Team sport players who can accelerate faster tend to have an advantage due to the high frequency of short sprint accelerations (e.g., 5–20 m, 2–3 seconds) during field-based team sports (48). Players rarely cover a large enough distance to reach maximum velocity, with 68% of sprints in rugby (15) and 90% of sprints in soccer (49) being less than 20 m. Acceleration performance can be vital during decisive periods of a game such as breaking a tackle, moving into open space, or accelerating away from or towards an opponent (23,31,39). The ability to accelerate can be effected by an individual's sprinting technique (19,27), force production capability (19,23), and the ability to apply that force in the horizontal direction (24,35,41). The general mechanical ability to produce horizontal external force during sprint running is portrayed by the linear force–velocity (F–v) relationship (11,44). The mechanical capabilities of the lower limbs are characterized by the variables: theoretical maximum velocity (V0), theoretical maximum force (F0), and peak power production (Pmax) (21,33,41). Given that mechanical power is the product of force and velocity, the slope of the linear F–v relationship (21,38) may signify the relative importance of force and velocity qualities in determining the maximal power output and an individual's F–v profile (33).
During sprint running, the initial start (first ground contact) and subsequent acceleration phase may warrant separate investigation. Research is lacking in this area and previous studies have found conflicting results due to differing methodologies (20,23,32). Kawamori et al. (23), for example, found no significant correlation between sprint times and impulses during first ground contact from a standing start in team sport athletes, whereasMero (32) reported a significant correlation between velocity and horizontal propulsive forces (r = 0.62–0.71) and vertical forces (r = 0.41–0.50) from a block start in track sprinters. Acceleration phase performance was significantly correlated with net horizontal (r = −0.52) and propulsive impulse (r = −0.66) at 8 m (23), whereas Hunter et al. (20) reported a significant correlation with sprint velocity at 16 m and net horizontal (r = −0.78), propulsive (r = −0.75), and vertical (r = −0.41) impulse in both track and field athletes and team sport athletes.
Several training methods have been used to optimize the kinematic and kinetic factors relating to enhanced sprint acceleration, including resistance training, plyometric training, and resisted sprinting modalities such as sled pulling or pushing, and vest and limb loading (1,8,10,14,42,46). Specificity of velocity and movement pattern training are fundamental components of an athlete's prescribed exercise program that can affect sporting performance (6,43), therefore, resisted sprint training provides a movement-specific overload to sprinting. Faccioni (14) suggested that resisted sprint running provided sports specificity for neuromuscular adaptation that may enhance velocity during the acceleration phase of sprinting. The authors of a review of resisted sprint running reported that resisted sprint running performance increased velocity, but results were similar to normal sprint training (18). However, the researchers of 2 resisted sprint studies stated that velocity increased in the acceleration phase more so than normal sprint training, whereas for distances >20 m normal sprint training increased velocity more so than resisted sprint running (18).
Previous resisted sprint running studies have used sled pulling (8,50), parachutes (1,40) and wearable resistance (WR) attached to the trunk (1,12), legs (46), foot (30), or ankle (42). A potential change in sprint technique that was induced by a sled or parachute was a greater forward lean of the trunk, as the load applied to the athlete was directed backwards (1,10,40). Wearable resistance (i.e., external loading attached directly to the trunk or limbs) is thought to provide a vertical load that increases braking forces and may overload the stretch-shortening cycle to greater effect (10). The methods for attaching WR to subjects has evolved from boots being filled with a mercury load (47); lead pellets placed in bags being taped to footwear (22,29,30); loaded belts being strapped above or around the ankle joint (7,42); small sandbags being placed in the pockets of a vest (3,8,12) to loads attached with Velcro to compression garments around the lower limbs (46).
