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Abstract The study sheds some light on how correct training can allow athletes to continue accelerting anf atttain a higher maximum speed. Forces applied to the ground during sprinting are vital to performance. This study aimed to understand how specific aspects of ground reaction force waveforms allow some individuals to continue to accelerate beyond the velocity plateau of others. Twenty‐eight male sprint specialists and 24 male soccer players performed maximal‐effort 60‐m sprints. A 54‐force‐plate system captured ground reaction forces, which were used to calculate horizontal velocity profiles. Touchdown velocities of steps were matched (8.00, 8.25, and 8.50 m/s), and the subsequent ground contact forces were analyzed. Mean forces were compared across groups and statistical parametric mapping (t tests) assessed for differences between entire force waveforms. When individuals contacted the ground with matched horizontal velocity, ground contact durations were similar. Despite this, sprinters produced higher average horizontal power (15.7‐17.9 W/kg) than the soccer players (7.9‐11.9 W/kg). Force waveforms did not differ in the initial braking phase (0%‐~20% of stance). However, sprinters attenuated eccentric force more in the late braking phase and produced a higher antero‐posterior component of force across the majority of the propulsive phase, for example, from 31%‐82% and 92%‐100% of stance at 8.5 m/s. At this velocity, resultant forces were also higher (33%‐83% and 86%‐100% of stance) and the force vector was more horizontally orientated (30%‐60% and 95%‐98% of stance) in the sprinters. These findings illustrate the mechanisms which allowed the sprinters to continue accelerating beyond the soccer players’ velocity plateau. Moreover, these force production demands provide new insight regarding athletes’ strength and technique training requirements to improve acceleration at high velocity.

Reference

Steffi L. Colyer, Ryu Nagahara, and Yohei Takai. 2018. How sprinters accelerate beyond the velocity plateau of soccer players: Waveform analysis of ground reaction forces. Scandinavian Journal of Medicine & Science in Sports 19 September 2018 https://doi.org/10.1111/sms.13302

The study reinforces the importance of ground reaction force GRF) and describes how athletes are able to apply greater amounts of mass-specific force to the ground in the shortest possible time.

Abstract

Sprint running performance can be investigated relatively simply at the whole-body level by examining the timing of the phases of the stride and the forces applied to the ground in relation to a runners body weight. Research using this approach has been used to address a number of basic questions regarding the limits and determinants of human running speed. The primary differentiating factor for the top speeds of human runners is how forcefully they can strike the ground in relation to body mass. A general relationship between mass-specific force application and maximum running speeds results from the similar durations of the aerial and swing phases of the stride for different runners. Recent work has elucidated the mechanism by which faster runners are able to apply greater mass-specific ground forces in the very brief foot-ground contact times sprinting requires.

Coaching Application

The amount of force an athlete can apply to the ground in relation to body weight and the speed with which the force can be applied (ground contact time) is a key factor in improving speed during each of the four phases of a short sprint (the start, acceleration, maximum speed, and deceleration phase). Both the amount of force production and the speed of force application can be improved with proper training.

Reference

Weyand, Peter (2017) “FORCE, MOTION, SPEED: A GROUNDED PERSPECTIVE ON HUMAN RUNNING PERFORMANCE,” ISBS Proceedings Archive: Vol. 35 : Iss. 1 , Article 289. Available at: https://commons.nmu.edu/isbs/vol35/iss1/289

This study takes a careful look at how a season of football affects the general health of Division I players.

Abstract
Among high school and collegiate football players, research shows that 56% of football players are considered obese. Training during a season involves rigorous cardio and strength conditioning to physically prepare for the necessary sport skills. While a preseason physical exam is required, minimal health information is gathered post season.

PURPOSE: The purpose of this pilot study was to compare health markers during a pre and post collegiate football season among Division 1 football players.

METHODS: Eleven male freshman players (18±0.24 years) volunteered to be tested before the start of the competitive season. Testing for health markers included demographics (age, height, and weight), fasting Low Density Lipoprotein (LDL), High Density Lipoprotein (HDL), blood glucose (BG), blood pressure (BP), and triglycerides (TRI). Body Mass Index (BMI) was calculated and additional measures included percent body fat (%BF) and subcutaneous (SCAT) and visceral fat (VAT) depths. Players were reassessed at the end of the football season. Descriptive statistics and comparison between pre and post season were analyzed in SPSS (v24). A repeated measures ANOVA was conducted to compare health markers; alpha level was set at 0.05.

