), P-values, effect sizes (ES), and mean differences (MD) for step frequency main effects
Step Frequency Comparison
Measure Step
Frequency Step Frequency Main
Effect
P-
value –5% vs
Preferred Preferred vs
+5% –5% vs +5%
Peak AT stress (MPa) –5% 67.31 ± 18.77 < .001 MD 1.47 2.38 3.85
Preferred 65.84 ± 18.23 ES 0.08 0.13 0.21
+5% 63.46 ± 18.05
Strain (%) –5% 6.62 ± 1.85 < .001 MD 0.15 0.23 0.38
Preferred 6.47 ± 1.79 ES 0.08 0.13 0.21
+5% 6.24 ± 1.78
Strain rate (%·s–1) –5% 96.40 ± 19.69 = .395 MD 0.9 1.15 2.05
Preferred 95.50 ± 18.25 ES 0.05 0.06 0.10
+5% 94.35 ± 19.36
Peak AT force (BW) –5% 6.67 ± 1.92 < .001 MD 0.14 0.24 0.38
Preferred 6.53 ± 1.91 ES 0.07 0.13 0.20
+5% 6.29 ± 1.88
COM heel distance at initial
contact (m) –5% 0.39 ± 0.04 = .156 MD 0 0.01 0.01
Preferred 0.39 ± 0.04 ES 0 0.22 0.22
+5% 0.38 ± 0.05
Knee angle at initial contact
(°) –5% 16.30 ± 8.45 = .001 MD –0.35 –2.27 –2.62
Preferred 16.65 ± 7.77 ES 0.04 0.28 0.31
+5% 18.92 ± 8.45
Ankle angle at initial contact
(°) –5% –2.67 ± 16.17 = .283 MD –0.14 0.79 0.65
Preferred –2.53 ± 15.92 ES 0.01 0.05 0.04
+5% –3.32 ± 14.94
Peak plantar exor moment
(N·m) –5% 119.67 ± 31.15 < .001 MD 2.62 3.7 6.32
Preferred 117.05 ± 30.87 ES 0.08 0.12 0.21
+5% 113.35 ± 30.01
Abbreviations: AT = Achilles tendon; BW = body weight; COM = center of mass.
Note. Sign convention for ankle angle: negative = dorsiexion, positive = plantar exion.
Figure 1 Ensemble average Achilles tendon (AT) stress during the stance running for forefoot strike and rearfoot strike patterns.
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370 Lyght et al
JAB Vol. 32, No. 4, 2016
Figure 3 Average Achilles tendon (AT) stress of the 3 step frequency conditions within the rearfoot strike pattern during the stance phase of running.
Figure 2 Average Achilles tendon (AT) stress of the 3 step frequency conditions within the forefoot strike pattern during the stance phase of running.
difference, 24.0%) compared with the RFS pattern.38 However, only
the +5% condition was different within both RFS (P = .001, ES =
0.14) and FFS (P = .029, ES = 0.11) patterns. We suspect that this
is due to the difference in location of the ground reaction force
vector relative to the foot. During a FFS running pattern, the ankle
is in a greater plantar exion angle at initial contact, causing the
resulting impact force vector to be anterior to the ankle. As such,
the triceps surae must produce a larger plantar exor moment to
control the heel descent during the stance phase.37 The contractions
by the triceps surae produce a force that is transmitted through the
AT as it stretched, resulting in an increase in mechanical stress.
Almonroeder et al reported a reduction in AT force by about 8%
and impulse of 11% per step and with the use of a RFS pattern
compared with a non-RFS.18 While they did not directly evaluate
a FFS as described in our study, they suspected that these changes
were accounted by greater ankle energy absorption when the COP
was located more anterior during stance. In our investigation, the
triceps surae during FFS running produced 32% greater force com-
pared with the RFS pattern.