Two previous sprint running studies used lower-body WR: loads strapped around the ankle during over ground sprint running (42) and loads attached around the thigh and shank with Velcro to compression garments during nonmotorised treadmill sprint running (46). From these 2 studies, lower-body WR resulted in significantly increased contact time (4.3–4.7% leg load: 5%BM) (46) and significantly decreased stride frequency (−1.9 to 2.4% ankle load: 0.60%BM) (42), acceleration (−1.8 to −3.7% ankle load: 0.6 to 1.8%BM) (42), maximum velocity (−5.9 to −6.3% ankle load: 0.6 to 1.8%BM (42); −5.3% leg load: 5%BM (46)), and step frequency (−3.6% leg load: 5%BM) (46). However, to date, no research has examined lower-limb loading using an anterior or posterior loading during sprint running. When a load is attached to the anterior surface of the lower limbs it may theoretically elicit greater recruitment of the hip flexor musculature resulting in improved front side force production mechanics during sprint acceleration. Previous research has shown that increased hip flexor strength improved sprint performance (9,13), whereas posterior limb loading may enhance hip extensor musculature recruitment during sprint running with strong hip extensors potentially enabling greater horizontal force production. As previously mentioned, increased horizontal force production may enhance sprint running performance (24,35,41), and the retention of a high ratio of horizontal to total force production, a central factor in improving acceleration (35).
Given the treatise of the literature and associated limitations the purpose of this study was to determine the acute changes in kinematics and kinetics when WR equivalent of 3%BM was attached to the anterior or posterior surface of the lower limbs during over-ground short-distance (20 m) maximal sprint running. It was hypothesized that loads of 3%BM would have no effect on the variables of interest and that the comparative effects of anterior and posterior loading would be nonsignificant.
Experimental Approach to the Problem
A cross-sectional design was used to investigate the effects of WR attached to the lower limbs (anterior: quadriceps and tibialis anterior or posterior: hamstring and gastrocnemius) on the kinematics and kinetics of sprint running. Subjects performed maximum effort 20 m sprints with and without WR attached to either the anterior (anterior wearable resistance: AWR) surface or posterior (posterior wearable resistance: PWR) surface of the legs. Wearable resistance sprint results were compared with the unloaded (UL) control condition and AWR was compared with PWR using repeated measures analysis of variance with Bonferroni post hoc comparisons used to determine statistical difference between conditions.
Nineteen male amateur to semi-professional rugby athletes (rugby league, n = 6, rugby union, n = 13) volunteered to participate in the study (age: 19.7 ± 2.3 years; body mass: 96.1 ± 16.5 kg; height: 181 ± 6.5 cm). Subjects were aged between 18 and 22 years. Based on an effect size of 0.25, an alpha level of 0.05, statistical power of 0.80 using a repeated measures within interaction design, a sample size of 18 was determined adequate for this study. All subjects were currently engaged in periodized strength and conditioning programs and had at least 2 years experience in sprint training. The Institutional Ethics Committee of Auckland University of Technology provided approval for this study. Subjects were informed of the protocol and procedures before their involvement, and written consent to participate was obtained.
Testing was performed on an indoor track. Subjects performed a 15 minutes standardized warm-up followed by 6 trials of a 20 m sprint, comprising 2 repetitions under each of the 3 loading conditions: (a) 3%BM AWR; (b) 3%BM PWR; and (c) unloaded (i.e., 0%BM) (UL). The order of the loading conditions was randomized and 3–6 subjects performed the testing protocol in a cycled format to maximize time efficiency and allow appropriate rest. Athletes started from a split-stance position with their preferred lead-foot on the starting line. Two repetitions with each condition were performed before the subject changed to the next condition. Each trial was separated by 4 minutes of passive rest. The average data from the 2 repetitions under each condition were used for analysis.
Kinematic variables were recorded over the initial 15 m of each sprint with an Optojump Next system (Microgate, Bolzano, Italy). Optojump is an optical measurement system consisting of 2 parallel bars containing LEDs. The system detects any interruptions in communication between the bars and calculates the duration to obtain kinematic variables such as step length, step frequency, contact time, and flight time (16). High validity in step parameters was reported during running (intraclass correlation coefficient = 0.96–0.99; mean bias = 0.4–2.7%) (17).