RESULTS: Ten out of the 11 subjects had a significant decrease in their systolic BP (pre: 131±6 mmHg, post: 118±7 mmHg; p=.001) and TRI (pre: 168.27±42.28 mg/dL, post: 116.82±42.28 mg/dL;p=.025). All other health markers had no significant change.

CONCLUSION: This data suggests that there has been a significant change from pre to post season in certain health markers related to cardiovascular health. This study opens the discussion of appropriate fitness training among football players beginning at an early stage. Does the training overlook the athlete’s health in preparation for the sport? This data also highlights the importance of dietary education for the players, as well as, requiring additional health assessments during a preseason physical exam. Further research on dietary consumption during a season is necessary to understand the impact of nutrition on the athlete’s health and fitness.

Coaching Application

Although findings were mostly favorable, the study highlights the need for improved in-
season nutritional guidance and perhaps more in-season fitness attention to cardiovascular health

Reference

Swanson, C; Vahk, A; Babcock, G; and Repovich, FACSM, W (2018) “HEALTH MARKERS IN DIVISION 1 FOOTBALL PLAYERS COMPARED PRE- TO POST-SEASON: A PILOT STUDY,” International Journal of Exercise Science: Conference Proceedings: Vol. 8 : Iss. 6 , Article 52. Available at: https://digitalcommons.wku.edu/ijesab/vol8/iss6/52

Injuries to the ankle and knee are common in the sport of basketball for both boys and girls. The study below compares the type and frequency of injuries between both genders.
Abstract

This prospective study investigated the incidence and pattern of acute time‐loss injuries in young female and male basketball players. Eight basketball teams (n=201; mean age 14.85±1.5) participated in the follow‐up study (2011‐2014). The coaches recorded player participation in practices and games on a team diary. A study physician contacted the teams once a week to check new injuries and interviewed the injured players. In total, 158 injuries occurred. The overall rate of injury (per 1000 hours) was 2.64 (95% CI 2.23‐3.05). Injury rate was 34.47 (95% CI 26.59‐42.34) in basketball games and 1.51 (95% CI 1.19‐1.82) in team practices. Incidence rate ratio (IRR) between game and practice was 22.87 (95% CI 16.71‐31.29). Seventy‐eight percent of the injuries affected the lower limbs. The ankle (48%) and knee (15%) were the most commonly injured body sites. The majority of injuries involved joint or ligaments (67%). Twenty‐three percent of the injuries were severe causing more than 28 days abseThe numberom sports. Number of recurrent injuries was high (28% of all injuries), and most of them were ankle sprains (35 of 44, 79%). No significant differences were found in injury rates between females and males during games (IRR 0.88, 0.55, to 1.40) and practices (IRR 1.06, 0.69, to 1.62). In conclusion, ankle and knee ligament injuries were the most common injuries in this study. Moreover, the rate of recurrent ankle sprains was alarming.

Coaching Application

Coaches could benefit by consulting athletic trainers concerning ankle injury prevention modalities that favorably reduce the incidence of initial and reoccurring injuries that are likely to result in lost practice and game time. Reseaerch indicates that prophylactic programs effectively reduce the risk of general lower extremity injuries and ankle sprains.

Reference

K. Pasanen, T. Ekola, T. Vasankai, P. Kannus, A. Heinonen, U.M. Kujala, and J. Parkari. 2016. High ankle injury rate in adolescent basketball: A 3‐year prospective follow‐up study. Scandinavian Journal of Medicine and Science in Sports Vol. 27, Issue 6.

The warm-up period prior to competition is often loosely and routinely completed with the dual purposes of preparing the body for more vigorous activity and decreasing the chance of soft tissue injury. The study described in the abstract below added a third dimension of possible performance enhancement by utilizing 3 depth jumps one minute after the dynamic stretching exercises to determine the immediate effect on 20-meter sprint performance.

