Matt R. Cross

Mechanical Properties of Sprinting in Elite Rugby Union and Rugby League

PURPOSE To compare mechanical properties of overground sprint running in elite rugby union and rugby league athletes. METHODS Thirty elite rugby code (15 rugby union and 15 rugby league) athletes participated in this cross-sectional analysis. Radar was used to measure maximal overground sprint performance over 20 or 30 m (forwards and backs, respectively). In addition to time at 2, 5, 10, 20, and 30 m, velocity-time signals were analyzed to derive external horizontal force-velocity relationships with a recently validated method. From this relationship, the maximal theoretical velocity, external relative and absolute horizontal force, horizontal power, and optimal horizontal force for peak power production were determined. RESULTS While differences in maximal velocity were unclear between codes, rugby union backs produced moderately faster split times, with the most substantial differences occurring at 2 and 5 m (ES 0.95 and 0.86, respectively). In addition, rugby union backs produced moderately larger relative horizontal force, optimal force, and peak power capabilities than rugby league backs (ES 0.73-0.77). Rugby union forwards had a higher absolute force (ES 0.77) despite having ~12% more body weight than rugby league forwards. CONCLUSIONS In this elite sample, rugby union athletes typically displayed greater short-distance sprint performance, which may be linked to an ability to generate high levels of horizontal force and power. The acceleration characteristics presented in this study could be a result of the individual movement and positional demands of each code.

Field monitoring of sprinting power–force–velocity profile before, during and after hamstring injury: two case reports

ABSTRACT Very little is currently known about the effects of acute hamstring injury on over-ground sprinting mechanics. The aim of this research was to describe changes in power–force–velocity properties of sprinting in two injury case studies related to hamstring strain management: Case 1: during a repeated sprint task (10 sprints of 40 m) when an injury occurred (5th sprint) in a professional rugby player; and Case 2: prior to (8 days) and after (33 days) an acute hamstring injury in a professional soccer player. A sports radar system was used to measure instantaneous velocity–time data, from which individual mechanical profiles were derived using a recently validated method based on a macroscopic biomechanical model. Variables of interest included: maximum theoretical velocity (V0) and horizontal force (FH0), slope of the force–velocity (F–v) relationship, maximal power, and split times over 5 and 20 m. For Case 1, during the injury sprint (sprint 5), there was a clear change in the F–v profile with a 14% greater value of FH0 (7.6–8.7 N/kg) and a 6% decrease in V0 (10.1 to 9.5 m/s). For Case 2, at return to sport, the F–v profile clearly changed with a 20.5% lower value of FH0 (8.3 vs. 6.6 N/kg) and no change in V0. The results suggest that the capability to produce horizontal force at low speed (FH0) (i.e. first metres of the acceleration phase) is altered both before and after return to sport from a hamstring injury in these two elite athletes with little or no change of maximal velocity capabilities (V0), as evidenced in on-field conditions. Practitioners should consider regularly monitoring horizontal force production during sprint running both from a performance and injury prevention perspective.

EFFECTS OF VEST LOADING ON SPRINT KINETICS AND KINEMATICS

Abstract Cross, MR, Brughelli, ME, and Cronin, JB. Effects of vest loading on sprint kinetics and kinematics. J Strength Cond Res 28(7): 1867–1874, 2014—The effects of vest loading on sprint kinetics and kinematics during the acceleration and maximum velocity phases of sprinting are relatively unknown. A repeated measures analysis of variance with post hoc contrasts was used to determine whether performing 6-second maximal exertion sprints on a nonmotorized force treadmill, under 2 weighted vest loading conditions (9 and 18 kg) and an unloaded baseline condition, affected the sprint mechanics of 13 males from varying sporting backgrounds. Neither vest load promoted significant change in peak vertical ground reaction force (GRF-z) outputs compared with baseline during acceleration, and only 18-kg loading increased GRF-z at the maximum velocity (8.8%; effect size [ES] = 0.70). The mean GRF-z significantly increased with 18-kg loading during acceleration and maximum velocity (11.8–12.4%; ES = 1.17–1.33). Horizontal force output was unaffected, although horizontal power was decreased with the 18-kg vest during maximum velocity (−14.3%; ES = −0.48). Kinematic analysis revealed decreasing velocity (−3.6 to −5.6%; ES = −0.38 to −0.61), decreasing step length (−4.2%; ES = −0.33 to −0.34), increasing contact time (5.9–10.0%; ES = 1.01–1.71), and decreasing flight time (−17.4 to −26.7%; ES = −0.89 to −1.50) with increased loading. As a vertical vector-training stimulus, it seems that vest loading decreases flight time, which in turn reduces GRF-z. Furthermore, it seems that heavier loads than that are traditionally recommended are needed to promote increases in the GRF-z output during maximum velocity sprinting. Finally, vest loading offers little as a horizontal vector-training stimulus and actually compromises horizontal power output.

Optimal Loading for Maximising Power During Sled-resisted Sprinting.