The following section outlines the variables of interest in this study and the method of calculation. Vertical stiffness (kvert) was calculated based on the spring mass model paradigm (5,34) as follows:where Fmax = maximal ground reaction force during contact (in kilo Newtons); ∆y = the vertical displacement of the center of mass (in meters).
The modeled maximal ground reaction force and the total vertical displacement of the center of mass were calculated from:where m = subject's body mass (in kilogram); g = acceleration due to gravity (in meter per squared second); FT = flight time (in seconds); CT = contact time (in seconds).
Instantaneous horizontal velocity data were collected with a radar device (Stalker ATS II; Applied Concepts, Dallas, TX, USA). The device was positioned directly behind the starting point and at a vertical height of 1 m to approximately align with the subject's center of mass; data were collected at a sampling rate 47 Hz. All data were collected using STATS software (Model: Stalker ATS II Version 18.104.22.168; Applied Concepts) supplied by the radar device manufacturer. A custom made LabVIEW program (Version 13.0; National Instruments Corp., Austin, TX, USA) was developed to calculate the variables based on the raw horizontal velocity data: V0; F0; Pmax; and sprint split times (2, 5, 10, and 20 m). A high level of reliability (coefficient of variation: 1.11–2.93% and standard error of measurement: 1.40–3.57%), for both intraindividual and interindividual comparisons, was found for the variables during the ground-sprint running (44). The methods of obtaining these variables have been validated in previous research during maximal sprint running (21,33,36,44).
During sprint running acceleration (a), velocity (v)–time (t) curve has been shown to follow a monoexponential function:where vmax = the maximal velocity reached; t= the acceleration time constant.
The horizontal acceleration of the center of mass can be expressed as a function of time, after derivation of velocity over time:
Net horizontal force (Fh) was then modelled over time:where Fair = the aerodynamic friction force to overcome during sprint running computed from sprint velocity and an estimated body frontal area and drag coefficient (2).
Wearable Resistance Loading
Subjects wore Lila Exogen compression shorts and calf sleeves (Sportboleh Sdh Bhd, Kuala Lumpur, Malaysia) for the duration of the testing session (Figure 1). The Exogen exoskeleton suit enabled fusiform-shaped loads (with Velcro backing) of 50–300 g to be attached in numerous configurations. Wearable resistance of 3%BM was attached to either the anterior or posterior surface of the legs, with 2/3 of the load placed evenly around the thigh and the remaining 1/3 on the shank of the leg. Previous studies using loads attached to the lower limbs (foot, ankle, leg) have used added loads between 0.34 and 5.0%BM (30,42,46), and the loads chosen for this study align with such loading parameters.
Standard descriptive statistics (means and standard deviations) were reported for all statistical comparisons. Normal distribution of the data was checked using the Shapiro-Wilk statistic. Kinematic analysis was split into 2 phases: the average of the first 2 steps was used to represent the start phase; and the subsequent 6 steps were averaged (i.e., steps 3–8) representing the acceleration phase (26). Statistical differences in kinematic and kinetic variables across loaded and unloaded conditions were determined using repeated measures ANOVA with Bonferroni post hoc comparisons. Statistical significance was set at an alpha level of p ≤ 0.05.
No statistical differences were found between AWR and PWR in any variables of interest, therefore the ensuing discussion will focus on the WR conditions (AWR and PWR) compared with UL sprint running. There were no significant differences in sprint split times from the start to the 2, 5, 10 and 20 m marks between WR sprint running compared with the UL condition (Table 1). There was, however, a significant increase in the 10–20 m split time for both AWR and PWR (2.2 and 2.9%, respectively) compared with UL. Sprint running with 3%BM WR also resulted in a significant decrease in estimated V0 with AWR (−5.4%) and PWR (−6.5%) compared with the UL condition.
In terms of the start phase, step length and step frequency were not significantly different between the WR and UL conditions (Table 2). Similarly, flight time was not different (p ≥ 0.05) between conditions. However, the contact time was greater (3.4 and 4.4%, AWR and PWR, respectively, p ≤ 0.05) and vertical stiffness was decreased (−6.2 and −12%, AWR and PWR, respectively) compared with the UL condition (p ≤ 0.05).
