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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.

Age-Related Differences in Spatiotemporal Variables and Ground Reaction Forces During Sprinting in Boys: The age of the athlete (pre-pubescent and adolescent growth spurt periods) is a significant factor in sprint performance. The study described below by Nagahara and colleagues (2018) analyzed these and other periods of development in terms of ground reaction force application, stride rate, and sprinting speed.

Purpose: Researchers aimed to elucidate age-related differences in spatiotemporal and ground reaction force variables during sprinting in boys over a broad range of chronological ages.

Methods: Ground reaction force signals during 50-m sprinting were recorded in 99 boys aged 6.5–15.4 years. Step-to-step spatiotemporal variables and mean forces were then calculated.

Results: There was a slower rate of development in sprinting performance in the age span from 8.8 to 12.1 years compared with younger and older boys. During that age span, mean propulsive force was almost constant, and step frequency for older boys was lower regardless of sprinting phase. During the ages younger than 8.8 years and older than 12.1 years, sprint performance rapidly increased with increasing mean propulsive forces during the middle acceleration and maximal speed phases and during the initial acceleration phase.

Conclusion: There was a stage of temporal slower development of sprinting ability from age 8.8 to 12.1 years, being characterized by unchanged propulsive force and decreased step frequency. Moreover, increasing propulsive forces during the middle acceleration and maximal speed phases and during the initial acceleration phase are probably responsible for the rapid development of sprinting ability before and after the period of temporal slower development of sprinting ability.

Coaching Application: Prior to the adolescent growth spurt and the rapid increase in individual height and weight during puberty from the simultaneous release of growth hormones, thyroid hormones, and androgens resulting in additional strength and power, increases in ground reaction force (GRF) and stride rate may be absent, minimal or actually decline. During and after the growth spurt, GRF increases result in rapid improvement in sprint performances during the start, acceleration, and maximum speed phases of a short sprint. It is during this phase of development that strength and power training programs are extremely effective. Although modified strength training programs are encouraged, the pre-pubescent years are ideal for mastering proper sprinting form and technique.

References: Nagahara, Ryu/, Takai, Yohei, Haramura, Miki, Mizutani, Miral, Matsuo, Akifumi, Kanehisa, Hiroaki, and Fukunaga, Tesuo, 2018. Age-Related Differences in Spatiotemporal Variables and Ground Reaction Forces During Sprinting in Boys, Pediatric Exercise Science.

Kinetic Determinants of Reactive Strength in Highly Trained Sprint Athletes: Reactive strength involves the ability of an athlete to rapidly and efficiently change from an eccentric to a concentric contraction. For a sprinter, this involves the explosive action of the stretch-shortening cycle as GRF (ground reaction force) is applied each stride, during the pushing action away from the ground, to rapidly propel the body upward and forward. In team sports, movements that require a change of direction and the ability to rapidly move through the stretch shortening cycle are common throughout a competitive event.

The purpose of this study by Douglas, et. al. (2017) was to determine the braking and propulsive phase kinetic variables underpinning reactive strength in highly trained sprint athletes in comparison to a non-sprint trained control group. Twelve highly trained sprint athletes and twelve non-sprint trained participants performed drop jumps (DJs) from 0.25m, 0.50m and 0.75m onto a force plate. One familiarization session was followed by an experimental testing session within the same week. Reactive strength index (RSI), contact time, flight time, and leg stiffness were determined. Kinetic variables including force, power and impulse were assessed within the braking and propulsive phases. Sprint trained athletes demonstrated higher RSI versus non-sprint trained participants across all drop heights (3.02 vs 2.02; ES [±90% CL]: 3.11 ±0.86). This difference was primarily attained by briefer contact times (0.16 vs 0.22 s; ES: -1.49 ±0.53) with smaller differences observed for flight time (0.50 vs 0.46 s; ES: 0.53 ±0.58). Leg stiffness, braking and propulsive phase force and power were higher in sprint trained athletes. Very large differences were observed in mean braking force (51 vs 38 Nkg; ES: 2.57 ±0.73) which was closely associated with contact time (r ±90% CL: -0.93 ±0.05). Sprint trained athletes exhibited superior reactive strength than non-sprint trained participants. This was due to the ability to strike the ground with a stiffer leg spring, an enhanced expression of braking force, and possibly an increased utilization of elastic structures. The DJ kinetic analysis provides additional insight into the determinants of reactive strength which may inform subsequent testing and training.

