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J. Mendiguchia, P. Samozino, E. Martinez-Ruiz,
M. Brughelli, S. Schmikli, J.-B. Morin,
A. Mendez-Villanueva
Progression of Mechanical
Properties during On-field
Sprint Running after
Returning to Sports from a
Hamstring Muscle Injury in
Soccer Players
Int J Sports Med
DOI 10.1055/s-0033-1363192
0172-4622
Orthopedics & Biomechanics
Mendiguchia J et al. Progression of Mechanical Properties … Int J Sports Med
accepted after revision
October 25 , 2013
Bibliography
DOI http://dx.doi.org/
10.1055/s-0033-1363192
Published online: 2014
Int J Sports Med
© Georg Thieme
Verlag KG Stuttgart · New York
ISSN 0172-4622
Correspondence
Jurdan Mendiguchia
Department of Physical
Therapy
ZENTRUM Rehab and
Performance Center
Calle B nave 23
(Poligono Barañain)
31010 Barañain
Spain
Tel.: + 34/660/384 638
Fax: + 34/94/8229 459
jurdan24@hotmail.com
Key words
●
▶
hamstring strain
●
▶
injury
●
▶
sprinting
●
▶
soccer
Progression of Mechanical Properties during On-
fi eld Sprint Running after Returning to Sports from a
Hamstring Muscle Injury in Soccer Players
has been suggested that the bi-articular posterior
thigh muscles have a major infl uence on control-
ling the direction of external forces producing a
force that is directed horizontally but backwards,
causing the body to propel forwards during the
support phase [ 4 , 27 ] . In addition, the biarticular
hamstring muscles have been shown to contrib-
ute to a net transfer of power from proximal to
distal joints during explosive leg extensions. This
transfer of power allows for an effi cient conver-
sion of body segment rotations (during the fi rst
half of stance phase) into the translation of the
center of mass (CM) in the horizontal direction
[ 25 , 26 ] . The signifi cant increase in the electro-
myography activity of the biceps femoris muscle
with increasing running speed at foot-strike
could indicate that hamstring muscles are
responsible for generating additional force pro-
duction and pulling the body over the stance leg
with minimal loss of horizontal speed and, there-
fore, make a signifi cant contribution to propul-
sion [ 4 , 30 , 33 ] . In this regard, previously injured
hamstrings have been shown to exhibit substan-
tial weakness in eccentric strength despite ath-
letes returning to full training and competition
Introduction
▼
Hamstring muscle injuries are the most preva-
lent injuries in soccer, accounting for 12–16 % of
all injuries [ 7 , 10 , 13 , 14 , 19 , 20 , 45 ] . In addition to
the high incidence, a common problem concern-
ing this injury is the high risk of recurrence (12–
31 %) [ 7 , 14 , 45 ] . Most hamstring injuries in soccer
occur during high-speed and power actions such
as sprinting [ 2 , 37 , 45 ] . Whether the injury occurs
during swing or stance phase of the sprint still
remains controversial [ 9 , 33 , 38 ] . In soccer, rapid
acceleration and sprinting movements are com-
mon in many match-winning actions such as
winning possession of the ball, passing defending
players or gaining position to score a goal [ 12 , 15 ] .
Therefore, from this basic standpoint it seems
logical to expect sprinting to be a key parameter
in soccer both from a performance and injury
prevention perspective.