No significant changes were found during the acceleration phase in step length with WR condition compared with the UL condition, however, step frequency was significantly decreased (−3.4 and −3.6%, AWR and PWR, respectively) (Table 3). Flight time was nonsignificantly different between WR conditions. Contact time was greater (3.0% both AWR and PWR, p ≤ 0.05), whereas vertical stiffness decreased with PWR (−7.7%) during the acceleration phase compared with UL sprint running (p ≤ 0.05).
No significant differences were found in F0 or Pmax (absolute and relative) compared with UL sprint running (Table 4). However, WR resulted in differences (−10.9 to −10.5%, AWR and PWR, respectively, p ≤ 0.05) in the absolute and relative slopes of the F–v profile compared with the UL condition. The change to the slope of the F–v profile resulted in a more force dominant F–v profile.
This is the first study to compare the kinematics and kinetics of sprint running with lower body WR of 3%BM to an UL condition. The reader needs to be cognizant that there is a paucity of research in this area, particularly differentiating the start and acceleration phases, therefore, the integration of research findings from other studies within this discussion is problematic. The main findings were that no significant changes in sprint times over the initial 10 m occurred. However, there was a significantly slower split time between 10 and 20 m (∼ −2 to −3%) and a significantly lower theoretical maximum velocity achieved (∼ −5 to −6%). There were no differences (p ≥ 0.05) in kinematics or kinetics when the WR was positioned on the anterior compared with the posterior aspect of the legs. These findings and implications to the practitioner are discussed in more detail in the remainder of the discussion.
It would seem that the WR loading used in this study had little impact on step length (no change to 0.9%, AWR and PWR, respectively) and step frequency (−1.3% both conditions) during the start phase but did significantly affect contact time (3.4–4.4%). Given that step frequency is determined by flight and contact time, the increased contact times should have reduced step frequency, which was the case; however, the change was nonsignificant and most likely explained by the influence of flight time i.e., nonsignificant changes. As a consequence of the longer contact times and the importance of this kinematic variable in calculating stiffness, vertical stiffness was reduced (p ≤ 0.05). The additional loading most likely increases flexion at the knees or ankles.
Horizontal force production is important to the start and acceleration phases of sprint running (24,35,41). During the start phase it was observed that F0 and Pmax were statistically unaffected (relative F0: 4.9–5.3%, relative Pmax: no change to −1.4%, p ≥ 0.05) by the additional loading. Whether the same is true for force and power in the vertical direction is unknown given the methodological procedures used in this study. In summary, it would seem that the additional loading had little effect on start kinematics and kinetics particularly in the horizontal plane. For those practitioners who are concerned that loading an athlete may negatively affect technique factors, these findings with a 3%BM load should allay most concerns and such loading can take place with very little technique breakdown. For those practitioners who wish to significantly overload start kinematics and kinetics, it is suggested that heavier relative loading is most likely required. However, it needs to be noted that the 3%BM loading did provide a means to overload contact time and vertical stiffness. Therefore, care is needed, as it would seem that differential relative loading is needed to overload certain kinematic and kinetic determinants of the sprint start.
With regards to the acceleration phase, similar changes in the kinematics and kinetics were observed with the exception of a significant decrease in step frequency. It would seem with the additional steps of the acceleration phase, the influence of increased contact times was greater and hence the significant decrease in step frequency (−2.6 and −2.7%, AWR and PWR, respectively, p ≤ 0.05). Similar findings were reported by Ropret et al. (42) with 0.6%BM (attached to the foot), which resulted in reduced stride frequency (−1.9%, p ≤ 0.05) but no significant change to stride length. The significant increase in contact time during the acceleration phase (3% both AWR and PWR) is comparable with the findings of Simperingham and Cronin (46) who reported a 4.3% increase in acceleration phase contact time with 5%BM lower body loading attached to the legs during treadmill sprint running. Previous researchers have proposed that the greater the external load, the greater the alteration to sprint running kinematics. Therefore, some practitioners would posit that the optimal load for resisted sprint running should provide a suitable overload stimulus for adaption without negatively affecting sprint running technique (1,25). The findings from this study for the most part support this school of thought, minimal (i.e., less than 5%) alterations in acceleration kinematics being observed during sprint running with WR.
