Sled Towing: The Optimal Overload for Peak Power Production

Abstract

Purpose:
The effects of different loads on kinematic and kinetic variables during sled towing were investigated with the aim to identify the optimal overload for this specific sprint training.

Methods:
Thirteen male sprinters (100m PB: 10.91±0.14 s) performed 5 maximal trials over a 20m distance in the following conditions: unloaded (UL) and with loads from +15% to +40% of the athlete's body mass (BM). In these calculations the sled mass and friction were taken into account. Contact and flight times (CT, FT), step length (SL), horizontal hip velocity (vh) and relative angles of hip, knee and ankle (at touch-down and take-off) were measured step-by-step. In addition, the horizontal force (Fh) and power (Ph) and the maximal force (Fh0) and power (Ph0) were calculated.

Results:
vh, FT and SL decreased while CT increased with increasing load (P < .001). These variables changed significantly also as a function of the step number (P < .01) except between the two last steps. No differences were observed in Fh among loads but Fh was larger in sled towing compared to UL. Ph was unaffected by load up +20%BM but decreased with larger loads. Fh0 and Ph0 were achieved at +20%BM. Up to +20%BM no significant effects on joint angles were observed at touch-down and take-off, while at loads >+30%BM joint angles tend to decrease.

Conclusions:
The +20%BM condition represents the optimal overload for peak power production: at this load sprinters reach their highest power without significant changes in their running technique (e.g. joint angles).

Full text

“Sled Towing: The Optimal Overload for Peak Power Production” by Monte A, Nardello F, Zamparo P
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: Original Investigation
Article Title: Sled Towing: The Optimal Overload for Peak Power Production
Authors: Andrea Monte, Francesca Nardello, and Paola Zamparo
Affiliations: Department of Neuroscience, Biomedicine and Movement Sciences, University
of Verona, Verona, Italy.
Journal: International Journal of Sports Physiology and Performance
Acceptance Date: November 30, 2016
©2016 Human Kinetics, Inc.
DOI: http://dx.doi.org/10.1123/ijspp.2016-0602

“Sled Towing: The Optimal Overload for Peak Power Production” by Monte A, Nardello F, Zamparo P
International Journal of Sports Physiology and Performance
© 2016 Human Kinetics, Inc.
Sled towing: the optimal overload for peak power production
Andrea Monte*1, Francesca Nardello1, Paola Zamparo1
Original Investigation
1Department of Neuroscience, Biomedicine and Movement Sciences,
University of Verona, Verona, Italy
Running head: Sled towing: the optimal overload for training
Word count (manuscript): 3485
Word count (abstract): 249
3 Figures
2 Tables
*Corresponding author
Andrea Monte
Address: Department of Neuroscience, Biomedicine and Movement Sciences,
University of Verona. Via Felice Casorati, 43; 37131 - VERONA (ITALY)
Phone number: +39-3395306766
Fax number: +39-045-8425131
E-mail: andrea.monte92@hotmail.com
Downloaded by EDIGO Verona on 12/16/16, Volume 0, Article Number 0

“Sled Towing: The Optimal Overload for Peak Power Production” by Monte A, Nardello F, Zamparo P
International Journal of Sports Physiology and Performance
© 2016 Human Kinetics, Inc.
Abstract
Purpose: The effects of different loads on kinematic and kinetic variables during sled towing
were investigated with the aim to identify the optimal overload for this specific sprint
training. Methods: Thirteen male sprinters (100m PB: 10.91±0.14 s) performed 5 maximal
trials over a 20m distance in the following conditions: unloaded (UL) and with loads from
+15% to +40% of the athlete’s body mass (BM). In these calculations the sled mass and
friction were taken into account. Contact and flight times (CT, FT), step length (SL),
horizontal hip velocity (vh) and relative angles of hip, knee and ankle (at touch-down and
take-off) were measured step-by-step. In addition, the horizontal force (Fh) and power (Ph)
and the maximal force (Fh0) and power (Ph0) were calculated. Results: vh, FT and SL
decreased while CT increased with increasing load (P < .001). These variables changed
significantly also as a function of the step number (P < .01) except between the two last steps.
No differences were observed in Fh among loads but Fh was larger in sled towing compared
to UL. Ph was unaffected by load up +20%BM but decreased with larger loads. Fh0 and Ph0
were achieved at +20%BM. Up to +20%BM no significant effects on joint angles were
observed at touch-down and take-off, while at loads >+30%BM joint angles tend to decrease.
Conclusion: The +20%BM condition represents the optimal overload for peak power
production: at this load sprinters reach their highest power without significant changes in
their running technique (e.g. joint angles).
Keywords: Sprint running · Load · Coefficient of friction · Force-Velocity relationship
Downloaded by EDIGO Verona on 12/16/16, Volume 0, Article Number 0