Abstract

The purpose of this investigation was to determine whether the addition of 3 depth jumps to a dynamic warm-up (DYNDJ) protocol would significantly improve 20-m sprint performance when compared with a cardiovascular (C) warm-up protocol or a dynamic (DYN) stretching protocol alone. The first part of the study identified optimal drop height for all subjects using the maximum jump height method. The identified optimal drop heights were later used during the DYNDJ protocol. The second part compared the 3 warm-up protocols above to determine their effect on 20-m sprint performance. Twenty-nine subjects (age, 20.8 6 4.4 years; weight, 82.6 6 9.9 kg; height, 180.3 6 6.2 cm) performed 3 protocols of a C protocol, a DYN protocol, and a DYNDJ protocol in a randomized order. A 20-m sprint was performed 1 minute after the completion of each of the 3 protocols. Results displayed significant differences between each of the 3 protocols. A significant improvement (p = 0.001) of 2.2% was obtained in sprint time between the C protocol (3.300 6 0.105 seconds) and the DYN protocol (3.227 6 0.116 seconds), a further significant improvement of 5.01% was attained between the C and the DYNDJ protocols (3.300 6 0.10 vs. 3.132 6 0.120 seconds; p = 0.001). In addition, a significant improvement (p = 0.001) of 2.93% was observed between the DYN protocol (3.227 6 0.116 seconds) and the DYNDJ protocol (3.132 6 0.116 seconds). The data from this study advocate the use of DYNDJ protocol as a means of significantly improving 20-m sprint performance 1 minute after the DYNDJ protocol.

Coaching Application

Warm-up programs just prior to competition or testing in a 20-yard or 40-yard dash may favorably improve sprint performance when a different, unique exercise, such as depth jumps, are used. Other studies have observed improved sprint times when immediately preceded by a short session of sprint-assisted and sprint-resisted training.

This study also provides a reminder that the warm-up period prior each workout session offers the opportunity to meet multiple training objectives: core temperature elevation, dynamic stretching of key areas involved in the sprinting action, sprint form and technique improvement, and improved sprint performance. The NASE 5-Step Model utilizes a warm-up session with dynamic stretching exercises that also improve sprinting form and technique. This approach saves valuable coaching time, especially during the in-season period, and provides daily devotion to critical areas that often are neglected in team sports.

Reference

Byrne, PJ, Kenny, J, and O’ Rourke, B. Acute potentiating effect of depth jumps on sprint performance. J Strength Cond Res 28 (3): 610–615, 2014

The amount of force utilized during supra-maximal towing with surgical tubing and other devices determines training effectiveness. A force that causes athletes to break form is too much. Force that does not result in faster stride rates and shorter ground contact time is too little. The study below examines these and other factors to provide guidelines for coaches utilizing any sprint-assisted training program.

ABSTRACT

The purpose of this study was to determine the influence of towing force magnitude on the kinematics of supra-maximal sprinting. Ten high school and college-age track and field athletes (6 men, 4 women) ran 60-m maximal sprints under 5 different conditions: Non-towed, Tow A (2.0% body weight 1⁄2BW ), Tow B (2.8% BW), Tow C (3.8% BW), and Tow D (4.7% BW). Three-dimensional kinematics of a 4-segment model of the right side of the body were collected starting at the 35-m point of the trial using high-speed (250 Hz) optical cameras. Significant differences (p , 0.05) were observed in stride length and horizontal velocity of the center of mass during Tow C and Tow D. For Tow D, a significant increase (p = 0.046) in the distance from the center of mass to the foot at touchdown was also observed. Contact time decreased significantly in all towing conditions (p , 0.01), whereas stride rate increased only slightly (,2.0%) under towed conditions. There were no significant changes in joint or segment angles at touchdown, with the exception of a significant decrease (p = 0.044) in the flexion/extension angle at the hip during the Tow D condition. We conclude that towing force magnitude does influence the kinematics of supra-maximal running and that potentially negative training effects may arise from towing individuals with a force in excess of 3.8% BW. Therefore, we suggest that coaches and practitioners adjust towing force magnitude for each individual and avoid using towing forces in excess of 3.8% of the athlete’s BW.

Coaching Application

The ideal “training zone” for supra-maximal towing may vary slightly from one athlete to another. The amount of force applied must be enough to increase stride rate, decrease ground contact time, and avoid altering form and technique. The findings of this study provide a documented guideline for determining the force/body weight ratio for maximum effect.

Reference
Clark, DA, Sabick, MB, Pfeiffer, RP, Kuhlman, SM, Knigge, NA, and Shea, KG. 2009. Influence of towing force magnitude on the kinematics of supra-maximal sprinting.
J Strength Cond Res Jul;23(4):1162-8.