During the acceleration phase of sprinting, for-
ward orientation of ground reaction force (GRF)
has been shown to be a stronger determinant of
fi eld sprint performance than the overall magni-
tude of vertical or resultant GRF [ 29 , 34 , 35 ] . It
Authors J. Mendiguchia
1 , P. Samozino
2 , E. Martinez-Ruiz
3 , M. Brughelli
4 , S. Schmikli
5 , J.-B. Morin
6 , A. Mendez-Villanueva
7
Abstract
▼
The objectives of this study were to examine
the consequences of an acute hamstring injury
on performance and mechanical properties of
sprint-running at the time of returning to sports
and after the subsequent ~2 months of regular
soccer training after return. 28 semi-professional
male soccer players, 14 with a recent history of
unilateral hamstring injury and 14 without prior
injury, participated in the study. All players per-
formed two 50-m maximal sprints when cleared
to return to play (Test 1), and 11 injured players
performed the same sprint test about 2 months
after returning to play (Test 2). Sprint perform-
ance (i. e., speed) was measured via a radar
gun and used to derive linear horizontal force-
velocity relationships from which the following
variables obtained: theoretical maximal velocity
(V 0 ), horizontal force (F
H0 ) and horizontal power
(Pmax). Upon returning to sports the injured
players were moderately slower compared to the
uninjured players. F
H0 and Pmax were also sub-
stantially lower in the injured players. At Test 2,
the injured players showed a very likely increase
in F
H0 and Pmax concomitant with improvements
in early acceleration performance. Practitioners
should consider assessing and training horizon-
tal force production during sprint running after
acute hamstring injuries in soccer players before
they return to sports.
Affi liations Affi liation addresses are listed at the end of the article
Orthopedics & Biomechanics
Mendiguchia J et al. Progression of Mechanical Properties … Int J Sports Med
[ 31 , 41 ] . Thus, it is possible that athletes with a previous ham-
string injury can have a reduced ability to generate forward pro-
pulsion, and hence impaired performance, during sprinting.
However, no research has to date examined the mechanical
properties during sprint running at the time of return to sports
following a hamstring injury.
The ability to specifi cally produce and apply high amounts of
force onto the ground in the horizontal direction as a function of
running velocity is well described by linear force-velocity (F-v)
and parabolic power-velocity relationships [ 28 , 35 ] . In particu-
lar, since mechanical power is the product of force and velocity,
the slope of the linear F-v relationship [ 28 , 35 ] may indicate the
relative importance of force and velocity qualities in determin-
ing the maximal horizontal power output (Pmax), and the indi-
vidual F-v profi le of each subject. These individual F-v
relationships describe the changes in external horizontal force
generation with increasing running velocity. They may be sum-
marized by 2 theoretical extrema: the theoretical maximal hori-
zontal force the legs could produce over one contact phase at
null velocity (F
H0 ), and the theoretical maximal velocity the legs
could produce during the same phase under zero load (V
0 ).
These integrative parameters characterize the mechanical limits
of the entire neuromuscular system to produce horizontal force
during sprint running, and encompass numerous individual
muscle mechanical properties as well as other morphological,
neural and technical factors [ 6 ] . Therefore, they provide an inte-
grative view of the F-v mechanical profi le of a runner during his
or her specifi c sprint running task. Recently, a simple fi eld
method has been proposed to quantify these parameters from a
biomechanical model. The model requires only time and veloc-
ity measurements during a single sprint (
●
▶
Fig. 1 ), which can be
considered an economical and valid alternative to biomechani-
cal lab testing [ 40 ] . Following our hypothesis that athletes with
a recent hamstring injury can have a reduced ability to generate
forward propulsion upon returning to sports, the 2 mechanical
entities composing power output (i. e., force and velocity) ana-
lyzed through the linear F-v, could also be aff ected. Therefore,
the aim of the present study was to characterize sprinting per-
formance and mechanical properties of sprint running (i. e., V
0
and F
H0 and Pmax) at the time of returning to sports after the
rehabilitation phase for a hamstring injury in soccer players. In
addition, the assessment was then repeated after about 2
months of regular soccer training following the return to sports
to provide additional insights into the recovery process over
time.
Material and Methods
▼
Subjects
28 semi-professional male soccer players recruited telephoni-
cally from 13 Spanish teams participated voluntarily in the
study. 14 players (21.9 ± 2.5 years; 174.6 ± 4.7 cm; 69.3 ± 5.9 kg)
had no history of hamstring injury (i. e., uninjured group).