Coaching Application: The findings of this current research and other studies indicate that reactive strength is an important quality for acceleration, agility, and change of direction speed.

Reference: Douglas, J, Pearson, S., Ross, a, and McGuigan, M. 2017. The Kinetic Determinants of Reactive Strength in Highly Trained Sprint Athletes. J Strength Cond Res. Sept. 11.

A New Direction to Athletic Performance: Understanding the Acute and Longitudinal Responses to Backward Running

Backward running is an activity seldom used in the training of athletes in power sports, yet this movement pattern is common for linebackers and defensive backs in football and all players in basketball, soccer, lacrosse, field hockey, tennis, and some other sports. The abstract below describes a study by Oliver, et. al (2018) that examines the value of backward sprint training.

Backward running (BR) is a form of locomotion that occurs in short bursts during many overground field and court sports. It has also traditionally been used in clinical settings as a method to rehabilitate lower body injuries. Comparisons between BR and forward running (FR) have led to the discovery that both may be generated by the same neural circuitry. Comparisons of the acute responses to FR reveal that BR is characterized by a smaller ratio of braking to propulsive forces, increased step frequency, decreased step length, increased muscle activity and reliance on isometric and concentric muscle actions. These biomechanical differences have been critical in informing recent scientific explorations which have discovered that BR can be used as a method for reducing injury and improving a variety of physical attributes deemed advantageous to sports performance. This includes improved lower body strength and power, decreased injury prevalence and improvements in change of direction performance following BR training. The current findings from research help improve our understanding of BR biomechanics and provide evidence which supports BR as a useful method to improve athlete performance. However, further acute and longitudinal research is needed to better understand the utility of BR in athletic performance programs.

Coaching Application\: In the NASE 5-Step Model, both sprint-assisted and sprint-resisted training (weighted suits, vests, or pants) includes backward sprinting that both underload and overload the lower extremities during high speed work. The drills attempt to mimic the specific movements that occur during competition in the sport. For example, strength coach, Bob Otrando, recommends that defensive backs in football end each workout with repetitions of backward sprinting to improve back pedaling skills. Training loads should be kept light enough to allow athletes to reach a high backward sprinting speed. Backward sprinting repetitions with added weight are also an important aspect of improving the speed-strength of the hamstring muscle group.

Reference:
Jon Oliver, John Cronin, Craig Harrison, and Paul Winwood. , Craig, Cronin. 2018. A New Direction to Athletic Performance: Understanding the Acute and Longitudinal Responses to Backward Running. Sports Medicine May, Volume 48, Issue 5, pp 1083–1096.

Both resisted and assisted training programs are key parts of the NASE 5-Step Training Model for the speed improvement of athletes in power sports. The study by Wibowo, on the Impact of Assisted Sprinting training (AS) and Resisted Sprinting Training (RS) in Repetition Method on Improving Sprint Acceleration compared both methods for their effectiveness in improving early and late acceleration.