“Sled Towing: The Optimal Overload for Peak Power Production” by Monte A, Nardello F, Zamparo P
International Journal of Sports Physiology and Performance
© 2016 Human Kinetics, Inc.
Nomenclature
STS = Sled Towing Sprinting
BM = Body Mass (kg)
BCoM = Body Centre of Mass
CT = Contact Time (s)
FT = Flight Time (s)
SL = Stride Length (m)
UL = unloaded condition
vh = hip horizontal speed (m·s-1)
Fh = horizontal force (N)
Ph = horizontal Power (W)
vh0 = maximal horizontal velocity (m·s-1)
Fh0 = maximal horizontal force (N)
Ph0 = maximal horizontal power (W)
hipang = hip angle (°)
kneeang = knee angle (°)
ankleang = ankle angle (°)
Downloaded by EDIGO Verona on 12/16/16, Volume 0, Article Number 0

“Sled Towing: The Optimal Overload for Peak Power Production” by Monte A, Nardello F, Zamparo P
International Journal of Sports Physiology and Performance
© 2016 Human Kinetics, Inc.
Introduction
Sprint running is a fundamental activity in many sports (e.g. athletics, soccer, rugby).
To improve sprint performance programs devoted to increase maximal strength, maximal
power or reactive strength (plyometric exercises) can be proposed.1 As an example, heavy
back squat has been shown to have excellent transfer to sprint performance.2 A positive
transfer of training to sport performance may also be achieved if the conditioning programme
emphasises a similar motor pattern and contraction type according to the training principle of
specificity.1,3,4,5,6 Sled towing sprinting (STS) is a method that is consistent with such
philosophy. Ideally, STS should imitate the range of motion, body position, running
technique and the pattern of muscle activation used in competition.6,7 This training method is
reported to increase: i) the force output demand for the lower limb muscles during contact
time;8 ii) the stride length in the acceleration phase in team sport athletes;4 iii) maximum
velocity.9
From a kinematic point of view, previous studies have mainly focused on parameters
such as step length (SL), contact and flight times (CT, FT) and joint angles. For example,
Alcaraz et al.10 showed a decrease in SL and running velocity with a sled loaded to 16% of
body mass, but no significant changes in running technique (i.e. by analyzing the joint
angles). Moreover, Cronin et al.11, Lockie et al.7 and Maulder et al.12 showed a decrement in
FT and SL, and an increase in CT as a function of sled load.
However, there is no agreement in the literature regarding the exact determination of
the optimal training load and this could be due to the lack of normalization for the coefficient
of friction.13 Indeed, when the applied load is calculated, the sled mass and the coefficient of
friction between sled and ground surface must be taken into account.
Recently, research has also focused on the kinetic parameters of STS.14,15 For
example, Kawamori et al.14 showed differences in ground reactions forces (GRFs) between
Downloaded by EDIGO Verona on 12/16/16, Volume 0, Article Number 0

“Sled Towing: The Optimal Overload for Peak Power Production” by Monte A, Nardello F, Zamparo P
International Journal of Sports Physiology and Performance
© 2016 Human Kinetics, Inc.
the conditions without load and with a load equal to 30% of body mass during the third
running step. Cottle et al.16 showed differences in vertical GRF during the starting phase in
the front leg compared to the back one. However, the sled towing effect on force and power
output generated by athletes during a sprint of 20 m is still unknown. Measuring forces for
the entire length of a sprint is quite a difficult task but, recently, Samozino et al.17 have
proposed a method for estimating horizontal force (Fh) during a sprint based on spatio-
temporal parameters. From these data, and from data of hip velocity (vh) it is possible to
calculate the horizontal component of mechanical power output (Ph = Fh.vh). Sled towing is
expected to alter Fh, vh and thus to affect Ph, depending on the applied load.
The aim of this study was to establish which load would maximize mechanical power
(Ph, estimated according to the method of Samozino et al.17) without inducing detrimental
angular changes during sled sprinting with different loads. In this study, to determine the
load, the coefficient of friction was taken into account, as suggested by Linthorne &
Cooper,13. Our hypotheses were: i) sled towing will alter the kinematics and kinetics of sprint
running: but ii) there will be an optimal load that maximizes mechanical power output
without inducing changes in running technique (e.g. in joint angles).
Methods
Participants
Thirteen male sprint athletes participated in this study (see Table 1). To be recruited,
each athlete had at least two years of experience with sled towing training and was free from
any type of injury (this was verified by means of an interview). Moreover, the athletes had to
abstain from training in the 48 hours before the test session. All participants received written
and oral instructions before the study and gave their written informed consent to the
experimental procedure. The experimental protocol was approved by the local Institutional
Review Board.
Downloaded by EDIGO Verona on 12/16/16, Volume 0, Article Number 0