Similar to a previous study, we observed a reduced COM-to-
heel distance (percentage difference, 14.6%) at initial contact with
a corresponding greater knee exion (percentage difference, 8.8%)
at initial contact while running with a FFS pattern.38 This reduction
in COM-to-heel distance at initial contact may advantageously
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Achilles Tendon Stress During Running 371
JAB Vol. 32, No. 4, 2016
position the lower extremity in a spring-like posture with greater
knee flexion that assists with shock attenuation.39 One could
speculate that greater ankle plantar exion at initial contact with a
FFS pattern would result in a greater vertical displacement of the
COM if other corresponding joint angles were held constant. The
greater knee exion angle observed during the FFS pattern may be
coupled with the ankle position at initial contact to minimize the
displacement of the COM.
Our data suggests that a RFS pattern may be benecial to
female runners in the prevention and treatment of AT injuries by
reducing the mechanical stress, strain, and strain rate imposed on
the tendon. The manipulation of foot strike pattern may alter stress
at other joints and may result in different injury types.23,40 Thus, the
recommendation of altering foot strike patterns solely to prevent
injury should be viewed with some caution. A change in foot strike
pattern may introduce novel loading, therefore a gradual transition
is wise as mechanical loading has been shown to induce changes in
the properties of tendons.41 The amount of loading and time neces-
sary to provide an adaptive stimulus is highly variable and we do
not have a denitive recommendation.42
Training programs used to alter foot strike and step frequency
changes remains rather ambiguous, as does their association with AT
injury prevention. Williams et al investigated the effects of foot strike
conversion.43 These authors compared runners’ mechanics between
those with habitual patterns and those who converted their foot strike
patterns after instruction and reported that the joint kinematics, joint
kinetics, and ground reaction forces were similar between the 2
groups. Furthermore, Hafer et al examined the effects of a 6-week
cadence retraining protocol on running mechanics.39 These authors
offered informal instructions and provided a metronome to each
participant to assist with their learning. Findings demonstrate that
runners may modify their preferred running mechanics in 6 weeks
with corresponding reductions in kinematics and kinetics that have
been associated with running-related injuries.19,40 Furthermore,
the results of these studies suggest that runners may be able to
successfully alter their foot strike pattern and step frequency after
minimal instruction.
Certain limitations should be considered when interpreting the
ndings of our study. The images obtained utilizing the 2D ultra-
sound technique may be prone to measurement error; however, the
intraclass correlation coefcient (ICC = 0.95) attained in this study
suggests excellent interrater reliability among assessors when mea-
suring the AT cross-sectional area of the participants.44 Additionally,
restricting our 2D images to a single transverse view may neglect
regional differences in the AT cross-sectional area and overestimate
(10%) tendon thickness.14,21 As such, our data may not portray the
precise amount of stress throughout the length of the tendon. Par-
ticipants’ competencies regarding step frequency adherence was
determined visually as in other clinically-related studies.19,20,39 Since
our data were collected during overground running along a 20-m
runway, this may pose error in visually estimating if the runner truly
matched the running cadence However, our protocol was similar to
Hafer et al,39 with a runway of similar length. Others have mainly
used treadmill protocols.19,20 Despite the error potential in visually
estimating cadence, our ndings do appear to show a systematic
manipulation of cadence. In a post investigation of our cadence
manipulation, the cadence based on kinematic measures was 4.6%
less and 13% higher than determined by visual observation. This
supports the notion that there was a cadence manipulation based
on observation, just not exactly the changes as posed based on
visual observation. Finally, we used a musculoskeletal modeling
approach to determine muscle force estimates in running that are
derived from many mathematical assumptions and approximations.
These parameters were not specic other than AT cross-sectional
area to our participants. While it is difcult to specically validate
these data, our data appear to largely coincide with published
results. Since our study used a repeated-measures design, many of
our study assumptions were consistently applied across all of our
participants that do not greatly limit our generalizations regarding
the manipulation of foot strike patterns and step frequency.
In conclusion, ndings suggest that running with a RFS pattern
may reduce peak AT stress, strain, and strain rate. Increases in step
frequency of 5% above preferred, regardless of foot strike pattern,
resulted in lowered peak AT stress and strain. The utilization of a
RFS pattern and, to a lesser extent, changes to step frequency may
be benecial in the treatment and prevention of AT-related injuries.
Acknowledgments
This work has received funding from a Graduate Student Research, Service,
and Educational Leadership Grant.
