Sprinting is essential for success in many sports (11–13,27). In field sports where the need to reach the ball first, or be in position for a play to develop is decisive, speed is a crucial factor (22,29). Sprint velocity is a product of stride length and stride frequency. To increase velocity, one or both of these components must be increased (22,33). Stride length and stride frequency can be increased by exerting larger forces or increasing the rate of force development during the stance phase (15,24,35). It is generally accepted that although maximum velocity is important in field sports, the ability to accelerate is seen as being of greater significance (10,27).
The kinematic and kinetic characteristics of the acceleration and maximal velocity phases of sprinting are quite different. The acceleration phase requires a greater forward trunk lean (16). Kugler and Janshen (20) proposed that if the force vector points further forward (trunk lean) then the ratio of vertical to propulsive force will be biased toward forward propulsion. In this instance, greater ground reaction force (GRF) can be applied without the negative effects associated with high vertical force application, such as short contact times. In contrast, at maximum velocity, athletes must preserve optimal postural stability, minimizing braking, and increasing vertical forces. Greater vertical GRFs are essential in allowing faster sprinters to reduce foot contact time during the stance phase (36).
The development of various resisted sprint training modalities, such as sled, parachute, and bungees, is providing coaches with alternative or additional sport-specific training strategies to more traditional methods. During ST, the external resistance is provided by the mass of the sled and the coefficient of friction between the sled and the surface (9). Resisted sprint training is performed in a horizontal direction and involves the relevant muscles, velocities, and ranges of motion (ROM) similar to those of normal sprinting (2,19).
Sled loading strategies, as well as the sets and repetitions used to implement ST, remain equivocal (2,8,23,26,28). There are several different methods by which sleds can be loaded; sled loading based on an absolute load or relative load relating to body mass have been commonly used; however, these methods do not take the athlete's strength capabilities into consideration (14,34). As such, loading sleds based on a reduction of sprint velocity is the preferred method (1,7,25,34). Previous investigations have implemented various sled loadings ranging from a 5 kg of absolute load to 32.2% body mass (23,37). Many researchers have found lighter sled loads to be the most effective as they have been shown to have less impact on contact time variables, joint angles, and ROM (17,26,28). Several researchers have used sled loadings based on a 10% decrement in sprint velocity to improve performance (7,25,33). Although information on loading strategies is undergoing a process of confirmation, there is a dearth of literature relating to the practicalities of ST, notably regarding attachments for harness systems.
Lawrence et al. (21) investigated the effects of different harness attachment points (shoulder and waist) on walking sled pulls. They reported differences in joint moments between the different attachments, concluding that the shoulder harness would challenge the knee extensors and the waist harness would challenge the hip extensors. Over time, it is expected that the different harness attachments would lead to positive strength adaptations related to the aforementioned joints, thereby allowing coaches to tailor the sled pulls specifically to areas of weakness.
Generally, sleds are attached to the athletes through a lead (3 m) and harness system, the most common being a shoulder or waist attachment point. At present, it is not known how the different harness attachment points impact on ST kinematics and kinetics. Therefore, the purpose of this study was to investigate the sprint kinematics and kinetics of ST during the acceleration phase when sleds were loaded to cause a 10% reduction in sprint velocity. Subjects completed sprint trials under different conditions (normal sprinting, shoulder attachment, and waist attachment). It was hypothesized that (a) differences between the kinetic parameters would be negligible between conditions, (b) both sled trials would be significantly different from the normal sprint condition in terms of lower limb and trunk kinematics, and (c) the attachment point would impact trunk, hip, knee, and hip joint kinematics differently. The findings will allow coaches to alter their use of ST to better suit the acceleration phase.
Experimental Approach to the Problem
This study used a crossover design to compare the effects of different harness attachments during ST. Fourteen resistance-trained men performed a series of 6 m sprints in 3 different conditions (normal, with shoulder, and waist attachments). The key dependant variables were the sagittal plane kinematic measures of the lower extremities and trunk, the kinetic data obtained from the force platform and various contact time measures.