In all types of training to improve the speed of power athletes, the concept of specificity is applied to make the movement patterns as close as possible to those that occur during competition. The study below applies this concept with resisted sprint training.

Abstract
Resisted sprint running is a common training method for improving sprint-specific strength. For maximum specificity of training, the athlete’s movement patterns during the training exercise should closely resemble those used when performing the sport. The purpose of this study was to compare the kinematics of sprinting at maximum velocity to the kinematics of sprinting when using three of types of resisted sprint training devices (sled, parachute, and weight belt). Eleven men and 7 women participated in the study. Flying sprints greater than 30 m were recorded by video and digitized with the use of biomechanical analysis software. The test conditions were compared using a 2-way analysis of variance with a post-hoc (done after the event) Tukey test of honestly significant differences. We found that the 3 types of resisted sprint training devices are appropriate devices for training the maximum velocity phase in sprinting. These devices exerted a substantial overload on the athlete, as indicated by reductions in stride length and running velocity, but induced only minor changes in the athlete’s running technique. When training with resisted sprint training devices, the coach should use a high resistance so that the athlete experiences a large training stimulus, but not so high that the device induces substantial changes in sprinting technique. We recommend using a video overlay system to visually compare the movement patterns of the athlete in unloaded sprinting to sprinting with the training device. In particular, the coach should look for changes in the athlete’s forward lean and changes in the angles of the support leg during the ground contact phase of the stride.

Coaching Application
The key to effective resistance sprint training is to find the sweet spot or “speed zone”, at or beyond an athlete’s top speed but below the point where mechanics are compromised. This concept requires finding the maximum amount of weight that each athlete can tolerate without breaking form. Observation by a video analysis video analysis will detect form changes.

Reference
Alcarez, PE, Palao, JM. Linthome, NP. 2008. Effects of three types of resisted sprint training devices on the kinematics of sprinting at maximum velocity. J Strength Cond Res. May;22(3):890-7.

Ground Force Direction for Optimal Performance Using a Standing Start. Both the moving start and variations of the standing start are utilized in baseball, basketball, soccer, field hockey, lacrosse, rugby and other sports. Athletes who either lack the strength needed for the forceful push off both feet required during the 3- or 4-point starting stance, or have not mastered the correct form and technique, can often improve their times in a short sprint by utilizing a standing start. This also places the front foot closer to the starting line, is more forgiving, easier to master, and moves the body into the correct sprint position within the first two steps with less ground reaction force than what is required from a crouched position. The study described below examined the connection between ground reaction impulses and sprint acceleration when the standing start was used and offers training advice for the improvement of horizontally-directed ground reaction force.

Abstract

Large horizontal acceleration in short sprints is a critical performance parameter for many team sports athletes. It is often stated that producing large horizontal impulse at each ground contact is essential for high short sprint performance, but the optimal pattern of horizontal and vertical impulses is not well understood, especially when the sprints are initiated from a standing start.

This study was an investigation of the relationships between ground reaction impulses and sprint acceleration performance from a standing start in team sports athletes. Thirty physically active young men with team sports backgrounds performed 10-m sprints from a standing start, whereas sprint time and ground reaction forces were recorded during the fifirst ground contact and at 8 m from the start. Associations between sprint time and ground reaction impulses (normalized to body mass) were determined by a Pearson’s correlation coefficient (r) analysis. The 10-m sprint time was significantly ( p 0.01) correlated with net horizontal impulse (r =20.52) and propulsive impulse (r =20.66) measured at 8 m from the start. No significant correlations were found between sprint time and impulses recorded during the first ground contact after the start. These results suggest that applying ground reaction impulse in a more horizontal direction is important for sprint acceleration from a standing start. This is consistent with the hypothesis of training to increase net horizontal impulse production using sled towing or using elastic resistance devices, which needs to be validated by future longitudinal training studies.

Coaching Application

Increasing ground reaction force in a horizontal direction, using the Austin leg-drive machine, weighted sleds, and band resistance, is recommended to improve speed in the standing start and early acceleration phase of a short sprint.

Reference

Naoki Kawamori, Kazunori Nosaka, and Robert U. Newton. 2013. Relationships between ground reaction impulse and sprint acceleration performance in team sport athletes. Journal of strength and Conditioning research 27(3)568-573.