Abstract

The purpose of this research was to determine the impact of assisted sprinting training (AS) and resisted sprinting training (RS) in repetition method on improving sprint acceleration capabilities. This research used an experimental method in the pre-test and post-test design. The research sample were twelve male collegiates track sprinters, athletic division Indonesia University of Education, Bandung. Six male collegiates track sprinters for AS and six male collegiates track sprinters for RS. It used simple random sampling. The instrument used was 30 m sprint test. After training three times per week for six week, data were obtained from pre-test and post-test processed statistically by t-test. The AS group and RS group showed significant changes on improving sprint acceleration capabilities. No significant different between AS and RS on improving sprint acceleration capabilities. In AS the increase was better than RS at a distance of 10 m from a distance of 30 m. While, in RS the increase was better than AS at a distance of 10-20 m and 20-30 m from a distance of 30 m. Accordingly, to improve acceleration at a distance 10 m use AS, while to improve acceleration at a distance of 10-20 m and 20-30 m from a distance of 30 m use RS.

Coaching Application: Although both AS and RS training improved acceleration, AS was slightly more effective during early and RS during late acceleration. The use of “contrast training” combines both AS and RS to alter motor patterns by using programs that impose demands easier (sprint-assisted training) and harder (sprint-resisted training) than the normal sprinting action during the same workout session. This approach may trick the neuromuscular system into performing at a higher level by making the task of sprinting more difficult or a bit easier than normal.

For both approaches (harder and easier), the resistance load is performed first, following by assisted training that makes sprinting an easier task. The heavy load is thought to excite the nervous system and allow for greater recruitment of motor neurons (post-activation potentiation) in the set that follows. Resisted sprints immediately follow the general warm-up and dynamic warm-up sessions. Three repetitions of maximum resisted sprints are performed, using a 2-5 minute recovery period between each. The contrast training session ends with one set of three repetitions over the same distance with no resistance or assistance. It is also acceptable to complete one resisted sprint, followed by one assisted sprint, and ending with one normal sprint. Another formula for contrast training is to complete 2-3 sets of one resisted effort, and finally a normal sprint.

Reference

Wibowo, Ricky. 2017 The Impact of Assisted Sprinting Training (AS) and Resisted Sprinting Training (RS) in Repetition Method on Improving Sprint Acceleration Capabilities. Jurnal Pendidikan Jasmani dan Olahra ga Volume 9 Nomor 1.

Hang clean and hang snatches produce similar improvements in female collegiate athletes: The study by Ayers and colleagues described in the Abstract below focused on hang clean and hang snatches to determine the training effects on the power, strength, and speed of female collegiate athletes.

Olympic weightlifting movements and their variations are believed to be among the most effective ways to improve power, strength, and speed in athletes. This study investigated the effects of two Olympic weightlifting variations (hang cleans and hang snatches), on power (vertical jump height), strength (1RM back squat), and speed (40-yard sprint) in female collegiate athletes. Twenty-three NCAA Division I female athletes were randomly assigned to either a hang clean group or hang snatch group. Athletes participated in two workout sessions a week for six weeks, performing either hang cleans or hang snatches for five sets of three repetitions with a load of 80-85% 1RM, concurrent with their existing, season-specific, resistance training program. Vertical jump height, 1RM back squat, and 40-yard sprint all had a significant, positive improvement from pre-training to post-training in both groups (p≤0.01). However, when comparing the gain scores between groups, there was no significant difference between the hang clean and hang snatch groups for any of the three dependent variables (i.e., vertical jump height, p=0.46; 1RM back squat, p=0.20; and 40-yard sprint, p=0.46). Short-term training emphasizing hang cleans or hang snatches produced similar improvements in power, strength, and speed in female collegiate athletes. This provides strength and conditioning professionals with two viable programmatic options in athletic-based exercises to improve power, strength, and speed.

Coaching Application: To improve sprinting speed, strength training exercise choices should be selected that train the movement patterns involved in sprinting, rather than the involved muscle groups. These exercises should mimic the movements that produce hip extension and involve multi-joint rather than single movements. Squats, dead lifts, lunges, step-ups and numerous variations of these weight room exercises, the Olympic Lifts, and plyometrics (hopping, jumping, and bounding) are recommended for a complete strength training program designed to increase overall strength, core strength, ground reaction force, and mass specific force (ratio of body weight/ground reaction force).