“Sled Towing: The Optimal Overload for Peak Power Production” by Monte A, Nardello F, Zamparo P
International Journal of Sports Physiology and Performance
© 2016 Human Kinetics, Inc.
Set-up and instrumentation
Kinematic parameters (step length: SL, contact time: CT, flight time: FT, hip
horizontal velocity: vh, hip horizontal acceleration: ah) and relative angles of hip (hipang),
knee (kneeang) and ankle (ankleang) were recorded at each step. To record these data two video
cameras (Elixim Casio, 100 Hz), positioned 18 m from the sprinting direction were interlaced
and synchronized (Figure 1).
The sled had a mass of 8 kg: it was connected to the athletes by means of a 3 m long
rope and a hip belt. Loads could be added on the sled with increments of 0.5 kg.
Experimental Procedure
All sprint tests were conducted in an indoor gym at 20°C and 60% of humidity. A
standardized 20-min warm-up (i.e. running, specific gaits and dynamic stretching) was
undertaken by each participant; it was followed by an initial familiarization to the
instrumentation set-up consisting in 2-3 submaximal sprints with the sledge. Each athlete was
then asked to perform 5 maximal trials over the 20 m distance. All sprint conditions were
randomized; at least 8 min of recovery were observed between trials. In the unloaded trial
(UL) the subjects ran, at their maximal speed, without the sled. In the loaded conditions they
were requested to run, as fast as possible, with an additional mass on the sled corresponding
to +15, +20, +30 and +40% of the athlete’s body mass; to calculate these values the sled mass
as well as the friction force were taken into account (see below).
Friction coefficient
The coefficient of friction () between the PVC floor and the sled was calculated by
means of a load cell (System Pese srl, Magenta (MI), Italy; model: S1, capacity: 50 kg,
accuracy 2 ± 0.1% mV·V-1). To determine  we used the “friction sled” method proposed by
Linthorne & Cooper13.
Downloaded by EDIGO Verona on 12/16/16, Volume 0, Article Number 0

“Sled Towing: The Optimal Overload for Peak Power Production” by Monte A, Nardello F, Zamparo P
International Journal of Sports Physiology and Performance
© 2016 Human Kinetics, Inc.
The sled was moved over the PVC surface at three different speeds in two conditions:
without load and with an added mass of 20 kg. The static coefficient of friction was on
average of 0.20 ± 0.01. Since friction force is given by the product of vertical (normal) force
times the coefficient of friction this “additional force” corresponds to 20% of the total weight
of the load + sled. For a 70 kg sprinter in the +20%BM condition the applied mass should be
of 14 kg; this mass should be diminished by 20% to take into account friction. Since the sled
mass is 8 kg the mass applied on the sled would be: 14 – (14 · 0.2) - 8 = 3.2 kg.
The angle of the rope with the horizontal was calculated for all sprinters, in all
conditions and for all steps at touch down and take off and found to be of 18 ± 1.2°, on the
average. The vertical force component can thus be neglected since the horizontal component
is 95% of the resultant force.
Data analysis
CT, FT, SL, vh, ah, hipang, kneeang, ankleang were recorded step-by-step. Data were
analyzed with SIMI Motion (version 16.1).
The net horizontal antero-posterior force (Fh) applied to BCoM and the corresponding
horizontal mechanical power (Ph) were modeled according to Samozino et al.17:
)()()( tFaerotahmtFh 
1)
vhFhPh 
2)
where vh and ah are the horizontal hip velocity and acceleration and m is the total mass (e.g.
subject’s mass + overload). The assumption made in this study was that the overload is
applied to BCoM (as implicitly assumed in other studies on this topic: Winwood et al.,18;
Kawamori et al.4; Valencia et al.15). Faero is the aerodynamic resistance and was calculated
as described in detail by Samozino et al17 based on measures of body mass and stature, vh
and air density.
Downloaded by EDIGO Verona on 12/16/16, Volume 0, Article Number 0

“Sled Towing: The Optimal Overload for Peak Power Production” by Monte A, Nardello F, Zamparo P
International Journal of Sports Physiology and Performance
© 2016 Human Kinetics, Inc.
The individual force-speed relationships were determined according to Rabita et al.19
and were extrapolated to obtain maximal velocity (vh0) and maximal force (Fh0), as the
intercepts of the force-speed curve with the force and velocity axis, respectively. Ph0 was
calculated according to Samozino et al.20 as follows:
400
0vhFh
Ph 

3)
All parameters, were computed step-by-step from the first complete step (the second)
to the last; indeed, contact times can not be calculated in the first step because of the need to
respect the starting line.19,21
Statistical Analysis
Average values and standard deviations were calculated for all variables. A repeated-
measure ANOVA (4 steps and 5 load conditions) was used to determine whether there was a
significant interaction between dependent variables under the various resisted conditions (i.e.
loads) or steps. When a significant main effect (P < .05) was found, a post-hoc test
(Bonferroni pairwise comparison) was conducted to determine where these differences lay.
All statistical analyses were computed using SPSS (Version 20.0).
Results
The mean values and corresponding standard deviations of the kinematic and kinetic
parameters are reported in Table 2 for all the investigated loads. The values of vh0, Fh0 and
Ph0 are reported in Figure 2. The angular parameters at touch-down and take-off are reported
in Figure 3. All subjects completed 13-14 steps: of these the first, the fifth, the tenth and the
last were analyzed.
On the average, all steps, all spatio-temporal parameters changed significantly with
external load (CT, FT, SL and vh: main effect, P < .01). From UL to +40%BM, velocity
decreased by 20.2%, CT increased by 19.0%, FT and SL decreased by 19.3 and 14.8%,
Downloaded by EDIGO Verona on 12/16/16, Volume 0, Article Number 0