PURPOSE To ascertain whether force-velocity-power relationships could be compiled from a battery of sled-resisted overground sprints and to clarify and compare the optimal loading conditions for maximizing power production for different athlete cohorts. METHODS Recreational mixed-sport athletes (n = 12) and sprinters (n = 15) performed multiple trials of maximal sprints unloaded and towing a selection of sled masses (20-120% body mass [BM]). Velocity data were collected by sports radar, and kinetics at peak velocity were quantified using friction coefficients and aerodynamic drag. Individual force-velocity and power-velocity relationships were generated using linear and quadratic relationships, respectively. Mechanical and optimal loading variables were subsequently calculated and test-retest reliability assessed. RESULTS Individual force-velocity and power-velocity relationships were accurately fitted with regression models (R2 > .977, P < .001) and were reliable (ES = 0.05-0.50, ICC = .73-.97, CV = 1.0-5.4%). The normal loading that maximized peak power was 78% ± 6% and 82% ± 8% of BM, representing a resistance of 3.37 and 3.62 N/kg at 4.19 ± 0.19 and 4.90 ± 0.18 m/s (recreational athletes and sprinters, respectively). Optimal force and normal load did not clearly differentiate between cohorts, although sprinters developed greater maximal power (17.2-26.5%, ES = 0.97-2.13, P < .02) at much greater velocities (16.9%, ES = 3.73, P < .001). CONCLUSIONS Mechanical relationships can be accurately profiled using common sled-training equipment. Notably, the optimal loading conditions determined in this study (69-96% of BM, dependent on friction conditions) represent much greater resistance than current guidelines (~7-20% of BM). This method has potential value in quantifying individualized training parameters for optimized development of horizontal power.

Very-Heavy Sled Training for Improving Horizontal-Force Output in Soccer Players

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

Methods of Power-Force-Velocity Profiling During Sprint Running: A Narrative Review

The ability of the human body to generate maximal power is linked to a host of performance outcomes and sporting success. Power-force-velocity relationships characterize limits of the neuromuscular system to produce power, and their measurement has been a common topic in research for the past century. Unfortunately, the narrative of the available literature is complex, with development occurring across a variety of methods and technology. This review focuses on the different equipment and methods used to determine mechanical characteristics of maximal exertion human sprinting. Stationary cycle ergometers have been the most common mode of assessment to date, followed by specialized treadmills used to profile the mechanical outputs of the limbs during sprint running. The most recent methods use complex multiple-force plate lengths in-ground to create a composite profile of over-ground sprint running kinetics across repeated sprints, and macroscopic inverse dynamic approaches to model mechanical variables during over-ground sprinting from simple time-distance measures during a single sprint. This review outlines these approaches chronologically, with particular emphasis on the computational theory developed and how this has shaped subsequent methodological approaches. Furthermore, training applications are presented, with emphasis on the theory underlying the assessment of optimal loading conditions for power production during resisted sprinting. Future implications for research, based on past and present methodological limitations, are also presented. It is our aim that this review will assist in the understanding of the convoluted literature surrounding mechanical sprint profiling, and consequently improve the implementation of such methods in future research and practice.

RPE and Velocity Relationships for the Back Squat, Bench Press, and Deadlift in Powerlifters.

Abstract Helms, ER, Storey, A, Cross, MR, Browm, SR, Lenetsky, S, Ramsay, H, Dillen, C, and Zourdos, MC. RPE and velocity relationships for the back squat, bench press, and deadlift in powerlifters. J Strength Cond Res 31(2): 292–297, 2017—The purpose of this study was to compare average concentric velocity (ACV) and rating of perceived exertion (RPE) based on repetitions in reserve on the squat, bench press, and deadlift. Fifteen powerlifters (3 women and 12 men, mean age 28.4 ± 8.5 years) worked up to a one repetition maximum (1RM) on each lift. Rating of perceived exertion was recorded on all sets, and the ACV was recorded for all sets performed at 80% of estimated 1RM and higher, up to 1RM. Rating of perceived exertion at 1RM on squat, bench press, and deadlift was 9.6 ± 0.5, 9.7 ± 0.4, and 9.6 ± 0.5, respectively and was not significantly different (p > 0.05). The ACV at 1RM on squat, bench press and deadlift was 0.23 ± 0.05, 0.10 ± 0.04, and 0.14 ± 0.05 m·second−1, respectively. Squat was faster than both bench press and deadlift (p > 0.001), and deadlift was faster than bench press (p = 0.05). Very strong relationships (r = 0.88–0.91) between percentage 1RM and RPE were observed on each lift. The ACV showed strong (r = −0.79 to −0.87) and very strong (r = −0.90 to 92) inverse relationships with RPE and percentage 1RM on each lift, respectively. We conclude that RPE may be a useful tool for prescribing intensity for squat, bench press, and deadlift in powerlifters, in addition to traditional methods such as percentage of 1RM. Despite high correlations between percentage 1RM and ACV, a “velocity load profile” should be developed to prescribe intensity on an individual basis with appropriate accuracy.