Fourteen resistance-trained men (age: 26.7 ± 3.5 years; mass: 84.2 ± 12.3 kg; stature: 174.4 ± 6.4 cm) participated in this study. All subjects were resistance trained (2 years minimum) with ST experience. The sample size was calculated based on previous acute ST investigations (14,21). All subjects gave written and informed consent before attending the testing sessions. The project was reviewed and approved by the institutional ethics committee of the University of Central Lancashire, in accordance with the principles of the Declaration of Helsinki. No external funding was provided by any of the harness or sled manufacturers used in this study.
One week before testing, all subjects completed a familiarization session. During this session, subjects were able to practice ST using the different harness attachment points. The same sled was used during all of the loaded trials. The sled was attached to the subjects using a 3-m nonelasticated attachment cord, and either a double shoulder strap or single waist belt. Using a 6-m sprint as a baseline, sleds were loaded so that sprint velocity was reduced by 10% (waist condition), as recommended by Kawamori et al. (17). Sprint velocity was monitored using infrared timing lights (Smartspeed Ltd., Fusionsports, Queensland, Australia).
Targeting occurs when participants deliberately lengthen or shorten the stride before force plate contact (32). These stride alterations have been shown to significantly impact on sagittal plane joint kinematics (6). Research shows that participants are able to run across an embedded force plate without significantly adjusting their stride mechanics (32). No studies have looked at how sprinting over an embedded force plate impacts on lower-body kinematics. However, in this study, measures were taken to ensure that no force plate targeting took place. First, the familiarization session was used to determine an individual starting position for each subject. Starting positions were adjusted so that each participant's right foot contacted the force plate on their third step. Starting positions of the ST trials were also adjusted accordingly and practiced until participants consistently landed on the force plate. In order to standardize starting positions, trials began in a 3 point position. Each participant chose to start with his left foot leading in the 3 point starting position. Regardless of the starting point, subjects sprinted a total distance of 6 m.
Subjects were asked not to participate in any physical activity 24 hours before the testing session. No food was allowed to be consumed during testing, although water was allowed. The testing session began with a standardized warm-up consisting of jogging (5 minutes), dynamic stretching (5 minutes), and a number of sprints building up to maximum intensity (2 × 75%, 2 × 90%, and 2 × maximum).
Previous research has shown that ST trials can impact on the kinematics of any subsequent normal sprint trials (17). Thus, the normal sprint trials were completed before either of the sled conditions (shoulder or waist). Once the normal sprint trials had been recorded, the ST trials were randomized. Testing procedures were identical to those described previously in the familiarization section. All subjects had 2 minutes recovery between each of the sprint trials. Five trials were collected for each of the conditions. Again, subjects sprinted a distance of 6 m in a 22-m laboratory. An embedded force platform, sampling at 1,000Hz, was positioned at approximately 3 m from the start (model 9281CA, dimensions = 0.6 × 0.4 m; Kistler Instruments Ltd., Winterthur, Switzerland). In order for the trials to be deemed successful, the whole foot had to contact the force platform. Trials were discarded in cases where any part of the foot did not land the force platform. Sprint times were generated for every trial, and any trials in which sprint velocity deviated more than ±5% of the initial trial in that condition were not used in the final analysis. In this instance, an extended recovery period of 4 minutes was implemented, and trails were repeated.
An 8 camera motion analysis system (Qualisys Medical AB, Gothenburg, Sweden) was used to capture kinematic data at 250Hz. The system was calibrated before every testing session. To determine stance leg kinematics (foot, shank, thigh, and trunk segments), retroreflective markers were placed on the following bony landmarks: the right calcaneus, first metatarsal head, fifth metatarsal head, medial malleolus, lateral malleolus, medial epicondyle, lateral epicondyle, acromion process (both), T12, and C7 (5). The pelvis segment was defined, using additional markers on the anterior superior iliac spines (ASIS) and posterior superior iliac spines (PSIS). Hip joint center was determined based on the Bell et al. (3) equations through the positions of the PSIS and ASIS markers. During dynamic trials, the foot segment was tracked using the calcaneus, first and fifth metatarsal heads. Rigid cluster tracking markers were also positioned on the right shank and thigh segments (4). The ASIS, PSIS, and greater trochanters were used as tracking markers for the pelvis. The trunk was tracked using markers at both acromion processes, as well as the T12 marker. A static calibration was completed and used as reference for anatomical marker placement in relation to the tracking markers after which all nontracking markers were removed.