“Sled Towing: The Optimal Overload for Peak Power Production” by Monte A, Nardello F, Zamparo P
International Journal of Sports Physiology and Performance
© 2016 Human Kinetics, Inc.
respectively. The differences between loads were significant (post hoc tests: P < .001) in all
parameters but for CT and FT (no significant differences were found between +15%BM and
UL).
On the average, all loads, all spatio-temporal variables changed when step number
increased (main effect: P < .01). CT decreased in all load conditions (-40% from the first to
the last step), whereas FT, SL and vh increased (+50.0, +75.8, +113.3% from the first to the
last step, respectively). The post hoc tests revealed no differences between the tenth step and
the last in all variables.
On the average, all steps, all kinetic variables changed significantly with external load
(main effect: P < .01). Ph decreased by 14.9% while Fh increased by 3.7% (from UL to
+40%BM). The changes in Fh were mainly due to differences between UL and the loaded
conditions; as indicated by the post hoc tests no differences in Fh were observed between
+15, +20, +30 and +40%BM.
On the average, all loads, all kinetic variables changed significantly with step number
(main effect: P < .01). Fh and Ph decreased by 82.0 and 62.5%, respectively, from the first to
the last step. The post hoc tests indicate significant differences between steps (P < .001) in all
parameters except for the fifth step compared to the tenth in Fh, and the tenth compared to the
last in Fh.
Finally, vh0, Fh0 and Ph0 were found to depend on the external load: vh0 linearly
decreased with it (vh0 = -0.59 load + 10.83, R2 = 0.99) where Fh0 and Ph0 were achieved at
+20%BM. Post hoc tests revealed no differences between +30 and +40%BM for Fh0 and Ph0
and between +15 and +30 or +40% for Fh0 (see Figure 2). Furthermore, a decrease in Ph0
was observed in the +40%BM condition compared to UL (P = 0.03). Because vh0, Fh0 and
Ph0 were extrapolated from the force-velocity relationship, they could not be expressed step-
by-step.
Downloaded by EDIGO Verona on 12/16/16, Volume 0, Article Number 0

“Sled Towing: The Optimal Overload for Peak Power Production” by Monte A, Nardello F, Zamparo P
International Journal of Sports Physiology and Performance
© 2016 Human Kinetics, Inc.
On the average, all steps, angular parameters changed significantly both at touch-
down and take-off when the load increased (main effect: hipang and kneeang: P < .001;
ankleang: P < .05). At touch-down, hipang decreased by 21.2%, ankleang decreased by 5.5% and
kneeang decreased by 7.9% from UL to +40%BM. At take-off, hipang decreased by 9.6% and
kneeang by 9.9% from UL to +40%BM; no differences in ankleang were observed in this phase.
Post hoc tests indicate that, during touch-down and take-off, no significant changes occur
when comparing UL with the +15 and +20%BM conditions (for all angles) nor between +30
and +40%BM (for all angles) (see Figure 3).
On the average, all loads, angular parameters showed significant differences as a
function of the step number both at take-off and touch-down (main effect: P < 0.01); hipang,
kneeang, ankleang all increased when step number increased in all load conditions. Post hoc
tests revealed no differences for kneeang between the 5th and the 10th step compared to the last
during take-off. Furthermore, no differences were observed for ankleang at the 10th step
compared to the last in both running phases.
Discussion
To our knowledge, this is the first study that analyzes kinematic, kinetic and angular
parameters during STS, over a distance of 20m. Moreover, in this study the effect of friction
in the calculation of the external load was taken into account. The main results of this study
indicate that STS affects all sprint parameters during the acceleration phase: indeed,
significant differences do exist at different loads on both spatio-temporal parameters and
lower limb angles. The main finding of this study, however, is that maximal horizontal power
(Ph0) in male sprint athletes reaches its maximal value at +20%BM and that, up to this load,
no significant changes are observed in the angular parameters (e.g. in running technique).
Further studies are needed to understand whether training at the optimal overload for
peak power production is related to longitudinal improvements in sprint performance. In the
Downloaded by EDIGO Verona on 12/16/16, Volume 0, Article Number 0