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... On the other hand, past studies have suggested that the properties of the AT could be affected by running strike patterns [6,44,45]. The plantarflexion torque exerted to resist dorsiflexion torque during running with a forefoot strike pattern was larger than that exerted to resist dorsiflexion torque during running with a rearfoot strike pattern and is important for energy release and absorption by plantar flexors [46]. ...
... The plantarflexion torque exerted to resist dorsiflexion torque during running with a forefoot strike pattern was larger than that exerted to resist dorsiflexion torque during running with a rearfoot strike pattern and is important for energy release and absorption by plantar flexors [46]. Lyght et al. [6] reported that the strain and stress associated with a rearfoot strike pattern were lower than those associated with a forefoot strike pattern because the large knee flexion angle can be attributed to the short distance between the center of mass and the heel. A springlike running posture is beneficial for shock absorption. ...
... Therefore, the authors suggested that the rearfoot strike pattern can reduce AT strain, stress, and strain rate compared to the forefoot strike pattern. Besides, increasing the stepping frequency by 5%, namely shortening the strike length, can reduce the peak AT stress and strain at a fixed speed of 3.5 m/s, regardless of foot strike pattern [6]. The muscle-tendon biomechanical differences of plantar flexors between the forefoot and rearfoot striking have been taken into account. ...
Article
Full-text available
The morphological and mechanical properties (e.g., stiffness, stress, and force) of the Achilles tendon (AT) are generally associated with its tendinosis and ruptures, particularly amongst runners. Interest in potential approaches to reduce or prevent the risk of AT injuries has grown exponentially as tendon mechanics have been efficiently improving. The following review aims to discuss the effect of different types of exercise on the AT properties. In this review article, we review literature showing the possibility to influence the mechanical properties of the AT from the perspective of acute exercise and long-term training interventions, and we discuss the reasons for inconsistent results. Finally, we review the role of the habitual state in the AT properties. The findings of the included studies suggest that physical exercise could efficiently improve the AT mechanical properties. In particular, relatively long-term and low-intensity eccentric training may be a useful adjunct to enhance the mechanical loading of the AT.
... All items were scored as 'Yes' (score = 1), 'No' (score = 0) or 'Unclear' (score = 0), except item 5, which was scored as 'Yes' (score = 2), 'Partial' (score = 1), 'No' (score = 0) or 'Unclear' (score = 0). Based on assessment scores, studies were categorised as high quality (≥ 20 out of maximum possible score 28), moderate quality (17)(18)(19) or low quality (≤ 16) [12]. The Downs and Black Quality Index has been shown to have high internal consistency, test-retest and inter-rater reliability, and high criterion validity [13]. ...
... Thirty-three studies investigated the immediate effects of changing step rate on performance and biomechanics, and four studies evaluated the longer-term effects of changing step rate on injury and biomechanics. The primary reasons for exclusion of studies were combined running retraining strategies [15][16][17][18], and manipulation of step length with no change in step rate [19][20][21]. In addition to data being extracted directly from the 37 included studies where possible, additional data were provided by 5 authors upon request [22][23][24][25][26]. ...