Motion files were exported as C3D files and quantified using Visual 3-D (C-Motion Inc., Germantown, MD, USA) and filtered at 12Hz using a Butterworth fourth order filter. Three-dimensional kinematics of the lower extremities and trunk were calculated using an XYZ cardan sequence of rotations (X represents the sagittal plane, Y represents the coronal plane, and Z represents the transverse plane). The relevant segments (thorax, thigh, shank, and virtual foot) and reference segments (pelvis, thigh, and shank) were used to calculate joint angles of the trunk, hip, knee, and ankle joints, respectively. All kinematic waveforms were normalized to 100% of the stance phase, and then processed trials were averaged. Various kinematic measures from the trunk, hip, knee, and ankle joints were investigated: angle at footstrike, angle at toe-off, peak angle, ROM from footstrike to toe-off, and the relative ROM (the angular displacement from footstrike to peak angle). Resultant velocity at toe-off was calculated using the vertical and horizontal center of mass. These variables were extracted from each of the 5 trials for each joint; data were then averaged within subjects for a comparative statistical analysis.
Contact time was determined as time over which 20 N or greater of vertical force was applied to the force platform (30). The durations of the braking and propulsive phases were based on anterior and posterior horizontal GRF. Peak GRF was determined for the following components: vertical, braking, and propulsive. Vertical impulse was calculated as the area under the vertical GRF-time curve minus body weight impulse over the time of ground contact. The braking and propulsive impulses were determined by integrating all the negative and positive values of horizontal GRF, respectively, over the time of ground contact (17). Net horizontal impulse was calculated as propulsive impulse minus the absolute value of braking impulse. Similarly, mean values of vertical and net horizontal GRF were obtained by dividing respective impulse values by the contact time, whereas mean braking and propulsive GRF were calculated by the time duration of braking and propulsive phases, respectively (17). All GRF measures were expressed relative to total body mass.
Descriptive statistics were calculated and presented as mean ± SD. One-way repeated-measures analysis of variance was used to compare the mean values of the different conditions (normal, waist, and shoulder) with the different outcome measures (velocity, contact time, kinematics, and kinetics). The significance level was set at p ≤ 0.05. Post hoc pairwise comparisons were conducted on all significant main effects using a Bonferroni adjustment to control for type I error. Effect sizes were calculated using partial Eta2 (pη2). All statistical analyses were undertaken using SPSS (Version 22, IBM SPSS Inc., Chicago, USA).
Table 1 presents the stance phase velocity and contact time data. The kinetic measures are presented in Table 2. Tables 3–6 present the sagittal plane kinematic parameters from the trunk, hip, knee, and ankle joints. Figure 1 presents the mean sagittal plane angular kinematics during the stance phase.
The mean sagittal kinematic waveforms were qualitatively similar (Figure 1), although statistical differences were observed at the trunk, hip, knee, and ankle joints (Tables 3–6).
The results indicate that a significant main effect was observed for sprint velocity (p < 0.01, pη2 = 0.87). Post hoc analysis revealed that sprint velocity was significantly reduced during the waist (p = 0.000) and shoulder (p = 0.000) trials compared with the normal trials. There was no significant difference between the ST conditions (p = 0.616).
Similarly, a significant main effect was observed for the contact time of the stance leg (p < 0.01, pη2 = 0.66). Post hoc analysis revealed that contact times of the stance leg were significantly shorter in the normal condition compared with the waist (p = 0.000) and shoulder (p = 0.000) attachments. There was no significant difference between ST conditions (p = 0.073). Results highlighted a significant main effect for the duration of the propulsive phase of the stance (p < 0.01, pη2 = 0.48). Post hoc tests indicated that the propulsive phase was significantly longer during the waist (p = 0.024) and shoulder (p = 0.002) attachment trials compared with the normal sprint trials. There was no significant difference between ST conditions (p = 0.841).
