An athlete's ability to accelerate and attain maximal velocity is reliant on the relationship between the force production capability of the body and the ability of the athlete to harness and effectively use that force in time periods relative to the activity (7). Furthermore, it has been proposed that the direction of force application plays a larger role in increasing the acceleration than the amount of force applied (24) and that greater accelerations and maximal velocities have been achieved through lower but more forward-oriented forces (18,23,24). Retaining a high ratio of horizontal to total force production seems to be key to enhance acceleration and maximum velocity (24). Understanding which training techniques may present such mechanical overload would seem important to optimize the development of these speed qualities.
Resisted sprint techniques, such as sled or vest weighted sprinting, offer a specific approach to overloading during common sport movements. (1,5,11,21,22). Potentially, these techniques could elicit positive effects on sprint kinematics (i.e., contact time, flight time, step length, and step frequency) by increasing the athlete's ability to generate vertical and horizontal forces. Although studies examining the acute and longitudinal effects of sled towing on sprint performance are increasingly common, there are few studies that examine the effects of vest loading on similar performance characteristics (6,8). Notably, it has been suggested that sled towing and vest loading may overload the neuromuscular system in a different manner. For example, sled towing, particularly at higher percentages of body mass (BM) (16), has been proposed to offer a greater horizontal vector-training stimulus whereas vest sprinting is more vertically oriented (7). Understanding the mechanical adaptations associated with vest loading in greater detail is of interest to these researchers.
It has been suggested that the heavier the external loading, the greater disruption to normal sprint kinematics—therefore, in best practice, the optimal loading for sprinting would be one that minimally affects running mechanics, while supplying an appropriate overload stimulus to promote adaption (2,22). To date, only 3 recent articles have examined the use of weighted vests on sprint performance and kinematics (2,6,8). Of these studies, 2 examined the acute effects of vest loading (9–20% of BM) on sprint kinematics and reported a decrease in velocity (−3.8%) and an increase in sprint time (7.5–11.7%) with loading in comparison with unresisted conditions (2,8). In addition, vest sprinting has been shown to affect step/stride length (−0.5 to −5.2%) and frequency (−2.4 to −2.7%) and several other kinematic variables in comparison with unloaded sprinting (2,8). A study by Bosco et al. (3), highlighted the ability to increase power through periodization involving external loading around the torso in the form of a weighted belt. However, as can be observed from this brief treatise of the literature, the kinetics-associated vest loading is relatively unexplored, and there is certainly no research profiling kinetics across a variety of vest loads.
The acceleration and maximum velocity phases of sprinting require different mechanical demands on the sprinter (7,9). The acceleration phase requires long contact periods, short flight times, positive net horizontal forces, and a greater forward trunk lean in comparison with the maximum velocity phase (1,9). Thus, each phase may respond differently to the same vest load and therefore warrant specific loading protocols (1,9,28). No previous studies have examined the effects of vest sprinting on acceleration or maximum velocity phase kinetics during sprint running. Thus, the means of effectively manipulating vest load to cause changes in the force profile for specific adaptation is currently unknown. Such information is important to strength and conditioning and sprint coaches for programming purposes. The aim of this study, therefore, was to analyze the effects of vest loading on sprint kinetics and kinematics during the acceleration and maximum velocity phases while sprinting on a nonmotorized treadmill (NMT) ergometer.
Experimental Approach to the Problem
A cross-sectional design was used to investigate the effects of vest loading on kinematics and kinetics during acceleration and maximum velocity sprinting. All subjects performed maximum effort 6-second sprints on an NMT with and without vest loading (9 and 18 kg). Data during the first 2 steps (i.e., first stride) of the 6-second sprint and 10 steps once the subjects attained maximum velocity were averaged for final values. Data were then compared using a repeated measures analysis of variance (ANOVA) with Bonferroni post hoc analyses to determine statistical difference between conditions.
