Sprint running has been described as consisting of an acceleration phase, a maximum speed phase, and a deceleration phase. In sporting activities such as soccer, rugby, and football, sprint efforts are typically of short duration (e.g. 10–20m) (Spencer, Bishop, Dawson, and Goodman,2005). Therefore, the ability to develop maximal sprint speed in as short a time as possible (i.e. acceleration) may be of greatest significance in those sporting activities (Cronin and Hansen, 2006). Hay (1985) supported such a contention stating that, in many sporting activities, the success enjoyed by athletes is directly related to their ability to increase or decrease their speed rapidly.
An athlete’s ability to accelerate is dependent upon various factors, including technique (kinematics) and the force production capability (kinetics) of the body, in particular the lower limb musculature. Examples of kinematic factors that are considered important to sprint acceleration performance include step frequency and step length (Hunter, Marshall, and McNair, 2004), the duration of the stance phase, the position of foot strike relative to a vertical line through the athlete’s centre of mass, knee flexion angle at and immediately after footstrike, the magnitude of hip extension at toe-off, and the angle of take-off of the athlete’s centre of mass at toe-off. Examples of kinetic factors that are considered important to sprint acceleration performance include horizontal propulsive impulse of the ground reaction force.
Procedures
Sprint times for 10 and 30m were recorded for all runs filmed. Testing consisted of videoing (sagittal plane) athletes sprinting over a distance of 30m, from a standing start. Baseline film of an unresisted 30-m sprint was collected first to provide a baseline measure of sprint kinematics. Athletes were then filmed with two different loads (15% and 20% of body mass) wearing the vest, and with two different loads (15% and 20% of bodymass) towing a sled. Each athlete was therefore tested a total of five times (once for each condition) with 8-min rests between trials; the order of resisted conditions was randomized to avoid any test order bias. The load of 15% of body mass was chosen as it is commonly used for vest sprinting and sled towing, whereas the load of 20% was chosen to examine the effect of a heavier external load on sprint kinematics. Starting position for all sprints was standardized with athletes starting with the left foot forward. The left toe was placed 0.5m behind the starting line. The right toe was placed approximately level with the heel of the left foot.
Data analysis
The kinematics examined can be divided into two categories: step variables and joint angles. Step variables included step length (point of foot strike of one foot to the point of foot strike of the contralateral foot), step frequency (number of steps per second), stance phase duration (the time spent on the ground during each step), and swing phase duration (the time that the foot was not in contact with the ground). The joint kinematics examined included trunk, thigh, knee, and ankle joint angles at the beginning and at the end of the stance phase (i.e. footstrike and toe-off).
Results
Sprint times
The mean 10-m times ranged from 1.72s to 2.06s. Baseline 10-m times were significantly faster than in all the loaded conditions. Significantly slower 10-m sprint times were found for sled towing at 20% of body mass compared with both vest conditions, and for sled towing at 15% of body mass compared with vest sprinting at 15% of body mass. The differences in 10-m sprint times within the two vest conditions, within the two sled conditions, and between the sled (15% body mass) and vest (20% body mass) were found to be non-significant.
The 30-m times became slower as load increased in both the vest and sled conditions. The mean 30-m times ranged from 4.12s for the baseline sprint to 4.90s when towing a sled loaded at 20% of body mass. Baseline times were significantly faster than in all the loaded conditions. All other comparisons at 30m were found to differ significantly. That is, the heavier loads were slower within conditions and both sled loads were slower than both vest loads at 30m.
A significant training technique–distance effect was also found. The effect of resisted technique was different between 10 and 30m. Sled towing resulted in a greater overall sprint time (by 18.0–22.9%) at 10m compared with vest sprinting (by 7.5-10.0%). The effect of sled towing decreased (by 14.7–18.9%) as distance increased. However, the effect of the vests at both loads increased (by 9.3–11.7%) as distance increased.
Step variables
There was an increase in step length through the acceleration phase of sprinting. Significant technique and distance effects were found for step length. Step length differed significantly between baseline and all loaded conditions at all three distances (5, 15, and 25m). Step length during sled towing at 20% of body mass was significantly different from sled towing at 15% of body mass and both vest loads at all distances.
Significant differences were also observed with step frequency. Significant decreases (by 2.7–6.1%) in step frequency between the baseline sprint and all loaded conditions were found.
Stance phase and swing phase
Stance phase duration and swing phase duration showed a significant distance effect. Mean stance phase duration decreased as the athletes progressed over the 30-m distance. This was coupled with concurrent increases in swing phase duration. The stance phase duration was longer than the swing phase duration in the baseline condition at 5 and 15m but not at 25m. However, for all loaded conditions at all distances, swing phase duration did not exceed stance phase duration.