A grounded perspective on human running: The information below provides accurate responses to two key questions about speed improvement from Dr. Peter Weyand, one of the leading researchers in the world on this topic. A careful read will result in a better understanding of how speed can be improved for sports competition, including two-mass model, spring-mass model, impact-deceleration, contact time, aerial time, swing time.

Sprint running performance can be investigated relatively simply at the whole-body level by examining the timing of the phases of the stride and the forces applied to the ground in relation to a runners body weight. Research using this approach has been used to address a number of basic questions regarding the limits and determinants of human running speed. The primary differentiating factor for the top speeds of human runners is how forcefully they can strike the ground in relation to body mass. A general relationship between mass-specific force application and maximum running speeds results from from the similar durations of the aerial and swing phases of the stride for different runners. Recent work has elucidated the mechanism by which faster runners are able to apply greater mass-specific ground forces in the very brief foot-ground contact times sprinting requires.

INTRODUCTION: The primary requirement of human locomotion is supporting the body’s weight against gravity. This requirement has provided insights into locomotor metabolism (Kram & Taylor, 1990; Roberts et al, 1998; Taylor, 1985), gait mechanics (Blickhan , 1989; Weyand et al., 2001) and the sprinting performance of humans (Kuitunen et al., 2002; Rabita et al., 2015) and other animal runners (Taylor, 1980). During steady-speed running on a level surface, the stride-averaged vertical force runners apply to the surface equals their body weight. Faster runners can satisfy this force requirement with greater forces that are applied in shorter periods of time. Consequently, the maximal speeds of human and other runners are largely explained by the maximum forces they can apply to the ground in relation to body mass (Weyand et al., 2010). Here, two questions are considered: 1) why does the general relationship between mass-specific force and top running speed exist? and 2) what is the mechanism by which faster runners are able to apply greater mass-specific ground forces?

QUESTION 1: Why does the general relationship between mass-specific force and top running speed exist? Elite human sprinters can apply peak forces to the running surface as large as 5.0 times body mass and stance-averaged forces that are as large as 2.5 times body mass. In contrast, capable, but less swift human runners typically apply peak forces of 3.5 times their body weight and stance-averaged forces of up to 2.0 times body weight (Clark & Weyand, 2014). The strength of the relationship between the mass-specific forces runners apply and how swiftly they can run at top speed results from the similar durations of the non-contact portion of the stride. At tops speed, 28 35th Conference of the International Society of Biomachanics in Sports, Cologne, Germany, June 14-18, 2017 1 Weyand: FORCE, MOTION, SPEED: A GROUNDED PERSPECTIVE ON HUMAN RUNNING PER Published by NMU Commons, 2017 fast and slow runners alike typically spend 0.12 s in the air between steps and take slightly longer than one-third of a second to reposition the limbs (Weyand et al., 2000). These observations suggest that maximizing force application may be the only viable mechanical option by which human runners can maximize speed (Clark et al., 2014).

QUESTION 2: What is the mechanism by which faster runners are able to apply greater forces to the ground? If the speeds of the swiftest human runners are largely determined by the magnitude of the massspecific forces they can apply to the ground, what confers the ability to apply relatively larger forces? Do the swiftest sprinters have intrinsically stronger limbs and limb muscles? Or, alternatively, do they use the motion of the running stride to maximize the forces applied to the ground? Recent research indicates that speed athletes exploit a motion-to-force mechanism during the impact portion of the contact period to maximize ground reaction forces applied during the brief contact periods sprinting requires (Calrk & Weyand, 2014; Clark et al., 2017). These athletes attain greater limb velocities before the foot contacts the ground. They also stop the limb more abruptly upon impact. This impact-deceleration mechanism results in a rapid rising edge of the force-time relationship and a peak force that occurs well before the mid-point of the contact period. The resulting asymmetrical pattern of force application deviates substantially from the ground force application predicted by the spring-mass model (Blickhan, 1989) indicating that the model does not include the force-motion elements responsible for sprinting performance. The variation present in the patterns of ground force application for sprinters and non-sprinters alike can be predicted from body mass and three stride-specific prameters (contact time, aerial time and ankle acceleration) using an anatomically based two-mass model of the human body. These observations indicte that sprinters exploit a motion-based deceleration mechanism to maximize ground force application. To what extent the dynamic mechanism relies on intrinsic limb strength vs. motor control and timing precision remains to be determined.