Article
Full-text available
Background Running-related injuries are prevalent among distance runners. Changing step rate is a commonly used running retraining strategy in the management and prevention of running-related injuries. Objective The aims of this review were to synthesise the evidence relating to the effects of changing running step rate on injury, performance and biomechanics. Design Systematic review and meta-analysis. Data Sources MEDLINE, EMBASE, CINAHL, and SPORTDiscus. Results Thirty-seven studies were included that related to injury ( n = 2), performance ( n = 5), and biomechanics ( n = 36). Regarding injury, very limited evidence indicated that increasing running step rate is associated with improvements in pain (4 weeks: standard mean difference (SMD), 95% CI 2.68, 1.52 to 3.83; 12 weeks: 3.62, 2.24 to 4.99) and function (4 weeks: 2.31, 3.39 to 1.24); 12 weeks: 3.42, 4.75 to 2.09) in recreational runners with patellofemoral pain. Regarding performance, very limited evidence indicated that increasing step rate increases perceived exertion ( − 0.49, − 0.91 to − 0.07) and awkwardness (− 0.72, − 1.38 to − 0.06) and effort (− 0.69, − 1.34, − 0.03); and very limited evidence that an increase in preferred step rate is associated with increased metabolic energy consumption (− 0.84, − 1.57 to − 0.11). Regarding biomechanics, increasing running step rate was associated with strong evidence of reduced peak knee flexion angle (0.66, 0.40 to 0.92); moderate evidence of reduced step length (0.93, 0.49 to 1.37), peak hip adduction (0.40, 0.11 to 0.69), and peak knee extensor moment (0.50, 0.18 to 0.81); moderate evidence of reduced foot strike angle (0.62, 034 to 0.90); limited evidence of reduced braking impulse (0.64, 0.29 to 1.00), peak hip flexion (0.42, 0.10 to 0.75), and peak patellofemoral joint stress (0.56, 0.07 to 1.05); and limited evidence of reduced negative hip (0.55, 0.20 to 0.91) and knee work (0.84, 0.48 to 1.20). Decreasing running step rate was associated with moderate evidence of increased step length (− 0.76, − 1.31 to − 0.21); limited evidence of increased contact time (− 0.95, − 1.49 to − 0.40), braking impulse (− 0.73, − 1.08 to − 0.37), and negative knee work (− 0.88, − 1.25 to − 0.52); and limited evidence of reduced negative ankle work (0.38, 0.03 to 0.73) and negative hip work (0.49, 0.07 to 0.91). Conclusion In general, increasing running step rate results in a reduction (or no change), and reducing step rate results in an increase (or no change), to kinetic, kinematic, and loading rate variables at the ankle, knee and hip. At present there is insufficient evidence to conclusively determine the effects of altering running step rate on injury and performance. As most studies included in this review investigated the immediate effects of changing running step rate, the longer-term effects remain largely unknown. Prospero Registration CRD42020167657.
... Much research has been conducted on different strike patterns during running and how they can affect Achilles tendon biomechanics associated with running-related Achilles tendon injury [34,35]. Several studies have shown that Achilles tendon force during running can be calculated by dividing net ankle joint moments and Achilles tendon moments arm using the inverse dynamics approach. ...
... Several studies have shown that Achilles tendon force during running can be calculated by dividing net ankle joint moments and Achilles tendon moments arm using the inverse dynamics approach. A previous study has confirmed that runners who are rearfoot strikers had less peak AT stress, strain, and strain rate compared with forefoot strike patterns [34]. Furthermore, previous studies demonstrated that foot strike patterns were not related to Achilles tendon cross-sectional area and stiffness in highly trained longdistance runners [36]. ...
Article
Objectives: Although overuse running injury risks for the ankle and knee are high, the effect of different shoe designs on Achilles tendon force (ATF) and Patellofemoral joint contact force (PTF) loading rates are unclear. Therefore, the primary objective of this study was to compare the ATF at the ankle and the PTF and Patellofe-moral joint stress force (PP) at the knee using different running shoe designs (forefoot shoes vs. normal shoes). Methods: Fourteen healthy recreational male runners were recruited to run over a force plate under two shoe conditions (forefoot shoes vs. normal shoes). Sagittal plane ankle and knee kinematics and ground reaction forces were simultaneously recorded. Ankle joint mechanics (ankle joint angle, velocity, moment and power) and the ATF were calculated. Knee joint mechanics (knee joint angle velocity, moment and power) and the PTF and PP were also calculated. Results: No significant differences were observed in the PTF, ankle plantarflexion angle, ankle dorsiflexion power, peak vertical active force, contact time and PTF between the two shoe conditions. Compared to wearing normal shoes, wearing the forefoot shoes demonstrated that the ankle dorsiflexion angle, knee flexion velocity, ankle dorsiflexion moment extension, knee extension moment, knee extension power, knee flex-ion power and the peak patellofemoral contact stress were significantly reduced. However, the ankle dorsiflexion velocity, ankle plantarflexion velocity, ankle plantarflexion moment and Achilles tendons force increased significantly. Conclusions: These findings suggest that wearing forefoot shoes significantly decreases the patellofemoral joint stress by reducing the moment of knee extension, however the shoes increased the ankle plantarflexion moment and ATF force. The forefoot shoes effectively reduced the load on the patellofemoral joint during the stance phase of running. However, it is not recommended for new and novice runners and patients with Achilles tendon injuries to wear forefoot shoes.
