Very heavy sled training

“Very-Heavy Sled Training for Improving Horizontal Force Output in Soccer Players” by Morin JB et al.
International Journal of Sports Physiology and Performance
© 2016 Human Kinetics, Inc.
Note. This article will be published in a forthcoming issue of the
International Journal of Sports Physiology and Performance. The
article appears here in its accepted, peer-reviewed form, as it was
provided by the submitting author. It has not been copyedited,
proofread, or formatted by the publisher.
Section: Brief Report
Article Title: Very-Heavy Sled Training for Improving Horizontal Force Output in Soccer
Players
Authors: Jean-Benoît Morin1,4
, George Petrakos2
, Pedro Jimenez-Reyes3
, Scott R Brown4
,
Pierre Samozino5
, and Matt R Cross4
Affiliations: 1
Université Côte d’Azur, LAMHESS, Nice, France. 2
Glasgow Warriors,
Scotstoun Stadium, Glasgow, United Kingdom. 3
Faculty of Physical Sciences and Sport,
Catholic University of San Antonio, Murcia, Spain. 4
Sports Performance Research Institute
New Zealand (SPRINZ), Auckland University of Technology, Auckland, New-Zealand.
5
Université Savoie Mont Blanc, Laboratoire Interuniversitaire de Biologie de la Motricité,
EA 7424, F-73000 Chambéry, France.
Journal: International Journal of Sports Physiology and Performance
Acceptance Date: October 6, 2016
©2016 Human Kinetics, Inc.
DOI: http://dx.doi.org/10.1123/ijspp.2016-0444

BRIEF REPORT
Very-heavy sled training for improving horizontal force output in soccer players
Jean-Benoît Morin1,4
, George Petrakos2
, Pedro Jimenez-Reyes3
, Scott R Brown4
, Pierre
Samozino5
, Matt R Cross4
1
Université Côte d’Azur, LAMHESS, Nice, France
2
Glasgow Warriors, Scotstoun Stadium, Glasgow, United Kingdom
3
Faculty of Physical Sciences and Sport, Catholic University of San Antonio, Murcia, Spain
4
Sports Performance Research Institute New Zealand (SPRINZ), Auckland University of
Technology, Auckland, New-Zealand
5
Université Savoie Mont Blanc, Laboratoire Interuniversitaire de Biologie de la Motricité, EA
7424, F-73000 Chambéry, France
Corresponding author:
Pr Jean-Benoît Morin, PhD
Laboratoire Motricité Humaine, Education Sport Santé (EA6312)
Faculté des Sciences du Sport, 261 route de Grenoble
06205 NICE Cedex 3
Phone : +33-489-836633
jean-benoit.morin@unice.fr
DownloadedbyWesternUniversityon11/17/16,Volume0,ArticleNumber0

ABSTRACT
Sprint running acceleration is a key feature of physical performance in team sports, and recent
literature shows that the ability to generate large magnitudes of horizontal ground reaction force
and mechanical effectiveness of force application are paramount. We tested the hypothesis that
very-heavy loaded sled sprint training would induce an improvement in horizontal force
production, via an increased effectiveness of application. Training-induced changes in sprint
performance and mechanical outputs were computed using a field method based on velocity-
time data, before and after an 8-week protocol (16 sessions of 10x20-m sprints). 16 male
amateur soccer players were assigned to either a very-heavy sled (80% body-mass sled load)
or a control group (unresisted sprints). The main outcome of this pilot study is that very-heavy
sled resisted sprint training, using much greater loads than traditionally recommended, clearly
increased maximal horizontal force production compared to standard unloaded sprint training
(effect size of 0.80 vs 0.20 for controls, unclear between-group difference) and mechanical
effectiveness (i.e. more horizontally applied force; effect size of 0.95 vs -0.11, moderate
between-group difference). In addition, 5-m and 20-m sprint performance improvement were
moderate and small for the very-heavy sled group, and small and trivial for the control group,
respectively. This brief report highlights the usefulness of very-heavy sled (80% body-mass)
training, which may suggest value for practical improvement of mechanical effectiveness and
maximal horizontal force capabilities in soccer players and other team sport athletes. This study
may encourage further research to confirm the usefulness of very-heavy sled in this context.
Keywords: resistance training; acceleration; performance; power; football