Reference

JL Ayers, M DeBeliso, TG Sevene, and KJ Adams. 2018. Hang cleans and hang snatches produce similar improvements in female collegiate athletes. Biol Sport. September: 33(3): 252-56

Predictors of Sprint Performance in Professional Rugby Players: Relative strength and power are key factors that affect the speed of athletes during the start, acceleration, and maximum speed phase of a short sprint. The Abstract of a study by Cunningham and colleagues, described below, reinforces this concept.

The ability to accelerate and attain high speed is an essential component of success in team sports; however, the physical qualities that underpin these activities remain unclear. This study aimed to determine some of the key strength and power predictors of speed with professional rugby players.

Methods: Twenty professional players were tested for speed (0-10-meter sprint and a flying 10-meter sprint), strength (3 repetitions maximum squat), lower body power countermovement jumps (CMJ, and drop jumps (DJ), reactive strength and leg spring stiffness. The strength and power variables were expressed as absolute values and relative values for analysis.

Results: Both relative strength (r=.55, P<0.05) and relative power (-.082, P<0.01) were negatively correlated with 10-meter time. Leg spring stiffness and DJ contact time were also related to the flying 10-meter time (r=.046 and 0.47 respectively, P(<0.05) while relative strength index was negatively related to both the 10-meter and flying 10-meter Tims (r=0.60 and r=0.62, P<0.05). Acceleration was significantly related to relative strength, relative power and jump height from a 40 cm DJ. Maximum velocity sprinting was significantly related to relative power, contact time, height and leg stiffness. The study provides an insight into those physical attributes that underpin sprinting performance in professional rugby union players and specifically highlights the importance of relative strength and power in the expression and development of different speed components (e.g. acceleration, maximum velocity). Coaching Application: Findings on “relative strength” reinforce the importance of acquiring a favorable ratio of Ground Reaction Force/Body Weight. Acceleration and maximum speed improve when GRF increases and/or body weight decreases. Team sport athletes should strive to become as strong as possible with minimum body fat. In the above study, absolute strength was not related to 10-meter or flying 10-meter speed. When 1RM strength was expressed relative to body mass, significant relationships wwere identified with jump height and 10-meter speed. In addition, acceleration and maximum speed are separate entities and require different training approaches to improve.

Reference

Cunningham, D.J., West, D.J., Owen, N.J., Shearer, D.A., Finn, C.V., Bracken, R.M., Crewther, B.T., Scott, P., Cook, C.J., and Kilduff, L.P. 2013. Strength and Power Predictors of Sprinting Performance in Professional Rugby Players. The Journal of Sports Medicine and Physical Fitness. 53: 1-2

The NFL Combine and Performance on the Football Field: Do high scores on the NFL Combine physical tests predict success in the NFL? Several studies have been conducted to answer these and other questions. The investigation by Kuzmits and colleagues described in the abstract below is perhaps one of the most negative.
Abstract

Kuzmits and Adams investigated the correlation between National Football League (NFL) combine test results and NFL success for players drafted at three different offensive positions (quarterback, running back, and wide receiver) during a 6-year period, 1999-2004. The combine consisted of series of drills, exercises, interviews, aptitude tests, and physical exams designed to assess the skills of promising college football players and to predict their performance in the NFL. Combine measures examined in this study included 10-, 20-, and 40-yard dashes, bench press, vertical jump, broad jump, 20- and 60-yard shuttles, three-cone drill, and the Wonderlic Personnel Test. Performance criteria include 10 variables: draft order; 3 years each of salary received and games played; and position-specific data. Using correlation analysis, we find no consistent statistical relationship between combine tests and professional football performance, with the notable exception of sprint tests for running backs. We put forth possible explanations for the general lack of statistical relations detected, and, consequently, we question the overall usefulness of the combine. We also offer suggestions for improving the prediction of success in the NFL, primarily the use of more rigorous psychological tests and the examination of collegiate performance as a job sample test. Finally, from a practical standpoint, the results of the study should encourage NFL team personnel to reevaluate the usefulness of the combine’s physical tests and exercises as predictors of player performance. This study should encourage team personnel to consider the weighting and importance of various combine measures and the potential benefits of overhauling the combine process, with the goal of creating a more valid system for predicting player success.