... Kinematic changes associated with an increase in cadence include decreased contralateral pelvic drop, hip adduction, and peak knee flexion during stance phase (Bramah, Preece, Gill, & Herrington, 2019). The changes in kinematics and kinetics due to an increase in cadence are associated with a decrease in internal loads such as a decreased peak load on the patellofemoral joint (Lenhart et al., 2015;Lenhart, Thelen, Wille, Chumanov, & Heiderscheit, 2014), tibiofemoral joint (Willy, Meardon, et al., 2016), heel, metatarsal (Wellenkotter, Kernozek, Meardon, & Suchomel, 2014), and Achilles tendon (Lyght, Nockerts, Kernozek, & Ragan, 2016). Furthermore, it has been shown that a 10% reduction in preferred stride length (and therefore increased cadence) reduces tibial damage and therefore the likelihood of stress fractures (Edwards, Taylor, Rudolphi, Gillette, & Derrick, 2009). ...
... This is in line with a study from Chan et al (Chan et al., 2018) where there were significantly more calf injuries in the gait-retraining intervention group compared to the control group. Increasing cadence results in an increased load on the calf muscles (Ahn, Brayton, Bhatia, & Martin, 2014;Nunns, House, Fallowfield, Allsopp, & Dixon, 2013) but lowers the load on almost all others structures of the lower extremity, which potentially can result in a reduced overall injury risk (Edwards et al., 2009;Heiderscheit et al., 2011;Lenhart et al., 2014;Lenhart et al., 2015;Lyght et al., 2016;Napier et al., 2018aNapier et al., , 2018bWellenkotter et al., 2014;Willy, Buchenic, et al., 2016). Nevertheless, follow-up studies should take into account drop-out of participants due to the possible higher load and damage on the calf muscles/Achilles tendon. ...
Article
Full-text available
Running with music has been shown to acutely change cadence. However, it is unclear if the increased cadence remains long-term when running without music in an in-field situation. The aim of this 12-week study was to investigate the effect of a 4-week music running program on cadence, speed and heartrate during and after the music running program. Seven recreational runners with a cadence of <170 steps per minute were randomly assigned to a baseline and post-intervention period of different durations. During the intervention phase, the participants ran with a musical beat that was 7.5-10% higher than their mean cadence at the start of the study. Cadence, heartrate and running speed were measured twice a week during a 5-kilometer run with a watch, and were analyzed using randomization tests and visual data inspection. Two participants dropped-out due to shortage of time (n = 1) and an acute calf injury (n = 1). Cadence significantly increased during the intervention period (+8.5%), and remained elevated during the post-intervention period (+7.9% (p = .001)) in comparison with the baseline period. Heartrate and running speed did not significantly change during any period. This study among five participants shows that four weeks of running with a musical beat that is 7.5-10% higher than the preferred cadence may be an effective and feasible intervention to increase running cadence. Importantly, the increased cadence occurred without simultaneous increases in running speed and heartrate, hereby potentially reducing mechanical loading without increasing metabolic load.
... 17,18 Interestingly, cadence increases have also been shown to reduce sagittal plane knee joint moment magnitudes, 19,20 albeit without increasing moments at the ankle. 21 Increasing cadence by 10% above one's preferred cadence is commonly advocated given its greater effects on reducing sagittal plane knee joint moment magnitudes. 22 Biomechanical interventions may be useful for shifting forces to different body parts in Masters runners with running-related pain or injuries. ...