1. Introduction
Sprint running acceleration is a key feature of physical performance in team sports (e.g.
soccer, rugby). Recent literature have shown that the ability to generate large magnitudes of
ground reaction force in the horizontal direction, particularly with respect to the mechanical
efficiency of overall capacity, is key in determining acceleration performance1,2
.
Resisted sled training provides a specific means of providing overload to horizontal
force capacities3
. Moreover, this practical and cost-effective training modality can be used very
easily by soccer players of all levels, from elite to amateur. Authors using the latter methods
have used comparatively light loading protocols (7 to 5% of body-mass (BM)), based on
misinterpretation of guidelines suggesting selection of loading parameters that minimise
kinematic changes4
. Contrastingly, a recent review of literature3
, and notably a study showing
horizontal power output was maximized at much greater loads (69-96% BM)5
, highlight
heavier loads may represent a more effective stimulus for improving sprinting acceleration.
This has also been suggested in an experimental training study6
showing the potential benefit
of using heavier loads. Our own observations (unpublished) show that resisted sprint
acceleration training with very-heavy loaded sleds (VHS) clearly force the athlete to run slow,
and thus allow for an enhanced opportunity to produce force during a forward-oriented body
position throughout the sprint, which is not possible during free or light load sled sprint
accelerations. Consequently, we propose that VHS could provide a practical solution to
develop both the specific force output and the ability to orient this force output with
effectiveness. To our knowledge no research has used greater loads than 43%6
for longitudinal
adaptations.
A recent experimental study indicates that the hip extensor muscles (hamstrings in
particular) play a crucial role in the production of horizontal force during acceleration bouts7
.
Sprint-related hamstring injuries have been the focus of many studies in recent years, with no

clear reduction in injury rates8
. Recently, a retrospective study9
showed that the maximal net
horizontal ground reaction force produced during sprint acceleration (F0) was clearly impaired
in soccer athletes proceeding their return-to-sport after hamstring injury, compared to their
non-injured counterparts. Furthermore, a prospective pilot study involving two athletes showed
similar results, with a clearly lower F0 observed pre-injury occurrence in a soccer player, and
a lower-than-the-average F0 in a rugby player directly preceding injury (compared to their
teammates)10
. Although more prospective evidence is currently in development (works in
progress), it is possible that a reduced F0 may indicate a greater risk of sprint-related hamstring
injury. It stands to reason that the targeted development of the ability to produce and effectively
apply high amounts of F0 could both improve sprint performance and reduce injury risk in
soccer players. Our rationale is that VHS towing is better at affecting F0 than typically
recommended light load sled towing or un-resisted sprinting because (i) the higher resistance
in itself constitutes a higher force overload, and (ii) it allows the athlete to keep on pushing
onto the ground in a more incline, horizontally-oriented and thus mechanically effective body
posture, over a longer time, which is not possible with lighter loads or unresisted sprints. The
results of a study investigating the specific overload, and subsequent improvement in these
capacities, would be of interest to all team-sport athletes and practitioners in which these
injuries are common.
The aim of this study was to test the hypothesis that VHS sprint training would induce
an improvement in horizontal force production, mainly via a more effective ground force
application, in amateur soccer players.
2. Methods
Athletes
20 male amateur soccer players initially volunteered to participate in this study. Due to
injuries (unrelated) or personal reasons (studies, work), four athletes in the original control

group could not perform the entire protocol and/or the post-testing session, and were removed
from the study. Consequently, the outcome statistics were performed on 10 players in the VHS
group (age: 26.3±4.0 years; body-height: 1.77±0.08 m; body-mass: 74.5±5.3 kg) versus 6
controls (age: 26.8±4.2 years; body-height: 1.75±0.08 m; body-mass: 70.7±6.5 kg), which was
performed in accordance with the declaration of Helsinki, and received the approval of the
local ethics committee. All players had over 10 yrs of competitive practice in soccer, and were
performing two 2-h training sessions a week plus one game at the time of the study. They were
not involved in any type of weight training at the time of the study.
Design
Athletes were randomly assigned to the VHS or control group. Both groups performed
the same sprinting program after collective standardised warm-up at the beginning of each
training session (on Tuesdays and Thursdays) during 8 consecutive weeks (16 sessions in total).
Pre- and post-training testing of sprint performance and mechanics occurred respectively one
week before and one week after the first and last training sessions. During each training session,
the control group performed 2 blocks of 5x20-m sprints with no resistance (2-min recovery
between sprints and 5-min recovery between the two blocks). The VHS group performed the
same sprint protocol as the control group, except that they ran towing a resisted sled attached
to their waist, with a load corresponding to 80% BM. This relative load common to all athletes
was selected for means of practicality, and due to the homogeneity of the athlete sample
characteristics on the relative load needed to induce a similar decrement in maximal running
speed among players (preliminary testing). The magnitude of loading was selected based on
speed decrement pilot data to approximate resistance for maximizing power5
. The VHS was
assigned a mixed content of un-resisted and resisted sprints with an increasing amount of
resisted sprint over the training intervention: 5 VHS sprints out of 10 during sessions 1 to 4, 6
during sessions 5 to 8, 7 during sessions 9 to 12, and 8 during the last four sessions. This