“Sled Towing: The Optimal Overload for Peak Power Production” by Monte A, Nardello F, Zamparo P
International Journal of Sports Physiology and Performance
© 2016 Human Kinetics, Inc.
following parameters the results reported in this study will be discussed and compared with
those reported in the literature
Load determination and coefficient of friction
There are three methods that can be utilized to determine sled load: based on its effect
on sprinting speed, based on a percentage of 1RM or % body mass.1 The latter method was
utilized in this study because it is simpler and easily reproducible. The major criticism to this
method is that a given load would translate in different resistive forces when sprinting over
different surfaces because of differences in the coefficient of friction. This “confounding
factor” was taking in to account in this study.
The static coefficient of friction () measured in our experimental conditions was of
0.20 ± 0.01. This value is comparable to that reported by Linthorne & Cooper,13 for a sled
towed on artificial grass (0.21 ± 0.01). According to these authors  can be as large as 0.58 ±
0.01 on a less smooth surface (e.g. on an athletic track). This implies that, when training on
different surfaces, the coefficient of friction could dramatically change and this could have a
strong effect on the calculations of the applied load. For these reasons the different results
reported in the literature so far regarding the optimal load in STS could be attributed to
differences in surface friction.
Kinematic data
Regarding the kinematic parameters, an increase in load during sled towing was
reported to decrease sprint velocity, to increase CT and decrease FT, SL and v.10,12 The
decrease in FT was associated with a decrease in SL: the athlete would spend less time in the
air with shortened strides.7 This increase in CT seems necessary to allow more time to
produce muscular power, in order to overcome the higher resistance.1 Furthermore, a larger
CT could mean more time for the sprinters to orient their resultant force vector more forward.
Downloaded by EDIGO Verona on 12/16/16, Volume 0, Article Number 0

“Sled Towing: The Optimal Overload for Peak Power Production” by Monte A, Nardello F, Zamparo P
International Journal of Sports Physiology and Performance
© 2016 Human Kinetics, Inc.
The ability to maximize the anteroposterior impulse plays an important role in sprint
acceleration performance.21,22
In agreement with other studies, we found a decrease in CT and a significant
increment in FT, in SL and in hip velocity as a function of the step number.23 Longer step
lengths are indicative of higher strength and power in the leg muscles: this is peculiar of the
sprinting step.24 Furthermore, shorter contact times were related to faster speeds during
maximum velocity and acceleration phases.25,26 FT is a function of SL: when the step length
is larger, the time of flight increases. However, during the initial steps, the higher CT is
useful to increase the horizontal force production, while the small SL could increase the step
frequency and improve the acceleration of BCoM.24,27
Kinetic data
This study is the first that report data of Fh, Ph, vh0, Fh0 and Ph0 during sled towing
running over a distance of 20m. Indeed, previous studies measured kinetic parameters in the
first steps only.4,15,18
Our results indicate that, although Fh increased in STS compared to the UL condition,
no changes are observed when comparing different loads. This is interesting especially
because many studies have shown that STS may provide a stimulus for increasing muscle
strength, peak force or rate of force development.15,16 The lack of significant differences in Fh
among different loads suggests that the “load effect” in STS does not depend on how heavy
the load is but only on its presence (at least in the range of loads considered in this study).
No significant differences were observed in Ph at loads of +15 and +20%BM
compared to the unloaded condition, while Ph was lower than in UL at loads > +30%BM.
This decrease should be attributed to the steady decrease in running velocity (vh) and seems
to be attenuated by the lack of differences in Fh while running loaded.
Downloaded by EDIGO Verona on 12/16/16, Volume 0, Article Number 0

“Sled Towing: The Optimal Overload for Peak Power Production” by Monte A, Nardello F, Zamparo P
International Journal of Sports Physiology and Performance
© 2016 Human Kinetics, Inc.
In agreement with other studies, we found a significant decrease in Fh and Ph as a
function of step number.17 This could be explained by the lower number of steps in the
ascending part of the Fh vs. vh relationship inherent to the high acceleration capability of
sprinters: they produce their maximal power immediately after the initial steps.19 The higher
Ph during the initial steps is useful to increase SL without reducing step frequency. In fact,
the acceleration capability is, afterwards, quasi-exclusively related with the increase in step
length.23 All these results suggest that the ability to generate high Fh and Ph during the initial
steps is essential to improve sprint acceleration.
Finally, our results indicate that maximal velocity, vh0, decreased linearly as a
function of the load, as previously found by Alcaraz et al.6 while maximal force, Fh0, and
power, Ph0, were achieved +20%BM which is, therefore, the optimal overload for
maximizing horizontal power. Data of vh0, Fh0 and Ph0 reported in this study in the unload
condition are in line with data reported in the literature on trained male sprinters.17,19
Angular parameters
Many studies reported angular data during STS. We found significant increases in all
angles at touch-down and take-off when the load increased. During both phases, loads
corresponding to +15 and +20%BM had no significant effects on all angles, while at +30 and
40% BM all angles decreased. This finding is consistent with previous studies were similar
trends of decreasing trunk angle were observed when increasing sled load.7,28 A larger trunk
lean during sled towing is specific to sprint acceleration movements.11 Therefore, sled towing
may be effective in improving sprint acceleration ability.
A significant alteration in the angular parameters affects negatively running technique
and the consequent “positive” transfer on training. Because of the reduction in kneeang and
ankleang, there were greater knee and plantar flexions at touch-down and take-off. This
Downloaded by EDIGO Verona on 12/16/16, Volume 0, Article Number 0