CONCLUSIONS: The conformation of both the running gait mechanics and ground reaction force patterns of human sprinters to a common pattern: 1) is further evidence that human sprinting performance is constrained by the brief duration of foot-ground force application at very fast running speeds, and 2) implies a convergence that results from the physics of motion and the properties of the tissues that generate and transmit musculoskeletal forces to the ground.

REFERENCES: Blickhan, R., “The Spring-Mass Model for Running and Hopping,” Journal of Biomechanics, vol. 22, pp. 1217- 1227, 1989. Clark, K.P. & Weyand, P.G., “Are running speeds maximized with simple-spring stance mechanics?,” Journal of Applied Physiology, vol. 117, pp. 604-615, 2014. Clark, K.P., Ryan, L.J., & Weyand, P.G., “Foot speed, foot-strike and footwear: linking gait mechanics and running ground reaction forces,” Journal of Experimental Biology, vol. 217, pp. 2037-2040, 2014. Clark, K.P., Ryan, L.J., & Weyand, P.G., “A general relationship links gait mechanics and running ground reaction forces,” Journal of Experimental Biology, vol.220, pp.247-258, 2017. Kram R. & Taylor C.R. Energetics of running: a new perspective, Nature 346(6281): 265-267,1990. Kuituen, S., Komi, P.V., & Kyrolainen, H., “Knee and ankle joint stiffness in sprint running,” Medicine and Science in Sports and Exercise, vol. 34, pp. 166-173, 2002. Rabita G., Dorel, S., Slawinski, J. Saez de Villareal, Couturier, P, Morin J.B. (2015) Sprint mechanics in worldclass athletes: new insight into the limits of human locomotion, Scandinavian Journal of Medicine and Science in Sports, 25(5): 583-594. Roberts T.J., Kram R., Weyand P.G., Taylor C.R. “Energetics of bipedal running I: energetic cost of generating force. Journal of Experimental Biology, 201(19) 2745-2751, 1998. Taylor, C.R., “Force development during locomotion: a determinant of gait, speed and metabolic power”, Journal of Experimental Biology, 115: 255-262, 1985.

Blog Reference

Peter Weyand Locomotor Performance Laboratory, Department of Applied Physiology and Wellness, Southern Methodist University, Dallas, Texas, USA. 35th Conference of the International Society of Biomachanics in Sports, Cologne, Germany, June 14-18, 2017
https://commons.nmu.edu/cgi/viewcontent.cgi referer=https://scholar.google.com/&httpsredir=1&article=1302&context=isbs.

Effects of weighted sled towing on ground reaction force during the acceleration phase of sprint running: There is a need for additional research on the proper use and variable control for sprint-resisted training (weighted sleds, Austin leg-drive machine, uphill and staircase sprinting, and resistance bands) to effectively improve horizontally- and vertically-directed ground reaction force (GRF). The study described below analyzed force direction and sled weight to provide valuable information on specific training to increase horizontally directed force.

Athletes use weighted sled towing to improve sprint ability, but little is known about its biomechanics. The purpose of this study was to investigate the effect of weighted sled towing with two different loads on ground reaction force. Ten physically active men (mean ± SD: age 27.9 ± 1.9 years; stature 1.76 ± 0.06 m; body mass 80.2 ± 9.6 kg) performed 5 m sprints under three conditions; (a) unresisted, (b) towing a sled weighing 10% of body mass (10% condition) and (c) towing a sled weighing 30% of body mass (30% condition). Ground reaction force data during the second ground contact after the start were recorded and compared across the three conditions. No significant differences between the unresisted and 10% conditions were evident, whereas the 30% condition resulted in significantly greater values for the net horizontal and propulsive impulses (P < 0.05) compared with the unresisted condition due to longer contact time and more horizontal direction of force application to the ground. It is concluded that towing a sled weighing 30% of body mass requires more horizontal force application and increases the demand for horizontal impulse production. In contrast, the use of 10% body mass has minimal impact on ground reaction force.

Coaching Application: Although 10% of body mass sled resistance was ineffective, a 30% resistance required more horizontally-directed ground reaction force beginning with the second ground contact after the start. Additional weight increased the demand for horizontal impulse direction.

References
Naoki Kawamori, Robert Newton, and Ken Nosaka. 2014. Effects of weighted sled towing on ground reaction force during the acceleration phase of sprint running. Journal of Sports Sciences, Vol. 32, 1139-1145.