Thirteen sport-active (rugby codes, n = 6; track sprinters, n = 2; field hockey n = 1; miscellaneous sporting codes, n = 4), weightlifting, healthy male university-level athletes volunteered to take part in this study (22.9 ± 3.3 years old; BM: 82.5 ± 8.4 kg; stature: 179.1 ± 6.6 cm). All athletes provided written informed consent before participating, and completed a health questionnaire to ensure that they were fit for testing. The Institutional Ethics Committee of Auckland University of Technology provided approval for this study.
A Woodway Force 3.0 (Eugene, OR, USA) NMT ergometer was used to quantify the sprint kinetics and kinematics (Figure 1). This ergometer is a modernized equivalent of the original NMT system introduced by Lakomy (19) and has been implemented (4,25) and validated (12). Nonmotorized treadmill ergometry pertaining to both the training and measurement of performance variables have been explored in depth during sprinting (13–15,20,26,27). The NMT system used in this study featured a user-driven vulcanized rubber belt, the mechanics of which feature 12 guiding rollers and 114 ball bearings. The subjects were harnessed around the waist to a vertical strut at the rear of the system. A nonelastic tether secured the harness to a load cell and a locking vertical-sliding gauge allowed the collection of horizontal force data. This sliding gauge was manually adjustable (and securable) to each subject's hip height to enable horizontal alignment of the tether to the load cell during running trials. Calibration of the load cell took place before the testing session by hanging a selection of known weights from the tether as instructed by the manufacturer. Vertical force output was collected using 4 load cells positioned beneath the NMT belt. The vertical load cells were calibrated before and after testing using known loads placed on top of the stationary treadmill belt. The velocity of the treadmill belt was collected by 2 optical speed microsensors located at the rear of the treadmill belt. Calibration for this measurement was not needed because the distance recorded per rotation of the belt did not change, and unit conversion was hardcoded into the NMT software. Power output was measured by the NMT as the product of the force exerted on the horizontal load cell and the velocity of the treadmill belt. All variables were collected at a sampling rate of 200 Hz, using a hardwired system interface (XPV7 PCB; Fitness Technology, Adelaide, Australia). Primary analysis took place within a custom-built LabVIEW software program (LabVIEW; National Instruments, Texas, USA).
Weighted vests were used to supply the vertical loading, and each subject performed the trials as usual on the NMT system. The vests were loaded with small sandbags that could be added or removed in increments of approximately 200 g. The load could be distributed evenly around the subject's torso, up to approximately 20 kg per vest. The 2 loading conditions included absolute loads of 9 and 18 kg. Previous researchers have used vest loads between 7 and 20% of relative BM (2,3,8), and the loads chosen for this study align with such loading parameters.
Participants were required to report on a single day for approximately 2.5 hours of preparation, familiarization, and testing. Close-fitting sports clothing and running shoes were worn throughout. First, the subjects' height, mass, and age were determined and recorded. The subjects were then required to undergo a standardized warm-up and familiarization. Initially, the subjects jogged unloaded on the treadmill for 90 seconds. During this time, the subjects were encouraged to vary their pace to familiarize themselves with the feeling of accelerating on the foreign running surface. Two build-up sprints of 70 and 80% of the subjects' expected maximum velocity were then performed based on the findings of semi-professional Australian Rules footballers sprinting on an NMT (4). This consisted of a 3-second submaximal acceleration to the determined velocity, holding that velocity for 5 seconds, and then decelerating. To conclude the warm-up protocol, a 3-second maximum acceleration was performed. This entailed a tester applying a stationary brake to the treadmill track to enable the subject to lean against the harness so as to simulate a block start. Based on their performance in this step of the protocol, subjects were allowed the opportunity to complete another trial of the blocked-start. Between each section of the warm-up, subjects were allowed to rest for 60 seconds, followed by a 2–4-minute rest period preceding the first trial.