Another study by Sierer, et. al. compared the NFL Combine performance differences between drafted and non-drafted players. Findings were slightly more positive. Although drafted athletes were found to perform better than non-drafted athletes, the success of each athlete in the NFL was not used as a criterion measure and predictive validity was not established. Boulier and Stekler used a data base from NFL drafts between 1974 and 2005 and a range of measures to determine the success of players selected in the draft. The study examined the success of drafting quarterbacks and wide receivers and also found combine test scores to be only slightly helpful in predicting NFL success at these positions.

It is understandable why it is so difficult to statistically determine success from field tests since football is skill specific and physical tests cannot mimic the many key situations for each player position. Clearly, there is room for improvement even in the area of speed tests where researchers found some predictive value. Football is a game of quickness, starting, stopping, and acceleration. An analysis of game play would reveal that it is a rare occasion when players in most positions sprint a distance of 40 yards. The First 3-step test and the 10-yard dash are much more sport specific for interior linemen (blocking, pass and run rushing) than the 40-yard dash. If the 40-yard dash is deemed necessary, the test should be changed to include split times at 5, 10, and 20 yards. Also, a plant, cut, and 10-yard acceleration test is also more football-appropriate for linebackers, defensive backs, and linemen.

Although the 2018 NFL Combine was more comprehensive and controlled than during the period the three studies above were conducted, it is time for further investigation to determine the predictive value of individual tests and combined scores on game performance in this modern era.

References

Boulier, Bryan Leslie and H.O. Stekler. 2010. Evaluating National Football League Draft Choices: The Passing Game. International Journal of Forecasting Vol. 26, Issue 3, July 589-605

Kuzmits, F.E., and A.J. Adams. 2008. The NFL Combine: Does it Predict Performance in the National Football League. J Strength Cond Res. Nov;22(6):1721-7.)

Sierer, S.P., Battaglini, C.L., Mihalik, J.P., Shields, E.W., and NT Tomasini. 2006. The National Football League Combine: performance differences between drafted and non-drafted players entering the 2004 and 2005 drafts. Journal of Strength & Conditioning Research: January 2008 – Volume 22 – Issue 1 – pp 6-12.

Effectiveness of a wireless Sensor Insole in measuring vertically directed ground reaction force (GRF) during the sprinting action: The availability of a device to accurately measure the amount and direction of force applied to the ground with each step during the sprinting action would be major breakthrough. The specific training programs and exercises that increase GRF, decrease ground contact time (GCT), and improve speed in short sprints could then be accurately identified. The technology to develop these sensors is available and researchers are now experimenting with various devices and protocols. An abstract of a study by Nagahara and Morin (2018), who tested one type of shoe insole sensor, follows.

Temporal variables and vertical ground reaction force have been used as measures characterizing sprinting. A recently developed wireless pressure sensor insole (sensor insole) could be useful for monitoring sprinting in terms of temporal variables and vertical ground reaction force during training sessions. The purpose of this study was to examine the concurrent validity of the sensor insole for measuring temporal and vertical force variables during sprinting. One athlete performed five 50-m sprints, and the step-to-step vertical ground reaction force and plantar pressure were simultaneously measured by a long-force platform system (reference device) and the sensor insole, respectively. The temporal and vertical ground reaction force variables were calculated using signals from both devices, and a comparison was made between values obtained with both devices for 125 steps analyzed. The percentage bias, 95% limits of agreement, and Bland–Altman plots showed low agreement with the reference device for all variables except for step frequency. For the vertical ground reaction force variables, the sensor insole underestimated the values (−18.9 to −48.3%) compared to the force platform. While support time and time to maximal vertical force from the foot strike were overestimated by the sensor insole (54.6 ± 8.0% and 94.2 ± 23.2%), flight time was underestimated (−48.2 ± 15.0%). Moreover, t-test revealed the significant difference in all variables between the sensor insole and force platform, except for step frequency. The bias for step frequency (0.4 ± 7.5%) was small. However, there was heteroscedasticity for all variables. The results from this study demonstrate that a wireless pressure sensor insole is generally not valid to measure the temporal and vertical force variables during sprinting. Thus, using the examined sensor insole for monitoring sprinting characteristics is not recommended at this time.