The relationship between kinetics and WR resulted in a significant change in the relative F–v profile (∼ −10 to −11%), but no significant changes were found in relative F0 or Pmax production. Although no significant changes were found in F0, WR sprint running did increase F0 production compared with UL sprint running with increases of 5.2% (AWR) and 4.9% (PWR) found. This increase in F0 is of interest, as previous researchers have suggested that the amount of force an athlete can produce is a key component in acceleration phase performance (10,28). In a study on soccer players, Buchheit et al. (4) found that the amount of horizontal force produced was of a beneficial impact during the acceleration phase but became less important during the maximum velocity phase, whereas Cross et al. (11) found that sprint performance in rugby players (0–30 m) seemed to be related to a more force dominant F–v profile. The increase in F0 found in this study may have resulted from the WR requiring the athlete to produce more horizontal force to overcome the external loading. Such findings are supported by the acute kinematic changes detailed earlier in this study. Therefore, it seems WR enhances an athlete's ability for horizontal force production in sprint running which may have partly contributed to maintaining split times over 0–10 m comparable with UL split times.
Morin and Samozino (37) have suggested that during the acceleration phase (<20 m), a greater relationship between sprint running performance and theoretical maximal horizontal force production exists. To determine this relationship in this study, the simple modelling of the derivation of the speed–time curve that lead to horizontal acceleration data was undertaken (37). Samozino et al. (45) proposed that by assessing the F–v profile, an individual's area of relative dominance can be identified (i.e., force or velocity dominant) which subsequently may be of importance for prescribing training loads, exercises, and schedules. With this in mind the addition of WR in this study resulted in a more force dominant F–v profile, this information is useful to the practitioner e.g., a velocity-dominant athlete may benefit from additional force training.
The significant change in the relative slope of the F–v profile, combined with the minimal change in sprint running kinematics, may indicate that lower body WR with 3%BM could be an effective training tool for improving sprint acceleration performance. This study provides descriptive stride variable information; however, it is limited in that it does not provide information on how WR may change joint angles during sprint running. Moreover, possible changes in sprint running technique that may occur as a result of the WR were beyond the methodological approach undertaken in this acute study and would need to be addressed through acute 3-dimensional motion capture and longitudinal research. Additional loading configurations (e.g., proximal compared with distal, or internal compared with external oblique loading) made possible by recent advances in WR technology could also be a focus of future research.
Lower body WR with 3%BM seems to reinforce ideal early sprint acceleration with speed maintained during the initial acceleration phase (0–10 m). Sprint running technique seems to remain comparatively unchanged by WR of this magnitude, with most kinematic variables found to be minimally altered (less than 5%). For practitioners who wish to significantly overload the start, it is proposed that heavier relative loading is needed. However, 3%BM loading did provide a means to overload contact time and vertical stiffness, therefore it would seem that differential relative loading is needed to overload certain kinematic and kinetic determinants of the sprint start. The acceleration phase of sprint running is typified by a longer stance phase resulting in greater propulsion and horizontal force production compared with the maximum velocity phase. Wearable resistance provided an overload during the stance phase resulting in the athlete having to produce a greater amount of force to overcome the additional loading. For athletes requiring a more force dominant F–v profile, and for relatively velocity dominant athletes, sprint running with WR may enable the athlete to improve their external horizontal force production. Lower body WR may be a suitable training tool for movement specific overload and maximizing transference to improved sprint running performance. Consideration should be given to the inclusion of WR in sports where sprint running is an important component, as it may provide a novel training stimulus resulting in positive adaptations. It is suggested that it is used as an adjunct training tool to heavy resistance training by promoting intermuscular co-ordination through the strategic placement of light variable resistance.
The authors wish to thank the group of subjects for their participation and to all those involved in assisting with the data collection process in this study. Kim Simperingham has received funding from Sportboleh Sdh Bhd.
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