progressive program was used in order to gradually introduce the stimulus of sled towing and
avoid potentially challenging increases in training load for the athletes. During each session,
the VHS were performed before the unresisted ones.
Methodology
After an appropriate warm-up, athletes performed two 30-m maximal accelerations
from a standing split start with 4 min of passive recovery between sprints. For the best time
trial, sprint performance and mechanical outputs were computed pre- and post-training using a
recently developed field method11
. Briefly, this computation method is based on a macroscopic
inverse dynamics analysis of the center-of-mass motion, and has been shown valid and reliable
in comparison to ground-embedded force plate measurements11
. Raw velocity-time (v) data
measured with a radar device (Stalker ATS Pro II, Applied Concepts, TX, USA) were fitted by
an exponential function. Instantaneous velocity was then derived to compute the net horizontal
antero-posterior ground reaction force (FH), and the power output in the horizontal direction
(P). Individual linear force-velocity relationships were then extrapolated to calculate
theoretical maximal force (F0) and velocity (V0) capabilities11,12
. Finally, the mechanical
effectiveness of force application was quantified over each step by the ratio of FH to the
corresponding resultant GRF (RF in %), and over the entire acceleration phase by the slope of
the linear decrease in RF when velocity increases (DRF)1,12
. The maximal RF value obtained
was termed RFmax.
Statistical analysis
Within- (Pre-post parallel groups trial.xls) and between-group (Post-only
crossover.xls) changes were analysed using magnitude-based inferences; implementing a
smallest worthwhile change value equal to a Cohen’s d of 0.2013
. Standardised effects were
then interpreted using threshold values of Cohen’s d <0.2, 0.2, 0.6, and 1.2 representing trivial,
small, moderate and large differences13
. If the probabilities of the true effect being substantially

positive and negative were both >5%, the effect was expressed as unclear; otherwise the effect
was clear and expressed as a magnitude of its observed value. Analysis was performed on both
raw and log transformed data, with no meaningful difference in outcome variables or
interpretation. The resultant analysis from the raw dataset is presented for the reader.
3. Results
Table 1 shows the outcomes of the main measurements and both within- and between-
group changes comparison. Figure 1 shows the magnitude of pre-post changes in both groups
for the main sprint performance and mechanical outputs.
4. Discussion
The main outcome of this pilot study is that VHS training using much greater loads
than traditionally recommended clearly increased maximal horizontal force production and
mechanical effectiveness (i.e. more horizontally applied force). This confirms our initial
hypothesis that VHS resistance sprint training is an effective and practical method to improve
F0 and RFmax in soccer players. The two latter variables showed small to moderate changes in
the VHS group versus unclear ones in the control group, and unclear to moderate (ES of
0.57±0.84 and 0.96±0.86, respectively) between group differences in the pre-post changes.
Furthermore, pre-post changes were of moderate magnitude, initial sprint acceleration
performance (5-m) improvement was moderate in the VHS group versus small in the control
group (Figure 1).
To our knowledge, no study has tested the training effect of sleds using resistance of a
high magnitude than 43% of BM6
on sprint performance and its underpinning mechanical
determinants (e.g. F0 and RFmax). This is likely due, in part, to common practice and
recommendation discouraging conditions that modify acute technique (e.g. >10-12.6% of BM,
or >10% velocity decrement4,3
), suggesting training would present negative adaptations.
Contrastingly, our results show that if one considers resisted sled training as a method of