Stance phase duration and swing phase duration were found to differ significantly across technique. Stance phase duration in all loaded conditions was significantly greater than in the baseline condition at all distances from the start. The 20% of body mass loads with the sled and vest resulted in significantly longer stance phases compared with the sled at 15% of body mass at all distances from the start; however, there was no significant difference between the two vest loads. Swing phase duration was significantly shorter for all conditions compared with baseline values. However, there was a significant difference between the sled at 15% of body mass and vest and sled at 20% of body mass.
Joint kinematics
The trunk angles at foot strike during vest sprinting with 20% of body mass were significantly smaller (i.e. more upright) than those at baseline at all three distances from the start. Trunk angles at both foot strike and toe-off during sled towing with 15% and 20% of body mass were significantly greater than those at baseline and vest sprinting with 15% and 20% of body mass at all three distances from the start.
The thigh angles in sled conditions with both 15% and 20% of body mass at toe-off were significantly smaller (i.e. greater thigh extension) than those in the vest conditions with both 15% and 20% of body mass at all three distances.
Knee angles at foot strike were significantly greater (i.e. greater knee flexion) in both sled conditions than in both vest conditions and the baseline sprint at all three distances from the start. No significant differences were observed between sprint conditions in knee angles at peak flexion and at toe-off.
Ankle angles at foot strike were significantly greater at 15 and 25m from the start than at 5m from the start.
Previous researchers have suggested that resisted sprinting techniques may have both acute and longitudinal effects on sprint technique. In the present study, we found that sled towing and vest sprinting influenced sprint kinematics during the acceleration phase of sprinting and that significant differences exist between the two resisted sprint-training techniques on some kinematic variables. As statistical analysis showed no athlete type effect, these findings can be applied to a variety of populations including athletes participating in team sports (such as rugby union) and both beach and track sprinting.
Sprint times and step variables
The effect of external load on sprint times was significant. Both sled towing and vest sprinting resulted in decreased sprint performance (i.e. increased sprint times of 7–23%) compared with unresisted sprinting in this study. To the authors’ knowledge, this is the first study to report the effect of vest loading on sprint times, whereas previous research has consistently reported decreased sprint performance during sled towing when compared with unresisted sprinting. However, the magnitudes of performance decrements differed across the studies (increases of 9-29%), which can be attributed to the differences in the load added, the sprint distance, the training levels of the athletes, and the design of the towing device used.
A greater effect on performance was observed when towing a sled compared with vest sprinting, when the same relative load was used. This finding is most likely related to the additional force required to overcome the effects of friction between the sled and the track surface. When the athlete attempts to move the sled, the ground exerts a horizontal force on the sled called the “force of static friction”. This frictional force acts in the opposite direction to the applied force; that is, it opposes the force that the athlete is applying. Once this force is overcome (by the athlete exerting a force greater than the force of static friction), the sled will begin to move. Therefore, in the initial stages of the sprint, the performance of the athlete was decreased significantly as he or she attempted to overcome the friction of the sled. Once the force of static friction was overcome, the effect of the towing load was decreased, as indicated by the 18% and 22.9% performance decreases for the 15% of body mass and 20% of body mass loads respectively at 10m, which were greater than the 14.7% and 18.9% decreases observed at 30m. A different pattern was observed during vest sprinting, with the performance decrease being greater at 30m than at 10m (7.5% and 10% decreases at 10m vs. 9.3% and 11.7% at 30m). These results suggest that the athletes had less additional force to overcome in the early stages of the sprint during vest sprinting; however, as they developed speed, the need to control the additional mass around their trunk resulted in decreased performance.
The increased sprint times (decreased sprint speed) during the resisted conditions was predominantly the result of decreased step length, with only small decreases in step frequency. With respect to sled towing, these results are consistent with previous studies, whereas no previous literature could be found regarding changes in step variables during vest sprinting.
Sled towing had a greater effect on step variables than did vest sprinting. However, again this only reached statistical significance when comparing sled towing and vest sprinting at 20% of body mass. The more marked effect of sled towing than vest sprinting on step variables at the same relative load (20% of body mass) was indicative of the extra frictional forces provided by this type of loading technique. It may also be a result of the changes in angle and height of take-off of the body’s centre of mass, which was indicated by joint kinematic data to be significantly adjusted during sled towing. The increased trunk flexion, and the increased thigh and knee extension at toe-off, are likely to have led to an anterior and inferior shift of the athlete’s centre of mass, resulting in lower horizontal trajectory of the athlete’s centre of mass.
Joint kinematics
The effects of the two different resisted sprinting techniques on step variables were very similar. That is, step length and step frequency were decreased while stance phase duration was increased. However, the data related to joint kinematics for the vest and sled conditions were significantly different. Due to the paucity of research into the kinematic effects of sled towing and particularly vest sprinting, very few of these findings have been reported previously.In this study, trunk angles during sled towing were significantly greater than at both baseline and in the vest condition at all distances from the start. This finding is consistent with previous research by Letzelter et al. (1995) and Lockie et al. (2003), who found similar trends of increasing trunk lean as the external load increased during sled towing. As suggested by Lockie et al. (2003) and Cronin and Hansen (2006), such a body position (i.e. large trunk lean) during sled towing seems to be specific to sprint acceleration movements. Therefore, sled towing may have the potential to overload the body in a manner specific to sprint acceleration and may be effective in improving sprint acceleration ability. In fact, a recent study by Zafeiridis et al. (2005) supports this hypothesis by showing that 8 weeks of sled towing improved sprint performance in the acceleration phase (0–20m) but not in the maximum speed phase (20–50m).