The data collection consisted of 6-second maximal velocity sprints under the 3 conditions. As per the warm-up protocol, the treadmill belt was blocked from behind (Figure 2). Starting position for all sprints was standardized with subjects starting with the right foot back, and then, they were instructed to lean into the harness. Throughout each 6-second trial, subjects were given continuous verbal encouragement to promote maximal effort. The loading protocols consisted of 2 absolute vest loads (9 and 18 kg). One to three trials, dependent on the success of the initial and subsequent collections, were performed for each condition. If more than 1 trial was collected, the trial with the highest maximum velocity was selected for analysis. The participant order was staggered and randomized, 4–6 subjects performed the testing protocol in a cycled format to maximize time efficiency and concurrently allow each subject appropriate rest. Rest between trials was less than 4 minutes for each participant after each trial.
Data was collected during the acceleration and maximum velocity phases of each 6-second sprint (Figure 3). The “first stride” was defined as the first 2 steps after the initial push-off. “Maximum constant velocity” was defined as the 10 steps after the maximum velocity was attained and maintained. The first 2 steps during the acceleration phase and 10 steps during the maximum velocity phase were averaged for final analysis (Figure 4). Velocity measures, peak horizontal force, peak GRF-z, mean GRF-z, and power output were obtained from the LabVIEW program as described previously. Contact time was determined from the time (in seconds) the treadmill registered force above 0 N to the moment it returned to a null reading (i.e., 0–0 N). Flight time was measured from the moment of last ground contact of one foot to the moment of first contact of the other foot. Stride frequency was determined by the following formula: 1/(contact time + flight time). Stride length was determined through the following formula: peak velocity/stride frequency.
Seven of the participants completed 2 trials at each loading condition to determine test-retest reliability of the sprint kinetics and kinematics variables. Intraclass correlation coefficient and coefficient of variation were calculated for each variable (Table 1). Means and SDs were determined for each variable under each loading condition and were used as measures of centrality and spread of data. Normal distribution of the data was checked using the Sharpio-Wilk statistic. A repeated measures ANOVA with Bonferroni post hoc contrasts was used to determine significant kinematic and kinetic differences between loads (body weight, 9 and 18 kg) for each condition (acceleration and maximum velocity). All variables, excluding step length, step frequency, and peak velocity, were analyzed in both the acceleration and maximum velocity phases. Statistical significance criterion was set at an alpha level of p ≤ 0.05. Additionally, effect sizes (ES) were calculated using the following equation: ES = (high value−low value)/((high value SD + low value SD)/2). Effect sizes were described as large (ES > 1.2), moderate (0.6 < ES < 1.2), small (0.2 < ES < 0.6), and trivial (ES < 0.2) (10).
Peak velocity decreased significantly between both vest conditions (−3.6 to −5.63%; ES = −0.38 to −0.61) in comparison with baseline unloaded sprinting (Table 2). Step frequency remained statistically unchanged by external loading; however, step length significantly decreased in both vested conditions (−4.2% [both conditions]; ES = −0.33 to −0.34) in comparison with baseline. Contact time during the acceleration phase was statistically unchanged respective to the baseline condition. During the maximum velocity phase, significant increases in contact time (5.9–10.0%; ES = 1.01–1.71) compared with baseline were observed in both vest conditions. The 18-kg vest condition flight time during the acceleration phase was significantly decreased (−26.7%; ES = −1.50) compared with baseline. During the maximum velocity phase, both vest conditions resulted in significantly decreased flight time (−17.4 to −18.8%; ES = −0.89 to −1.08).
Peak GRF-z output during the acceleration phase was statistically unchanged relative to the baseline condition. During the maximum velocity phase, only the 18-kg vest condition produced significantly greater peak GRF-z (8.8%; ES = 0.70) as compared with the baseline. During both the acceleration and maximum velocity phases, the 18-kg vest significantly increased the mean GRF-z compared with the baseline (11.8–12.4%; ES = 1.17–1.33). Vest loading had no significant effect on horizontal force output in either the acceleration or the maximum velocity phases. There were no significant variations from baseline for power output during the acceleration phase; however, during the maximum velocity phase, the 18-kg vest condition resulted in a significantly lower horizontal power output compared with the unloaded baseline conditions (−14.3%; ES = −0.48).