increasing movement specific horizontal force, power, and effectiveness, much heavier loads
may present an effective training stimulus5
. This is particularly justified in amateur players
without access to strength training facilities.
VHS training resulted in specific improvements in the F0 and RFmax variables, with only
trivial effect on v0, and a decrease in the ability to maintain the mechanical effectiveness
throughout the acceleration (DRF). Since VHS training at the magnitudes used in this study may
be considered as a training method to specifically develop the F0 and RFmax variables, it could
be interesting to use it in the context of individualised training based on the force-velocity
profile in sprinting for those athletes who are deficient in maximal horizontal force or
mechanical effectiveness12
. Further studies are necessary to test methods aiming at specifically
improving the opposing end of the force-velocity spectrum (v0) and the DRF index.
Our primary purpose was to provide pilot data on the effects of training at resistance of
magnitudes far outstripping the heaviest in current literature (~43% BM)6
. Given the
recommendations from recent authors3,5
, and the unclear to small effects shown by the
between-group comparison (Table 1), future studies should look to test the effects of
individualised loading prescription on horizontal force and sprint performance measures, and
clarify the inter-individual variability of responses observed for some variables.
5. Practical applications
VHS training may be used to specifically improve sprint maximal horizontal force
production. These results may not only have important implications regarding short distance
acceleration performance12
, but also hamstring injury prevention9,10
. We invite further research
to consider and address some limitations of the present study, and investigate (i) the effect of
individualized loading prescription using speed decrement instead of body mass, and (ii) the
generalizability of these results to higher-level athletes and other sports.

6. Conclusions
Very heavy sled training (80% BM) is effective in improving 5-m and 20-m sprint
performance and mechanical effectiveness and maximal horizontal force capabilities in soccer
players.
Acknowledgements
We are grateful to the athletes of this study and their coaching staff for their commitment and
enthusiasm in performing the assigned training and testing. We also thank Satya Vesseron for
his help in recruiting the athletes and supervising the training program. Scott R Brown was
partly funded by the International Society of Biomechanics (ISB) Student International Travel
Grant.

REFERENCES
1. Morin JB, Edouard P, Samozino P. Technical ability of force application as a
determinant factor of sprint performance. Med Sci Sport Exerc. 2011;43(9):1680-1688.
doi:10.1249/MSS.0b013e318216ea37.
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insight into the limits of human locomotion. Scand J Med Sci Sport. 2015;25(5):583-
594. doi:10.1111/sms.12389.
3. Petrakos G, Morin JB, Egan B. Resisted Sled Sprint Training to Improve Sprint
Performance: A Systematic Review. Sport Med. 2016;46(3):381-400.
doi:10.1007/s40279-015-0422-8.
4. Alcaraz PE, Palao JM, Elvira JLL. Determining the optimal load for resisted sprint
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doi:10.1519/JSC.0b013e318198f92c.
5. Cross MR, Brughelli ME, Samozino P, Morin J-B. Optimal loading for maximizing
power in resisted sled sprinting. In: European College of Sport Sciences, 6-9 July.
Vienna, Austria; 2016.
6. Kawamori N, Newton RU, Hori N, Nosaka K. Effects of weighted sled towing with
heavy versus light load on sprint acceleration ability. J Strength Cond Res.
2014;28(10):2738-2745. doi:10.1519/JSC.0b013e3182915ed4.
7. Morin J-B, Gimenez P, Edouard P, et al. Sprint acceleration mechanics: The major role
of hamstrings in horizontal force production. Front Physiol. 2015;6(DEC):e404.
doi:10.3389/fphys.2015.00404.
8. Ekstrand J, Waldén M, Hägglund M. Hamstring injuries have increased by 4%
annually in men’s professional football, since 2001: a 13-year longitudinal analysis of
the UEFA Elite Club injury study. Br J Sports Med. 2016:1-8. doi:10.1136/bjsports-
2015-095359.
9. Mendiguchia J, Samozino P, Martinez-Ruiz E, et al. 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. 2014;35(8):690-695. doi:10.1055/s-
0033-1363192.
10. Mendiguchia J, Edouard P, Samozino P, et al. Field monitoring of sprinting power-
force-velocity profile before, during and after hamstring injury: two case reports. J
Sports Sci. 2016;34(6):535-541. doi:10.1080/02640414.2015.1122207.
11. Samozino P, Rabita G, Dorel S, et al. A simple method for measuring power, force,
velocity properties, and mechanical effectiveness in sprint running. Scand J Med Sci
Sports. 2016;26(6):648-658. doi:10.1111/sms.12490.
12. Morin JB, Samozino P. Interpreting power-force-velocity profiles for individualized
and specific training. Int J Sports Physiol Perform. 2016;11(2):267-272.
doi:10.1123/ijspp.2015-0638.
13. Hopkins WG, Marshall SW, Batterham AM, Hanin J. Progressive statistics for studies
in sports medicine and exercise science. Med Sci Sports Exerc. 2009;41(1):3-12.
doi:10.1249/MSS.0b013e31818cb278.