Another 14 players (21.6 ± 2.5 years; 173.5 ± 4.7 cm; 72.4 ± 7.1 kg)
had experienced an acute recent hamstring injury (i. e., injured
group) with a recovery time ranging between 1.5 and 6 weeks
(3.5 ± 1.5 weeks). All hamstring injuries were diagnosed by the
doctors or physiotherapists for each team; moreover, these were
always checked by the same clinician (E.M.R.) during the fi rst
personal contact. At that time, each subject (injured and unin-
jured players) also completed a questionnaire in order to estab-
lish injury history, particularly in relation to hamstring injuries.
Inclusion criteria for the injured group included: 1) sudden
onset of posterior thigh pain of non-contact etiology during a
match or training which forced the player to leave that training
or match; 2) injury severe enough to have caused the player to
miss at least one offi cial match or week of regular training [ 42 ] ;
3) tenderness triggered by palpation, stretching and contraction
of the hamstring muscles [ 18 ] , with or without confi rmation by
imaging techniques; and, 4) an injury that should be assessed
(Test 1) within 4 weeks after returning to competition (2.8 ± 0.9
weeks). Inclusion criteria for the control group were 1) unknown
history of hamstring injury and 2) currently participating fully
in their regular training sessions or matches. Exclusion criteria
for both groups were 1) muscular, knee or lumbar-pelvic injury
that required professional medical intervention at least 2 years
prior to measurements and 2) any known neurological, cardi-
orespiratory or systemic disorder [ 42 ] . To reduce potential con-
founding, both groups included at least 1 injured and uninjured
player for each team. Additionally, the injured and uninjured
group were matched according to position (defenders and for-
wards), status (titular or substitute player) and leg dominance.
In addition, teams presented a similar physical load profi le. i. e.,
all subjects underwent three 90-min practices per week and
played one offi cial match on the weekend. All subjects provided
written informed consent, and ethics approval was granted by
the Catholic University of San Antonio (Spain) human research
ethics committee, which conforms to the ethical standards of
the International Journal of Sports Medicine [ 21 ] and those
established by the declaration of Helsinki.
Fig. 1 Performing an on-fi eld single sprint trial using the novel method.
Orthopedics & Biomechanics
Mendiguchia J et al. Progression of Mechanical Properties … Int J Sports Med
Experimental protocol
The soccer players involved in this study were asked not to train
or exercise vigorously for at least 2 days preceding testing. Before
the tests (Test 1 and 2), each player performed an identical warm
up comprising 5 min of low-pace (~10 km·h
-1
) running, followed
by 3 min of lower limb muscle stretching, 5 min of sprint-specifi c
warm-up exercises, and 3 progressive 6-s sprints separated by
2 min of passive rest. Subjects were then allowed 5 min of free
cool down before performing two 50-m maximal sprints from a
standing start on a natural grass fi eld (
●
▶
Fig. 1 ). These sprints
were separated by 6 min of passive rest and supervised by the
same clinician (E.M.R.), who assured that players wore their
usual soccer shoes and ran during similar times (i. e., same hour
and on diff erent days than their normal soccer training session)
and under similar temperature (22.0 ± 5.5 °) and wind conditions
(12.1 ± 9.5 km · h − 1 ), the latter being measured by a PCE-AM 82
anemometer (PCE Ibérica, Tobarra, Albacete, Spain).
Test 1 was carried out when the injured players returned to
sports after the rehabilitation phase (i.e., when they were cleared
by their doctors or physiotherapists) and participated in all
training/competition activities with the rest of the squad. Simul-
taneously to the assessment of each injured player, one or more
uninjured player of the same team was also assessed. Subse-
quently, 11 players for the injured group (21.6 ± 2.2 years;
172.6 ± 4.7 cm; 71.2 ± 5.7 kg) performed a second sprint test at
9.5 ± 1.5 weeks after returning to sports (Test 2). During this
period, none of the previously injured players was involved in
any specifi c supplementary or preventative training apart from
what was implemented in each squad. The 3 remaining players
did not undergo testing with Test 2 due to suff ering a hamstring
re-injury, an ankle sprain injury and due to a personal issue,
respectively.