“Sled Towing: The Optimal Overload for Peak Power Production” by Monte A, Nardello F, Zamparo P
International Journal of Sports Physiology and Performance
© 2016 Human Kinetics, Inc.
suggests that propulsive forces be greater and applied for a longer time during the stance
phase.
Cronin et al.29 suggested that during the stance phase of running, the most significant
braking forces are associated with eccentric contraction of the knee extensors and ankle
plantar flexors. In the current study, at +30 and +40%BM kneeang decreased: consequently,
the breaking phase increased. Hence, these loads may have a negative impact on performance
and they impair a positive transfer on training.
As expected, all angular parameters increased with the running steps, in accordance
with other studies. Cronin et al.29 showed an increase of trunk and knee angle in the first
running steps when the load is applied; in these same conditions Lockie et al.7 found
significant differences at knee angle between the first and the second step. This “closer” body
position during the initial steps could be more efficient for acceleration purposes and could
allow a greater application of horizontal force to the ground. It must also be recognized that
such position could reduce flight time and thus lead to a shortened stride.
Practical Applications
In our experimental conditions the +20%BM conditions constitutes the optimal load
for peak power production. As pointed out in this study, however, training loads in sled
towing must be calculated by taking into account friction forces since these change on
different surfaces. Differences in the results reported in different studies (e.g. in the %BM at
which significant kinematic changes occur) can indeed be explained, among the others, by
this factor.
Conclusion
In the present study we show how STS affects kinematic, kinetic and angular
parameters during the acceleration phase of a 20 m sprint, as a function of the load and of the
running step. Horizontal power output is an important determinant for sprinting ability and
Downloaded by EDIGO Verona on 12/16/16, Volume 0, Article Number 0

“Sled Towing: The Optimal Overload for Peak Power Production” by Monte A, Nardello F, Zamparo P
International Journal of Sports Physiology and Performance
© 2016 Human Kinetics, Inc.
present data show that this parameter is maximal in the +20%BM condition. In addition, no
changes in joint angles were observed up to this load, indicating that this load is not
detrimental for running technique.
Acknowledgments
We would like to thank Lorenzo Manca and Matteo Saoncella for their help in data
collection.
Conflict of Interest
The authors report no conflict of interest.
Downloaded by EDIGO Verona on 12/16/16, Volume 0, Article Number 0

“Sled Towing: The Optimal Overload for Peak Power Production” by Monte A, Nardello F, Zamparo P
International Journal of Sports Physiology and Performance
© 2016 Human Kinetics, Inc.
References
1. Petrakos G, Morin JB, Egan B. Resisted Sled sprint training to improve sprint
performance: a systematic review. Sports Med. 2016; doi: 10.1007/s40279-015-0422-
8.
2. Seitz LB, Reyes A, Tran TT, Saez de Villarreal E, Haff GG. Increases in lower-body
strength transfer positively to sprint performance: a systematic review with meta-
analysis. Sports Med. 2014; 44:1693–702.
3. Buchheit M1, Samozino P, Glynn JA, Michael BS, Al Haddad H, Mendez-Villanueva
A, Morin JB. Mechanical determinants of acceleration and maximal sprinting speed in
highly trained young soccer players. J Sports Sci. 2014; 32:1906–13.
4. 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: 2738-2745.
5. Young WB. Transfer of strength and power training to sports performance. Int. J.
Sports Physiol. Perform. 2006; 1: 74-83.
6. Alcaraz PE, Palao JM, Elvira JLL. Determining the optimal load for resisted sprint
training with sled towing. J. Strength Cond. Res. 2009; 23: 480-485.
7. Lockie RG, Murphy AJ, Spinks CD. Effects of resisted sled towing on sprint
kinematics in field-sport athletes. J. Strength Cond. Res. 2003; 17: 760-767.
8. Harrison AJ, Bourke G. The effect of resisted sprint training on speed and strength
performance in male rugby players. J. Strength Cond. Res. 2009; 23: 275-283.
9. Clark KP, Stearne DJ, Walts CT, Miller AD. The longitudinal effects of resisted
sprint training using weighted sleds vs. weighted vests. J. Strength Cond. Res. 2010;
24: 3287-3295.
10. Alcaraz PE, Palao JM, Elvira JLL, Linthorne NP. Effects of three types of resisted
sprint training devices on the kinematics of sprinting at maximum velocity. J.
Strength Cond. Res. 2008; 22: 890-897.
11. Cronin J, Hansen KT. Resisted sprint training for the acceleration phase of sprinting.
J. Strength Cond. Res. 2006; 28: 42-51.
12. Maulder PS, Bradshaw EJ, Keogh JWL. Kinematic alterations due to different loading
schemes in early acceleration sprint performance from starting blocks. J. Strength
Cond. Res. 2008; 22: 1992-2002.
13. Linthorne NP, Cooper JE. Effect of the coefficient of friction of a running surface on
sprint time in a sled-towing exercise. S. Biomech. 2013; 12:2, 175-185.
14. Kawamori N, Newton R, Nosaka K. Effects of weighted sled towing on ground
reaction force during the acceleration phase of sprint running. J. Sports Sci. 2015; 32:
1139-1145.
Downloaded by EDIGO Verona on 12/16/16, Volume 0, Article Number 0