Coaching Application. Although findings of the above study indicate that a wireless shoe sensor did not accurately measure temporal and vertical ground reaction forces, it is a first step toward the development of a low cost device that is as accurate or more accurate than the costly force plate technology currently available. Developing a low cost, accurate device is a challenge, however, the tremendous importance of finding ways to increase GRF and decrease GCT during the start, acceleration, and maximum speed phase of a short sprint, encourages researchers to continue their pursuit in this area of interest.

Reference: Ryu Nagahara and Jean-Benoit Morin. 2018. Sensor insole for measuring temporal variables and vertical force during sprinting. Proceedings of the Institution of Mechanical Engineers. Part P: Journal of Sports Engineering and Technology. First Published January 19, 2018.

Meta-analysis techniques have the advantage of combining the results from multiple studies to increase power, resolve uncertainty when studies disagree, and improve the accuracy of findings. Rumpf and colleagues (2016) used this technique to analyze the effects of various training programs on sprint performance. Findings reveal important information for team and individual sport coaches and athletes. An abstract of this study is provided below.

Linear sprinting speed is an essential physical quality for many athletes. There are a number of different training modalities that can be used to improve sprint performance. Strength and conditioning coaches must select the most appropriate modalities for their athletes, taking into consideration the sprint distances that typically occur during competition. The study purpose was to perform a brief review as to the effect of specific (free sprinting; resisted sprinting by sleds, bands, or incline running; assisted sprinting with a towing device or a downhill slope), nonspecific (resistance and plyometric training), and combined (a combination of specific and nonspecific) training methods on different sprint distances (0–10, 0–20, 0–30, and 31+ m). A total of 48 studies fulfilled the inclusion criteria, resulting in 1,485 subjects from a range of athletic backgrounds. The training effects associated with specific sprint training were classified as moderate (effect size [ES] = −1.00; %change = −3.23). Generally, the effect of specific sprint training tended to decrease with distance, although the largest training effects were observed for the 31+ m distance. The greatest training effects (ES = −0.43; %change = −1.65) of nonspecific training were observed for the 31+ m distance. The combined training revealed greatest effects (ES = −0.59; %change = −2.81) for the 0–10 m distance. After this review, specific sprint training methods seem the most beneficial over the investigated distances. However, the implementation of nonspecific training methods (e.g., strength and power training) could also benefit speed and athletic performance.

*Coaching Application: This analysis dealt primarily with the start and acceleration phase of a short sprint. Findings support the use of specific training (free sprinting: resisted and assisted), non-specific training (resistance and plyometrics), and a combination of each. Combined training was shown to be most effective for the 0-10 meter distance whereas non-specific training was more effective for distances in excess of 31 meters. Although specific training methods appeared to be more beneficial, the authors made it clear that nonspecific training (strength and power) also are important. This study did not examine the effects of each training program on ground reaction force (GRF) or rate of force development such as ground contact time (GCT) which remain as key aspects of all sprint training programs.

Reference
Rumph, Michael C., Lockie, Robert G.,Cronin, John B., Jalivand, Farzad. 2016. Effect of Different Sprint Training Methods on Sprint Performance Over Various Distances: A Brief Review. Journal of Strength and Conditioning Research, Vol. 30, Number 6, June, 1767-1785.