The performance for each sprint was measured by means of a
Radar Stalker ATS System
TM (Radar Sales, Minneapolis, MN). This
device measures the forward sprinting velocity of the subject at
a sampling rate of 33.25 Hz, and has been previously validated in
human sprint running experiments [ 8 , 11 , 36 ] . It was placed on a
tripod 10 m behind the subjects at a height of 1 m corresponding
approximately to the height of subjects’ CM.
Data analysis
Horizontal external force, velocity and power were obtained
using a recently validated computational method from speed
data measured during the acceleration phase of each sprint
(ranging from the sprint start to the maximal speed plateau)
[ 40 ] . For each acceleration phase, the velocity (v) – time curve
was fi tted by a monoexponential function using least square
regression [ 11 , 17 , 22 ] :
v (t) = vmax .(1-e ( − t/τ) ) (1)
w i t h vmax being the maximal velocity reached and τ the accel-
eration time constant. The horizontal acceleration ( a ) of the
body center of mass as a function of time during the acceleration
phase can be expressed, after derivation of v(t) over time, as fol-
lows:
a (t) = ( vmax / τ).e
( − t/τ) ) (2)
The net horizontal external force ( F H ) was modeled over time as:
F H (t) = m. a (t) + Fair (3)
w i t h Fair being the aerodynamic friction force runners have to
overcome during sprint running computed from running veloc-
ity and an estimation of runner’s frontal area and drag coeffi -
cient (for details, see Arsac and Locatelli [ 3 ] ). On the basis of
these F H and v values, individual linear force-velocity relation-
ships were determined by least-square linear regressions (33–
34) to obtain for each subject F H0 and V 0
(force and velocity-axis
intercepts of the force-velocity regression curves, respectively),
the F-v profi le (slope of the F-v curve) and the maximal horizon-
tal power output (Pmax = F
0
.V
0
/4) [ 39 , 40 ] .
Statistical methods
Data in the text are presented as means ± SD. Data were analyzed
for practical signifi cance using magnitude-based inferences [ 23 ]
with a modifi ed statistical Excel spreadsheet [ 24 ] . We used this
qualitative approach because traditional statistical approaches
often do not indicate the magnitude of an eff ect, which is typi-
cally more relevant to athletic performance than any statistically
signifi cant eff ect [ 23 ] . Inter-group standardized diff erences or
Cohen eff ect sizes (d) (90 % confi dence limits, CL) in the selected
performance variables were calculated using pooled standard
deviations. Eff ects were evaluated for practical signifi cance by
pre-specifying 0.2 between-subject SDs as the smallest worth-
while diff erence (SWD) [ 23 ] . Threshold values for d statistics
were < 0.20, 0.20, 0.60, 1.2 and 2.0 for trivial, small, moderate,
large and very large, respectively [ 23 ] . Probabilities were also
calculated to establish whether the true (unknown) diff erences
were lower, similar or higher than the SWD. Chances of higher or
lower diff erences were evaluated qualitatively as follows:
≤ 1 %, almost certainly not; > 1–5 %, very unlikely; > 5–25 %,
unlikely; > 25–75 %, possible; > 75–95 %, likely; > 95–99 %, very
likely; > 99 %, almost certain [ 23 ] . If the chance of both higher
and lower values was > 5 %, the true diff erence was assessed as
unclear [ 23 ] . Otherwise, we interpreted that change as the
observed chance.
Results
▼
Both injured and uninjured soccer players were similar in terms
of age, height and body mass except for slightly and moderate
greater body mass and BMI in the injured players at Test 1.