“Sled Towing: The Optimal Overload for Peak Power Production” by Monte A, Nardello F, Zamparo P
International Journal of Sports Physiology and Performance
© 2016 Human Kinetics, Inc.
15. Valencia MAA, Arenas SR, Elvira JLL, Ravé JMG, Navarro-Valdivielso R, Alcaraz
PE. Effects of sled towing on peak force, the rate of force development and sprint
performance during the acceleration phase. J. Hum. Kinet. 2015; 46: 139-148.
16. Cottle CA, Carlson LA, Lawrence MA. Effects of sled towing on sprint starts. J.
Strength Cond. Res. 2014; 28: 1241-1245.
17. Samozino P, Morin JB, Dorel S, Slawinski J, Peyrot N, Saez-de-Villareal E, Rabita G.
A simple method for measuring power, force and velocity properties of sprint
running. Scand. J. Med. Sci. Sports. 2015; doi: 10.1111/sms.12490.
18. Winwood PW, Cronin JB, Brown SR, Keogh JWL. A biomechanical analysis of the
heavy sprint-style sled pull and comparison with the back squat. International Journal
of Sports Science and Coaching. 2015; 10: 851 - 868.
19. Rabita G, Dorel S, Slawinski J, Saez de Villarreal E, Couturier A, Samozino P, Morin
JB. Sprint mechanics in world-class athletes: a new insight into the limits of human
locomotion. Scand. J. Med. Sci. Sports. 2015; doi: 10.1111/sms.12389.
20. Samozino P, Rejc E, di Prampero PE, Belli A, Morin JB. Optimal force-velocity
profile in ballistic movements. Altius: citius or fortius? Med. Sci. Sports Exerc. 2012;
44: 313-322.
21. Morin JB, Samozino P, Edouard P, Tomazin K. Effect of fatigue on force production
and force application technique during repeated sprints. J. Biomech. 2011; 44: 2719-
2723.
22. Hunter JP, Marshall RN, McNair PJ. Interaction of step length and step rate during
sprint running. Med Sci Sport Exerc. 2004; 36: 261–271.
23. Debaere S, Jonkers I, Delecluse C. The contribution of step characteristics to sprint
running performance in high-level male and female athletes. J. Strength Cond. Res.
2013; 27: 116-124.
24. Babic V, Harasin D, Dizdar D. Relations of the variables of power and morphological
characteristics to the kinematic indicators of maximal speed running. Kines. 2007; 39:
28-39.
25. Lockie RG, Murphy AJ, Knight TJ, Janse de Jonge XAK. Factors that differentiate
acceleration ability in field sport athletes. J. Strength Cond. Res. 2011; 25: 2704-
2714.
26. Murphy AJ, Lockie RG, Coutts AJ. Kinematic determinants of early acceleration in
field sport athletes. J. Sports Sci. Med. 2003; 2: 144-150.
27. Lockie RG, Murphy AJ, Schultz AB, Jeffriess MD, Callaghan SJ. Influence of sprint
acceleration stance kinetics on velocity and step kinematics in field sport athletes. J.
Strength Cond. Res. 2013; 27: 2494-2503.
28. Letzelter M, Sauerwein G, Burger R. Resistance runs in speed development. Modern
Athlete and Coach 1995; 33: 7-12.
Downloaded by EDIGO Verona on 12/16/16, Volume 0, Article Number 0

“Sled Towing: The Optimal Overload for Peak Power Production” by Monte A, Nardello F, Zamparo P
International Journal of Sports Physiology and Performance
© 2016 Human Kinetics, Inc.
29. Cronin J, Hansen KT, Kawamori N, Mcnair P. Effects of weighted vests and sled
towing on sprint kinematics. Sports Biomec. 2008; 7: 160-172.
Downloaded by EDIGO Verona on 12/16/16, Volume 0, Article Number 0

“Sled Towing: The Optimal Overload for Peak Power Production” by Monte A, Nardello F, Zamparo P
International Journal of Sports Physiology and Performance
© 2016 Human Kinetics, Inc.
Figure 1: Schematic drawing of the sled towing track configuration.
Downloaded by EDIGO Verona on 12/16/16, Volume 0, Article Number 0

“Sled Towing: The Optimal Overload for Peak Power Production” by Monte A, Nardello F, Zamparo P
International Journal of Sports Physiology and Performance
© 2016 Human Kinetics, Inc.
Figure 2: Maximal horizontal velocity (vh0), maximal horizontal force (Fh0) and maximal
horizontal power (Ph0) as a function of the load (UL, unload and with loads corresponding to
15, 20, 30 and 40% of the sprinter’s body mass).
* Significant difference between unload and BM% condition (P < .05)
** Significant difference between unload and BM% condition (P < .01)
*** Significant difference between unload and BM% condition (P < .001)
Downloaded by EDIGO Verona on 12/16/16, Volume 0, Article Number 0