Article
The objective of this study was to compare the immediate effects of modifications to footwear or cadence on lower limb biomechanics of female Masters runners. After analyzing habitual treadmill running biomechanics in 20 female runners (52.4 [8.3] y), we assessed the effects of 5 conditions: (1) barefoot running, (2) Merrell Vapor Glove, (3) Merrell Bare Access, (4) Brooks Pure Flow, and (5) increasing cadence by 10%. In comparison with habitual biomechanics, greater vertical loading rates of the ground reaction force were observed during running barefoot or with a Merrell Vapor Glove or Bare Access. There was high variability among participants as to changes in foot kinematics during the conditions. Running barefoot (−26.0%) and with a Merrell Vapor Glove (−12.5%) reduced sagittal plane knee moments, but increased sagittal plane ankle moments (both 6.1%). Increasing cadence by 10% resulted in a more modest decrease in knee flexion moments (−7.7%) without increasing peak external ankle dorsiflexion moments. When asked if they would prefer minimalist shoes or increasing cadence, 11 participants (55%) chose cadence and 9 (45%) chose footwear. Minimalist footwear decreased sagittal knee moments, but increased vertical loading rate and sagittal ankle moments. Increasing cadence may be useful to lower sagittal knee moments without increasing ankle moments.
... Similarly, tibiofemoral joint forces have been shown to decrease with a 10% decrease in step length [22]. Achilles tendon stress has also been shown to decrease with a 5% increase in step rate [43]. The present study supports these past findings: we found significant reductions in rearfoot motion, tibial rotation, and vertical and braking ground reaction forces with an increased step rate. ...
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... At the beginning of the experiment, the participants were asked to lie prone on a treatment bed with their dominant ankle (defined as the preferred kicking leg (Sun et al., 2018)) in the neutral position (90 • ). An ultrasound device (M7 Super, Mindray, China) was used to capture images of the CSA of the AT with an ML6-15-D probe (10 MHz maximum frequency) at 10 cm proximal to the calcaneal insertion (Lyght et al., 2016). ...
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Minimalist shod runners have reported greater material and mechanical properties of the Achilles tendon (AT) due to increased loading than runners who wear more cushioned running shoes. This study aimed to investigate the effects of 12-week transition training from conventional shoes to minimalist shoes on AT loading in habitual rearfoot strike runners. Seventeen healthy male habitual rearfoot strikers completed 12-week transition training. They were instructed either to run in minimalist shoes with a forefoot strike pattern (MIN+FFS, n = 9) or run in minimalist shoes but were free to develop their strike pattern (MIN, n = 8). Ultrasound images were captured to determine the cross-sectional area of the AT. Sagittal plane ankle kinematics and ground reaction forces were recorded simultaneously to quantify ankle joint mechanics and AT loading. The strike angle significantly decreased in MIN+FFS after the transition training, indicating a flatter foot at initial contact, whereas no changes were observed in MIN. After training, a significant increase in peak plantarflexion moment was observed for MIN+FFS (15.4%) and MIN (7.6%). Significantly increased peak AT force, peak loading rate and peak stress were observed after training in both groups. Specifically, MIN+FFS had a greater increase in peak AT force (20.3% versus 10.1%), peak loading rate (37.2% versus 25.4%) and peak AT stress (13.7% versus 8.1%) than MIN. Furthermore, for both groups, there were no significant differences in the moment arm and cross-sectional area of the AT observed before and after 12 weeks of training. The results of this study suggested that it was insufficient to promote the morphological adaptation of the AT, but the mechanical loading of the AT was adapted during running after 12-week transition training with minimalist shoes in MIN+FFS and MIN. Preliminary evidence showed that a gradual transition to minimalist shoes with a forefoot strike pattern may be beneficial to the mechanical loading of the AT.
... 21,22 A change from rearfoot to forefoot strike may have other beneficial effects including decreasing knee joint contact forces by an average of 1.2 BW 23 and decreasing patellofemoral joint stress by an 24 However, a forefoot strike may increase stress and loading at the Achilles tendon, ankle, and plantar Effect of Increasing Running Cadence on Peak Impact Force in an Outdoor Environment surface of the foot. [25][26][27] Therefore, the benefits of changing foot strike pattern may need to be considered alongside the potential negative effects when implementing a gait modification technique. Because of the maintained running speed between conditions, these findings may also be partially explained by the relationship between cadence and stride length in running. ...
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