Rating of Perceived Exertion as a Method of Volume Autoregulation Within a Periodized Program

Abstract Helms, ER, Cross, MR, Brown, SR, Storey, A, Cronin, J, and Zourdos, MC. Rating of perceived exertion as a method of volume autoregulation within a periodized program. J Strength Cond Res 32(6): 1627–1636, 2018—The purpose of this investigation was to observe how a rating of perceived exertion (RPE)-based autoregulation strategy impacted volume performed by powerlifters. Twelve (26 ± 7 years, n = 9 men, n = 3 women) nationally qualified powerlifters performed the back squat, bench press, and deadlift 3x per week on nonconsecutive days in a session order of hypertrophy, power, and then strength; for 3 weeks. Each session subjects performed an initial top set for a prescribed number of repetitions at a target RPE. A second top set was performed if the RPE score was too low, then subsequent back-off sets at a reduced load were performed for the same number of repetitions. When the prescribed RPE was reached or exceeded, sets stopped; known as an “RPE stop.” The percentage load reduction for back-off sets changed weekly: there were 2, 4, or 6% RPE stop reductions from the top set. The order in which RPE stop weeks were performed was counterbalanced among subjects. Weekly combined relative volume load (squat + bench press + deadlift), expressed as sets x repetitions x percentage 1-repetition maximum was different between weeks (p < 0.001): 2% = 74.6 ± 22.3; 4% = 88.4 ± 23.8; 6% = 114.4 ± 33.4. Combined weekly bench press volume (hypertrophy + power + strength) was significantly higher in accordance with load reduction magnitude (2% > 4% > 6%; p ⩽ 0.05), combined squat volume was greater in 6 vs. 2% (p ⩽ 0.05), and combined deadlift volume was greater in 6 vs. 2% and 4% (p ⩽ 0.05). Therefore, it does seem that volume can be effectively autoregulated using RPE stops as a method to dictate number of sets performed.

Determining friction and effective loading for sled sprinting

ABSTRACT Understanding the impact of friction in sled sprinting allows the quantification of kinetic outputs and the effective loading experienced by the athlete. This study assessed changes in the coefficient of friction (µk) of a sled sprint-training device with changing mass and speed to provide a means of quantifying effective loading for athletes. A common sled equipped with a load cell was towed across an athletics track using a motorised winch under variable sled mass (33.1–99.6 kg) with constant speeds (0.1 and 0.3 m · s−1), and with constant sled mass (55.6 kg) and varying speeds (0.1–6.0 m · s−1). Mean force data were analysed, with five trials performed for each condition to assess the reliability of measures. Variables were determined as reliable (ICC > 0.99, CV < 4.3%), with normal-force/friction-force and speed/coefficient of friction relationships well fitted with linear (R2 = 0.994–0.995) and quadratic regressions (R2 = 0.999), respectively (P < 0.001). The linearity of composite friction values determined at two speeds, and the range in values from the quadratic fit (µk = 0.35–0.47) suggested µk and effective loading were dependent on instantaneous speed on athletics track surfaces. This research provides a proof-of-concept for the assessment of friction characteristics during sled towing, with a practical example of its application in determining effective loading and sled-sprinting kinetics. The results clarify effects of friction during sled sprinting and improve the accuracy of loading applications in practice and transparency of reporting in research.

The Potential for a Targeted Strength-Training Program to Decrease Asymmetry and Increase Performance: A Proof of Concept in Sprinting

The global application of horizontal force (FH) via hip extension is related to improvements in sprint performance (eg, maximal velocity [vmax] and power [Pmax]). Little is known regarding the contribution of individual leg FH and how a difference between the legs (asymmetry) might subsequently affect sprint performance. The authors assessed a single male athlete for pre-post outcomes of a targeted hip-extension training program on FH asymmetry and sprint-performance metrics. An instrumented nonmotorized treadmill was used to obtain individual leg and global sprint kinetics and determine the athletes strong and weak leg, with regard to the ability to produce FH while sprinting. Following a 6-wk control block of testing, a 6-wk targeted training program was added to the athletes strength-training regimen, which aimed to strengthen the weak leg and improve hip-extension function during sprinting. Preintervention to postintervention, the athlete increased FH (standardized effect [ES] = 2.2; +26%) in his weak leg, decreased the FH asymmetry (ES = -0.64; -19%), and increased vmax (ES = 0.67; +2%) and Pmax (ES = 3.2; +15%). This case study highlighted a promising link between a targeted training intervention to decrease asymmetry in FH and subsequent improvement of sprint-performance metrics. These findings also strengthen the theoretical relationship between the contribution of individual leg FH and global FH while sprinting, indicating that reducing asymmetry may decrease injury risk and increase practical performance measures. This case study may stimulate further research investigating targeted training interventions in the field of strength and conditioning and injury prevention.