At Test 1 (i. e., return to sport), the injured players were very
likely, slower (moderate magnitude diff erences) at 5, 10 and
40-m than their uninjured counterparts (
●
▶
Table 1 ). Top speed
was also lower in the injured players, while the magnitude of the
inter-group diff erences was smaller. Among the mechanical var-
iables, Pmax and F
H0 were substantially lower (moderate magni-
tude diff erences) in the injured players (
●
▶
Table 1 ), while the
magnitude of the inter-group diff erences was smaller for V
0 .
After ~2 months following return to sports (i. e., Test 2) the
injured players presented a very likely increase of moderate
magnitude in Pmax and F
H0 concomitant with improvements in
sprint performance at 5 and 10 m (
●
▶
Table 1 ). Performance at
40 m and Top Speed and the remaining mechanical variable (V
0 )
( − 0.29 ± − 0.98 ± 0.40) presented small to trivial (typically
unclear) changes from Test 1 to Test 2. Thus, as a result of the
observed improvements in both sprint performance and
mechanical variables observed at Test 2 in the injured players,
most of the initially (i. e., Test 1) observed inter-group (i. e.,
injured vs. uninjured) diff erences resolved with ~2 months’ fol-
low-up (
●
▶
Table 1 ) .
Orthopedics & Biomechanics
Mendiguchia J et al. Progression of Mechanical Properties … Int J Sports Med
Discussion
▼
Hamstring injury recurrence rates have remained substantially
high in soccer [ 7 , 14 , 20 , 45 ] , which might be indicative of inef-
fective return-to-sport strategies. Thus, the aim of the present
study was to examine the eff ects of an acute hamstring strain
injury on sprinting performance and mechanical properties of
sprint running (i. e., V
0 and F H0 and Pmax) at the time of return to
sport and after the subsequent ~2 months. The main fi ndings of
the present study were: a) despite being cleared to play, soccer
players returning from a recent hamstring injury had substantial
lower sprinting speed performance and reduced mechanical
horizontal properties compared to the uninjured players, b) the
greater magnitude diff erences in F
H0 compared to V
0 suggested
that the lower maximal horizontal power observed in the
injured player was mainly related to the reduced maximal hori-
zontal force component, and c) approximately 2 months of regu-
lar soccer training after return to sports resulted in substantial
improvements in sprinting speed (acceleration) concomitant
with an increase in maximal horizontal force and power,
whereas the speed component (V
0 ) and top speed remained
unaltered. The present study is the fi rst, to the authors’ knowl-
edge, to assess mechanical horizontal properties during a com-
mon on-fi eld sprinting action at the time of returning to sports
in soccer players with prior hamstring injuries. Moreover, meas-
urements were performed during the acceleration phase of the
sprint. Until now, testing methods were restricted to the fl ying
top speed that could be maintained only for a few steps [ 5 ] , irre-
spective of the typically preceding acceleration phase that has
been shown to be fundamental to soccer performance and risk
of injury. The method used here allowed us to obtain horizontal
external force, velocity and power over ground and in fi eld con-
ditions, which could have been only possible using a 50-m long
force plate system. This method was recently validated in com-
parison to force plate measurements and presented very low
bias (absolute bias < 5 %) and good reliability (coeffi cient of vari-
ation < 4 %) on force, velocity and power parameters [ 40 ] .
