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: 1Université Côte d’Azur, LAMHESS, Nice, France. 2Glasgow Warriors,

Scotstoun Stadium, Glasgow, United Kingdom. 3Faculty of Physical Sciences and Sport,

Catholic University of San Antonio, Murcia, Spain. 4Sports Performance Research Institute

New Zealand (SPRINZ), Auckland University of Technology, Auckland, New-Zealand. 5Université 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

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 [email protected]

Dow

nloa

ded

by W

este

rn U

nive

rsity

on

11/1

7/16

, Vol

ume

0, A

rtic

le N

umbe

r 0

mailto:[email protected]




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

Dow

nloa

ded

by W

este

rn U

nive

rsity

on

11/1

7/16

, Vol

ume

0, A

rtic

le N

umbe

r 0




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

Dow

nloa

ded

by W

este

rn U

nive

rsity

on

11/1

7/16

, Vol

ume

0, A

rtic

le N

umbe

r 0




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

Dow

nloa

ded

by W

este

rn U

nive

rsity

on

11/1

7/16

, Vol

ume

0, A

rtic

le N

umbe

r 0




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

Dow

nloa

ded

by W

este

rn U

nive

rsity

on

11/1

7/16

, Vol

ume

0, A

rtic

le N

umbe

r 0




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

Dow

nloa

ded

by W

este

rn U

nive

rsity

on

11/1

7/16

, Vol

ume

0, A

rtic

le N

umbe

r 0




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

Dow

nloa

ded

by W

este

rn U

nive

rsity

on

11/1

7/16

, Vol

ume

0, A

rtic

le N

umbe

r 0




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.

Dow

nloa

ded

by W

este

rn U

nive

rsity

on

11/1

7/16

, Vol

ume

0, A

rtic

le N

umbe

r 0




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.

Dow

nloa

ded

by W

este

rn U

nive

rsity

on

11/1

7/16

, Vol

ume

0, A

rtic

le N

umbe

r 0




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.

2. Rabita G, Dorel S, Slawinski J, et al. Sprint mechanics in world-class athletes: A new

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

training with sled towing. J Strength Cond Res. 2009;23(2):480-485.

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.

Dow

nloa

ded

by W

este

rn U

nive

rsity

on

11/1

7/16

, Vol

ume

0, A

rtic

le N

umbe

r 0




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.

Dow

nloa

ded

by W

este

rn U

nive

rsity

on

11/1

7/16

, Vol

ume

0, A

rtic

le N

umbe

r 0




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.

Dow

nloa

ded

by W

este

rn U

nive

rsity

on

11/1

7/16

, Vol

ume

0, A

rtic

le N

umbe

r 0