“Sled Towing: The Optimal Overload for Peak Power Production” by Monte A, Nardello F, Zamparo P
International Journal of Sports Physiology and Performance
© 2016 Human Kinetics, Inc.
Figure 3: Relative angles of hip (i.e. the internal angle between trunk and thigh), knee (i.e.
the internal angle between thigh and shank) and ankle (i.e. the internal angle between shank
and foot) during touch-down and take-off as a function of the load (UL, unload and with
loads corresponding to 15, 20, 30 and 40% of the sprinter’s body mass) and the step number
(from the first to the last).
Downloaded by EDIGO Verona on 12/16/16, Volume 0, Article Number 0

“Sled Towing: The Optimal Overload for Peak Power Production” by Monte A, Nardello F, Zamparo P
International Journal of Sports Physiology and Performance
© 2016 Human Kinetics, Inc.
Table 1: Anthropometric characteristics of the sprinters. Data are means ± SD.
Range
Age (years)
19.4±2.3
18-23
Body Mass (kg)
71.5±2.9
68.5-74.5
Body Height (m)
1.77±0.03
1.73-1.85
100 m Personal Best (s)
Number of workouts/week
10.91±0.14
4.46±0.88
10.66-11.11
3.0-6.0
Sled towing Experience (years)
3.77±1.36
2.0-7.0
Downloaded by EDIGO Verona on 12/16/16, Volume 0, Article Number 0

“Sled Towing: The Optimal Overload for Peak Power Production” by Monte A, Nardello F, Zamparo P
International Journal of Sports Physiology and Performance
© 2016 Human Kinetics, Inc.
Table 2: Kinematic and kinetic data as a function of the imposed load (UL: unloaded; with
loads corresponding to +15, +20, +30 and +40% of BM) in the first, fifth, tenth and last step.
Data are means ± SD.
CT (s)
FT (s)
First
Fifth
Tenth
Last
First
Fifth
Tenth
Last
UL
0.18±0.0
1
0.13±0.0
1
0.11±0.0
1
0.11±0.0
1
0.09±0.0
1
0.11±0.0
1
0.12±0.0
2
0.12±0.0
1
+15
%
0.19±0.0
2
0.13±0.0
1
0.12±0.0
1
0.12±0.0
1
0.09±0.0
2
0.11±0.0
1
0.12±0.0
1
0.11±0.0
1
+20
%
0.19±0.0
2
0.14±0.0
1
0.12±0.0
1
0.12±0.0
1
0.08±0.0
1
0.10±0.0
1
0.11±0.0
1
0.12±0.0
1
+30
%
0.20±0.0
3
0.15±0.0
1
0.13±0.0
1
0.13±0.0
1
0.08±0.0
2
0.09±0.0
1
0.10±0.0
1
0.10±0.0
1
+40
%
0.21±0.0
2
0.16±0.0
1
0.15±0.0
1
0.13±0.0
1
0.07±0.0
2
0.09±0.0
1
0.09±0.0
1
0.09±0.0
1
SL (m)
vh (m⋅s-1)
First
Fifth
Tenth
Last
First
Fifth
Tenth
Last
UL
1.00±0.0
3
1.58±0.0
6
1.89±0.0
7
1.85±0.0
4
3.74±0.2
0
6.70±0.5
4
8.04±0.4
9
8.04±0.3
6
+15
%
0.97±0.0
4
1.52±0.0
9
1.78±0.1
4
1.69±0.0
6
3.54±0.4
2
6.35±0.4
4
7.55±0.5
6
7.36±0.4
1
+20
%
0.96±0.0
4
1.48±0.0
8
1.70±0.0
9
1.65±0.1
0
3.51±0.3
2
6.12±0.4
0
7.34±0.5
6
7.03±0.6
3
+30
%
0.92±0.0
4
1.35±0.0
8
1.59±0.0
9
1.62±0.1
1
3.34±0.3
2
5.54±0.5
0
6.78±0.5
0
6.97±0.4
8
+40
%
0.89±0.0
4
1.30±0.0
7
1.54±0.1
1
1.53±0.1
2
3.21±0.2
6
5.34±0.3
4
6.49±0.6
2
6.93±0.1
0
Ph (W)
Fh (N)
First
Fifth
Tenth
Last
First
Fifth
Tenth
Last
UL
1800±316
1327±313
1054±194
732±132
479±63
196±32
130±17
91±8
+15%
1863±616
1338±289
1044±218
764±144
514±117
209±31
137±19
103±14
+20%
1879±573
1269±272
1010±231
600±121
528±105
206±31
136±22
85±9
+30%
1836±608
1120±280
915±191
715±138
541±117
200±31
134±19
102±13
+40%
1788±393
1108±222
900±253
624±100
552±83
206±29
137±25
90±6
Footnote: CT: contact time; FT: flight time; SL: step length; vh: horizontal speed; Ph; horizontal power; Fh:
horizontal force.
Downloaded by EDIGO Verona on 12/16/16, Volume 0, Article Number 0