This is the first study, to the best of our knowledge, that has investigated the effects of vest loading on sprint kinetics along with sprint kinematics during the acceleration phase and maximum velocity phase of sprinting. Both vest loads significantly decreased the maximal velocity output. The 3.5% decrease with approximately a 10.9% load (9 kg) of mean subject BM is comparable with the findings of Alcaraz et al. (2) who reported a 3.8% decrease in velocity with a 9% BM load. Moreover, Cronin et al. (8) reported a 9.3–11.7% increase in 30-m sprint times with a vest load of 15–20% BM. Vest loading seems to have similar effects on peak velocity outputs on NMT ergometry as over-ground sprinting.
The effects of additional external loading on step frequency and step length may explain the decrements in velocity observed in this study. In over-ground running, vest or belt loading has been reported to significantly reduce the overall stride length, and typically decrease step frequency (2,8). Furthermore, the added loading will usually increase the ground contact time (2,8) and decrease the flight time (2) during the gait cycle. The results from this study concur with this trend, with the addition of external loading decreasing step length (−4.2%), increasing contact time at the maximum velocity (5.9–10.1%), and decreasing the flight time during both acceleration (−26.7%) and maximum velocity phases (−17.4 to −18.8%). The significant decrease in flight time observed at maximum velocity may suggest that the added vertical load supplied by the vest essentially limited the subjects' ability to propel their body into a flight phase.
The relationship between external loading and GRF-z output is of interest, particularly to those wishing to overload such forces for a specific training purpose. Interestingly, no significant increases in peak GRF-z relative to baseline with vest loading during the acceleration phase were observed in this study. Moreover, only the 18-kg vest resulted in a large increase in the mean GRF-z during acceleration (11.8%). It would seem that vest loading might not be an appropriate resisted method to overload the acceleration phase vertical GRF-z, especially in terms of lighter loading protocols similar to those used in this study. At maximum velocity, only the 18-kg vest load resulted in a moderate increase in GRF-z over the baseline condition (8.8%). Similar to the acceleration phase, a moderate increase in the mean GRF-z was observed with the 18-kg vest alone (12.4%). It seems that greater training loads than the traditional recommendation of 7–15% BM (2,8) may be needed to promote significant increases in GRF-z production. However, as previous loading parameters have been assigned on the basis of minimization of technical alterations, trainers will need to decide whether the possibility of compromising technique is an acceptable trade-off for an increased GRF-z training stimulus.
The fact that peak GRF-z did not increase during the acceleration phase highlights an interesting phenomenon whereby the addition of mass to the subject does not result in a corresponding increase in GRF-z. It would be expected that an increase in peak GRF-z would be similar to the mass added with each vest load (i.e., 88.3–176.6 N); however, this was not the case. It seems that additional mass may not provide as great a GRF-z training stimulus as initially thought, and other training methods need to be explored to overload GRF-z production capabilities, especially during acceleration. This phenomenon is most likely explained by the additional vertical loading affecting the rise and fall of the center of mass (COM) during the flight phase, i.e., significant reduction in flight phase time. If the flight height of the COM is reduced, then there will be a concomitant reduction in the GRF-z. It would seem that this reduction in flight time (rise and fall of COM) counters the effect of the additional mass. Similar findings have been reported during running at submaximum velocities while subjects carried compliant poles (18). Kram (17) found that an additional load of 19% body weight only increased peak GRF-z by 4.7% and reduced flight times by 35%. Future research quantifying the path of the COM is needed to validate this contention.
Vest loading did not result in any significant effects on horizontal force output during either the maximum velocity or acceleration phases. Cronin et al. (8) reported a decrease in forward trunk lean with vest sprinting, likely illustrating a reduction in the subjects' ability to control the added mass added to their frame. Because forward lean has been reported as a mechanical determinant of horizontal force production (18,23), it is likely that vest loading affected the subjects' ability to produce force in a horizontal direction through a limitation in their ability to effectively control the added mass and resultant forces.