Greater trunk angles at foot strike during sled towing may decrease the braking forces associated with landing. A number of authors (Alexander,1989; Hunter et al., 2005; Mann and Herman,1985; Mann et al.,1982) have commented that to minimize the negative effect of horizontal and vertical braking on velocity, foot strike underneath and not in front of the athlete’s centre of mass is preferable during sprinting (i.e. the landing distance is smaller). During sled towing, an increase in trunk angle, without a concurrent increase in thigh flexion, allows foot strike to occur closer to the athlete’s centre of mass, possibly reducing braking and increasing the time for the production of propulsive forces. Similarly, if an athlete runs more upright without a concurrent decrease in thigh flexion at foot strike, foot strike will occur further from the body’s centre of mass resulting in an increase in braking forces. Therefore, it is possible that braking forces were greater during baseline and vest sprinting than sled towing. Further studies that collect force data together with kinematics are needed to confirm these thoughts.
Although thigh angles at foot strike were not significantly different between the two training modalities, thigh angles at toe-off were significantly smaller (i.e. greater thigh extension) during sled towing than during vest sprinting. Several authors have commented that thigh extension provides the most significant propulsive forces during sprinting (Jonhagen, Nemeth, and Eriksson, 1994; Mann, 1981; Wiemann and Tidow, 1995). Whether increased thigh extension during the stance phase is desirable during sprinting is a point of much debate. Mann and Herman (1985) suggested that an abbreviated thigh extension is more desirable, as the increased ground contact required to increase thigh extension and subsequent increase in stance phase duration has a negative impact on step frequency. Interestingly in the current study, increased stance phase duration was found during sled towing. Other authors (Hay, 1985; Vonstein, 1996) have suggested that maximum thigh extension during ground support will increase propulsive forces thus increasing step length, and is therefore a desirable characteristic in sprint technique.
Sled towing resulted in significantly greater knee angles (i.e. less extension, greater flexion) at foot strike compared with baseline and vest sprinting at all stages of the 30-m sprint. Therefore, during sled towing there was greater knee flexion at foot strike and no change in extension at toe-off. This suggests that during sled towing propulsive forces may act through a greater range, and possibly comprise a greater proportion of the stance phase. Ito and colleagues (Ito, Komi, Sjoden, Bosco, and Karlsson,1983) suggested that during the stance phase of running, the most significant braking forces are associated with eccentric contraction of the knee extensors and ankle plantar flexors. In the current study, at both 5 and 15m from the start, sled towing resulted in limited additional knee flexion after foot strike (during the braking phase of stance). At 25m from the start there was an increase in knee flexion after foot strike, but only with the 15% of body mass load. Given that braking forces after foot strike are associated with knee flexion, its absence during sled towing may also indicate kinetic changes that warrant further investigation.
Although not the main focus of this study, it is of interest for future research to examine how the point of attachment or direction of resistance forces from vest or sled could influence the alterations of sprint kinematics. In sled towing, the length of the rope as well as the height of the rope attachment on an athlete (e.g. waist belt vs. shoulder harness) could change the angle of pull. In vest sprinting, the distribution of added weights could influence the position of the centre of mass. Such factors are likely to influence acute sprint kinematics and loading patterns, and may have potential implication for long-term training adaptations.
Conclusion
Sled towing and vest sprinting both resulted in acute changes in sprint kinematics during the acceleration phase of sprinting, but in a different manner when the same relative load (% of body mass) was added. These acute kinematic differences may reflect the different manner in which the two techniques overload the body and therefore provide insight into possible differential mechanisms by which vest and sled towing may act to improve sprint acceleration performance. Vest sprinting has less of an effect on trunk angle, with the athlete remaining more upright, and consequently long-term changes in sprint techniques are less likely. Furthermore, vest sprinting may result in a greater load on the eccentric braking phase at the beginning of the stance phase. As braking forces are a more significant component of the stance phase during the maximum speed phase of sprinting, it seems that vest sprinting may be a more appropriate mode of resistance training for the latter stages of the acceleration phase and the maximum speed phase. Sled towing resulted in greater thigh extension and trunk lean, enabling the athletes to place themselves in an optimal position to maximize propulsive and minimize braking forces. Furthermore, as the duration of the propulsive phase is greater during the stance phase of acceleration, sled towing may be a more appropriate training modality for the early stages of the acceleration phase of sprinting.
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