Figure 1. Magnitude of pre-post changes in the main sprint acceleration performance and mechanical
outputs. The standardised differences are expressed as a factor of the smallest worthwhile change
(SWC = effect size (Cohen’s d) of 0.2). Bars indicate the 90% confidence limits. BM: body-mass; v0:
maximal theoretical running velocity; F0: theoretical maximal horizontal force; Pmax: maximal power;
RFmax: maximal ratio of force; DRF: decrease in the ratio of force; 5m: 5-m sprint time; 20m: 20-m
sprint time.

Table 1. Athlete body-mass, mechanical, technical and performance sprint variable comparisons of post – pre changes within and between the
control and very heavy sled groups.
Control group (n = 6) Very heavy sled group (n = 10) Post ‒ pre group change
Pre Post Post ‒ Pre Pre Post Post ‒ Pre
Very heavy sled group –
Control group
x̅
± SD
x̅
± SD
%∆
± SD
ES;
±90% CL
Inference
x̅
± SD
x̅
± SD
%∆
± SD
ES;
±90%
CL
Inference
ES;
±90%
CL
Inference
Body-mass (kg)
70.7
± 6.5
70.5
± 6.3
-0.22
± 1.70
-0.02;
±0.13
Trivial***
(neutral)
74.5
± 5.3
74.8
± 5.4
0.41
±
2.08
0.05;
±0.15
Trivial**
(neutral)
0.08;
±0.19
Trivial**
(neutral)
v0 (m·s-1
)
8.67
± 0.45
8.72
± 0.71
0.60
± 6.55
0.09;
±0.86
Unclear
8.56
± 0.41
8.49
± 0.34
-0.77
±
2.75
-0.16;
±0.30
Trivial*
(negative)
-0.28;
±1.07
Unclear
F0 (N·kg-1
)
6.99
± 0.53
7.12
± 0.63
1.86
± 6.14
0.20;
±0.53
Unclear
6.91
± 0.47
7.33
± 0.66
6.12
±
7.99
0.80;
±0.61
Moderate**
(positive)
0.57;
±0.84
Unclear
Pmax (W·kg-1
)
15.1
± 1.8
15.4
± 1.6
2.14
± 1.51
0.14;
±0.08
Trivial**
(positive)
14.7
± 1.2
15.4
± 1.7
5.30
±
7.86
0.59;
±0.50
Small**
(positive)
0.32;
±0.45
Small*
(positive)
RFmax (%)
48.6
± 3.2
48.2
± 3.4
-0.73
± 5.30
-0.11;
±0.54
Unclear
46.8
± 2.3
49.1
± 3.0
5.13
±
6.09
0.95;
±0.66
Moderate***
(positive)
0.96;
±0.86
Moderate**
(positive)
DRF
-0.073
± 0.004
-0.075
± 0.009
2.06
± 11.34
-0.30;
±1.30
Unclear
-0.074
±
0.006
-0.079
±
0.006
6.09
±
8.48
-0.61;
±0.52
Moderate**
(negative)
-0.46;
±1.23
Unclear
5-m (s)
1.42
± 0.05
1.41
± 0.06
-0.98
± 1.37
-0.23;
±0.27
Small*
(positive)
1.43
± 0.04
1.40
± 0.06
-2.10
±
3.12
-0.68;
±0.59
Moderate**
(positive)
-0.36;
±0.64
Unclear
20-m (s)
3.51
± 0.14
3.49
± 0.13
-0.58
± 0.77
-0.12;
±0.13
Trivial**
(positive)
3.54
± 0.10
3.50
± 0.13
-1.21
±
2.27
-0.40;
±0.44
Small**
(positive)
-0.19;
±0.42
Unclear
Values are mean ± standard deviation, percent change ± standard deviation and standardised effect size; ±90% confidence limits. Abbreviations: n, sample size; x̅, mean; SD, standard deviation,
%∆, percent change; ES, effect size; 90% CL, 90% confidence limits; kg, kilogramme; v0, maximal theoretical running velocity; m, metre; s, second; F0, maximal theoretical horizontal force; N,
newton; Pmax, maximal power output; W, watt; RFmax, maximal ratio of force after 0.3 seconds; DRF, decrease in the ratio of force. Qualitative inferences are trivial (< 0.20), small (0.20 – <
0.60) and moderate (0.60 – < 1.20): * possibly, 25 – < 75; ** likely, 75 – < 95%; *** very likely, 95 – < 99.5. Positive, neutral and negative descriptors qualitatively describe the change
between post and pre values and its importance relative to the specific variable.

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