The addition of vest loading did not result in any significant effects on power output during the acceleration phase. Only during the maximum velocity phase did the 18-kg vest condition result in a small decrease in power compared with the baseline condition (−14.3%). Bosco et al. (3) highlighted the possibility for increases in vertical power from vest weighted training; however, the results of this study certainly do not support such a contention. This dissonance can be explained by the different power calculations in both studies; the power output for the NMT is calculated as the product of horizontal force and velocity. Because horizontal force production was unaffected and velocity reduced by vest loading, it is unlikely that vest loading would provide a horizontal power stimulus. It could be speculated that the same would be true of the value of vest loading as a vertical power-training stimulus—that is, given the nonsignificant changes in GRF-z production and the decrement in velocity with additional load, it is unlikely that vest loading would overload vertical power to any great extent. Therefore, if there is a strong relationship between power production and sprint performance, as some researchers have alluded to (11), then careful consideration needs to be given to the selection of training modalities that maximize horizontal and vertical power output.
There are several limitations in this study that should be noted. First, the athlete sample represents a collection of team, individual and casual sport athletes of varying abilities. Second, because the loads chosen in this study were absolute, the loading protocols chosen represented a different percentage of BM for each participant. In a practical sense, this may not be possible or probable in an application to the field—hence why absolute loading was chosen in this study—however future studies should either select a sample with similar characteristics (i.e., a team of rugby union forwards), or calculate loads based on a percentage of each subject mass. Third, although the Woodway treadmill offers a more realistic running experience to standard motorized treadmills, particularly for maximal sprinting, it differs to over-ground running. Maximum sprint performances are typically 25–30% slower on the Woodway NMT. Maximum velocity has however been shown to be valid in comparison with over-ground sprinting (12). Fourth, vertical and horizontal force data on the treadmill were not collected from the same location. This could be addressed in future studies with tri-axial force-plates imbedded beneath the track (25). Finally, mechanisms for the findings were not investigated because maximum sprint velocity significantly decreased with each load. A more detailed analysis could be conducted in the future if sprint velocity were maintained with increasing vest loading.
A fundamental tenet of strength and conditioning practice is matching the training stimulus to the individual needs of the athlete, which in turn should produce the desired adaptation. Important in such an approach is understanding the mechanical stimuli certain training methods provide. In terms of vest loading, it is thought that this type of training provides a means of improving the vertical eccentric/concentric force capability of athletes during cyclic activities such as hopping, bounding, and running. It would seem, however, that the interaction between load and the use of this training method in producing the desired adaptation may be more complicated than initially thought. Increasing vest loading using the loading parameters of this study had little effect on GRF-z. It would also seem that the vest loads offered little as a horizontal and vertical power-training stimulus. Given these findings, careful consideration needs to be given to the choice of load and utilization of this type of training if increasing GRF-z production of the athlete is a training goal. It may be that heavier vest loads than initially thought are needed to overload cyclic vertical strength and power, but the effect of such loading on sprint kinematics/technique needs investigation.
In summary, vest loading may have a place in the preparation of the athlete, however, understanding when and for what reason it should occur in the athlete plan is fundamental to targeted individualized programming. Furthermore, as maximal effort sprinting is a complex interplay between a number of different variables, the reader needs to be cognizant that the optimal training solution for sprinting performance is not a single modality, but a combination of several. Identifying individual needs and matching these with modalities that address the individual's mechanical/physiological limitations would seem fundamental in improving strength and conditioning practice and optimizing sprint performance.
The authors wish to thank the committed group of subjects for their participation in this study. This project did not receive external financial assistance, and none of the authors hold any conflict of interest.
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Keywords:Copyright © 2014 by the National Strength & Conditioning Association.
vest loading; sprint mechanics; horizontal force; vertical force; acceleration; maximum velocity