Specifi cally, upon return to sports, Pmax was moderately lower
in the injured players, primarily related to the reduced F
H0 com-
ponent. Present results concur with previous fi ndings showing a
decrease in horizontal force production, with no diff erences in
vertical forces, at 80 % of maximal velocity on a non-motorized
force treadmill in Australian Rules Football players with a previ-
ous hamstring injury [ 5 ] . The reduced horizontal force compo-
nent in the present study is particularly relevant for sprinting as
a large horizontal component of the force vector is desired in
order to maximize forward propulsion [ 4 , 29 , 34 , 35 ] . In this
regard, the role of the hamstring muscles in the initial contact
phase is believed to be essential for producing hip extension and
knee fl exion power and thereby a more forward-directed force
with increasing running speed [ 4 , 33 ] . Thus, the lower force
component (i. e., F
H0 ) at the time of return to sports observed in
the present study might be related to the reported hamstring
strength defi cits, both as a hip extensor and knee fl exor, in previ-
ously injured hamstrings athletes despite returning to full train-
ing and competition [ 31 , 41 , 43 ] . In addition, the injured athlete’s
apprehension of experiencing pain when producing a high level
of force [ 44 ] might also play a role in the reduced ability to gen-
erate horizontal force and hence forward momentum during
sprinting. Future studies should quantify whether the force
reductions on the single-joint level (e. g., knee fl exion) are caus-
ally linked with a limited ability to generate horizontal force
vectors during a more functional action (i. e., sprinting).
Following the fi rst assessment and approximately 2 months
after returning to sports, where soccer training and matches
were resumed and re-injury risk is higher [ 16 ] , the injured play-
ers presented a very likely, moderate increase in horizontal
power and theoretical maximal force compared to an unclear
Table 1 Anthropometric, sprinting performance and mechanical variables (mean ± SD) for each group and the standardized diff erences (with 90 % confi dent
limits) and probabilistic inferences about the true standardized magnitude in the means between groups.
Non-
injured T1
(n = 14)
Injured T1
(n = 14)
Injured T2
(n = 11)
Non-injured T1 vs.
Injured T1
Non-injured T1 vs. Injured
T2
Injured T2 vs. Injured T1
ES (90 % CL)
chances of better/trivial/
worst
ES (90 % CL)
chances of better/trivial/
worst
ES (90 % CL)
chances of better/trivial/
worst
Body mass
(kg)
69.3 ± 5.9 72.4 ± 7.1 71.2 ± 5.8 − 0.46 ( − 1.08;0.17) small
(4/20/76) likely ↓
− 0.31 ( − 0.98;0.35) small
(10/29/61) unclear
0.18 ( − 0.48;0.84) trivial
(17/35/48) unclear
Height (m) 1.75 ± 0.05 1.73 ± 0.05 1.73 ± 0.05 0.24 ( − 0.39;0.86) small
(54/34/12) unclear
0.41 ( − 0.26;1.08) small
(70/23/7) unclear
0.18 ( − 0.49;0.85) trivial
(48/35/17) unclear
BMI (kg/m 2 ) 22.7 ± 1.5 24.1 ± 2.4 23.9 ± 1.6 0.65 (0.02;1.28) moderate
(2/10/88) likely ↓
0.72 (0.05;1.39) moderate
(2/8/90) likely ↓
0.09 ( − 0.74;0.56) trivial
(22/39/39) unclear
5-m (s) 1.4 ± 0.05 1.5 ± 0.12 1.4 ± 0.07 0.90 (0.27;1.53) moderate
(97/3/0) very likely ↑
0.05 ( − 0.64;0.73) trivial
(35/38/27) unclear
0.81 (0.16;1.46) moderate
(94/5/1) likely ↑
10-m (s) 2.2 ± 0.07 2.3 ± 0.17 2.2 ± 0.11 0.87 (0.25;1.50) moderate
(96/4/0) very likely ↑
0.03 ( − 0.66;0.72) trivial
(34/38/28) unclear
0.79 (0.13;1.44) moderate
(93/6/1) likely ↑
40-m (s) 5.9 ± 0.18 6.1 ± 0.32 6.0 ± 0.26 0.83 (0.19;1.46) moderate
(95/5/0) very likely ↑
0.23 ( − 0.47;0.92) small
(53/32/15) unclear
0.56 ( − 0.10;1.22) small
(82/15/3) likely ↑
Top Speed
(km/h)
30.5 ± 1.1 29.8 ± 0.9 29.8 ± 1.3 0.64 (0.01;1.27) moderate
(88/11/1) likely ↑
0.55 (0.13;1.23) small
(81/16/3) likely ↑
− 0.02 ( − 0.70;0.67) trivial
(29/38/33) unclear
V 0
(km/h) 31.9 ± 1.31 31.4 ± 0.91 31.0 ± 1.45 0.46 ( − 0.17;1.09) small
(76/20/4) likely ↑
0.63 ( − 0.05;1.30) moderate
(86/12/2) likely ↑
− 0.29 ( − 0.98;0.40) small
(12/30/58) unclear
F H0
(N/kg) 6.8 ± 0.56 6.1 ± 1.04 6.9 ± 0.84 0.85 (0.23;1.48) moderate
(96/4/0) very likely ↑
− 0.21 ( − 0.90;0.48) small
(16/33/51) unclear
0.92 (0.26;1.58) moderate
(97/3/0) very likely ↑
Pmax (W/kg) 15.0 ± 1.44 13.1 ± 2.39 14.9 ± 2.15 0.91 (0.29;1.54) moderate
(97/3/0) very likely ↑
0.03 ( − 0.66;0.72) trivial
(34/38/28) unclear
0.77 (0.10;1.43) moderate
(92/7/1) likely ↑
Orthopedics & Biomechanics
Mendiguchia J et al. Progression of Mechanical Properties … Int J Sports Med
small and trivial unclear eff ect for V
0 and top speed. These
changes in the force profi le of the players were concomitant to
improvements of similar magnitude in the acceleration phase
(i. e., 5- and 10-m) of the sprint, while the magnitude of the
change in top speed and 40 m performance was much lower and
trivial (
●
▶
Table 1 ). The fact that the horizontal power and force
levels, with concomitant substantial improvements mainly in
the acceleration phase of sprinting performance, improved to
match the levels of uninjured players appears to indicate that the
initial diff erences between injured and uninjured players as dis-
cussed above, were most likely related to the hamstring injury
itself. Although indirectly these results seem to support the
importance of F
H0 for achieving greater horizontal power and
sprinting performance, especially during the acceleration phase,
in soccer. Indeed, it has been recently reported that to improve
acceleration in fi eld sport athletes, horizontal force and power
production should be developed [ 32 ] . In contrast, and given the
study design, it is diffi cult to know if V
0 remained lower and
unchanged after 2 months of returning to sports, since the train-
ing contents were not appropriate for improving this parameter
or because there were initial inter-group diff erences. It can be
speculated that because soccer training and match play involves
mainly short sprints [ 1 ] , supplemental exercises (e. g., long
sprints) might be required to overload the top speed factors.
In summary, our fi ndings indicate that running mechanics, spe-
cifi cally the ability to produce a high level of horizontal force at
low speed (F
H0 ) (i. e., fi rst meters of the acceleration phase), and
sprinting performance were impaired after returning to sports
from a hamstring injury in soccer players. Within ~2 months
after returning to sports, the horizontal force production and
acceleration capacity were both improved. Therefore, practition-
ers should consider assessing and training horizontal force pro-
duction during sprint running after acute hamstring injuries in
soccer players. Further research is required to improve our
understanding on the eff ect of vertical and horizontal strength
training on sprinting mechanics and performance.
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Affi liations
1 Department of Physical Therapy, ZENTRUM Rehab and Performance Center,
Barañain, Spain
2 Laboratory of Exercise Physiology (EA4338), University of Savoy, Le Bourget
du Lac, France
3 Chair of Sports Traumatology, Catholic University of San Antonio, Murcia,
Spain
4 Sports Performance Research Institute New Zealand, Auckland University of
Technology, Auckland, New Zealand
5 Department of Rehabilitation and Sports Medicine, Rudolf Magnus Institute
of Neuroscience, Utrecht, Netherlands
6 Laboratory of Exercise Physiology, University of Saint-Etienne, France
7 Sport Science, ASPIRE Academy for Sports Excellence, Doha, Qatar
Orthopedics & Biomechanics
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