The results (Table 2) show that there was a significant main effect for net horizontal mean force (p < 0.001, pη2 = 0.547). Post hoc tests revealed that the normal condition resulted in significantly lower net horizontal mean force than the shoulder attachment (p = 0.020) and the waist condition (p = 0.001). There was no significant difference between the ST conditions (p = 0.056). Similarly, there was a significant main effect for the net horizontal impulse between conditions (p < 0.001, pη2 = 0.742). Post hoc tests indicated that both ST conditions were significantly greater than the normal sprint trials (p = 0.000). The net horizontal impulses produced during the waist attachment condition were significantly larger than the shoulder condition (p = 0.045). There was a significant main effect for the propulsive mean force (p ≤ 0.05, pη2 = 0.329). Post hoc tests revealed that the waist condition led to significantly greater mean propulsive GRF than the normal condition (p = 0.004). There was no significant difference between the ST conditions (p = 0.056). Finally, a significant main effect was observed for propulsive impulse measures (p < 0.001, pη2 = 0.746). Post hoc tests revealed that the normal condition resulted in significantly lower propulsive impulse measures than the shoulder attachment (p = 0.000) and the waist condition (p = 0.000). There was no significant difference between the ST conditions (p = 0.063).
The results (Table 3) show that in the sagittal plane, there was a significant main effect for the magnitude of ROM for the trunk (p < 0.001, pη2 = 0.493). Post hoc tests revealed that trunk ROM was significantly lower during the shoulder condition compared with the normal (p = 0.000) and waist (p = 0.000) conditions. A significant main effect was observed for the relative ROM of the trunk (p > 0.001, pη2 = 0.410). Post hoc tests indicated that relative trunk ROM was significantly greater in the shoulder condition compared with the normal sprinting condition (p = 0.001).
The results (Table 4) show that in the sagittal plane, there was a significant main effect for hip joint angle at footstrike (p < 0.001, pη2 = 0.47). Flexion at the hip joint was significantly greater at footstrike during the waist (p = 0.015) and shoulder (p = 0.004) attachment trials compared with the normal trials. There was no significant difference between the ST trials (p = 1.000). Similarly, the results indicate that there was a main effect for hip joint angle at toe-off (p ≤ 0.05, pη2 = 0.38). Extension was greater in the normal trials compared with the waist (p = 0.015) and shoulder (p = 0.035) attachment trials. There was no significant difference between ST trials (p = 1.000). Finally, a significant main effect was found for peak hip flexion (p < 0.001, pη2 = 0.47). The peak hip joint angle was significantly lower in the normal sprint trials compared with the waist (p = 0.015) and shoulder (p = 0.004) attachment conditions. There was no significant difference between the ST sled trials (p = 1.000).
The results (Table 5) show that in the sagittal plane, there was a significant main effect for knee joint angle at footstrike (p < 0.001, pη2 = 0.73). Post hoc tests revealed that knee joint flexion was significantly greater at footstrike during the waist (p = 0.000) and shoulder (p = 0.000) attachment sled trials compared with the normal sprint trials. There was no significant difference between ST conditions (p = 0.441). The results indicate that there was a significant main effect for knee joint angle at toe-off (p ≤ 0.05, pη2 = 0.36). Knee joint extension was greater in the normal trials compared with the waist (p = 0.018) and shoulder (p = 0.016) attachment trials. There was no significant difference between ST trials (p = 1.000). A significant main effect was found for peak knee joint angle (p < 0.001, pη2 = 0.73). Post hoc analysis revealed that all the conditions were significantly different from one another. Knee flexion in the normal trials was lower than the waist (p = 0.001) and shoulder (p = 0.000) attachment trials. Knee flexion was significantly greater in the shoulder attachment condition compared with the waist attachment trials (p = 0.037). Finally, there was a significant main effect for the magnitude of ROM at the knee joint (p ≤ 0.05, pη2 = 0.29). Post hoc tests indicated that knee joint ROM was significantly smaller in the normal condition compared with the shoulder attachment condition (p = 0.036). There was no significant difference between the normal and waist attachment trials (p = 0.461).
The results (Table 6) show that in the sagittal plane, there was a significant main effect for ankle joint angle at footstrike (p < 0.001, pη2 = 0.4). Post hoc tests indicated that dorsiflexion was significantly greater at footstrike during the waist (p = 0.041) and shoulder (p = 0.006) attachment trials compared with the normal sprint trials. There was no significant difference between the ST conditions (p = 0.494). Finally, a significant main effect was found for peak ankle dorsiflexion (p < 0.001, pη2 = 0.46). Peak ankle dorsiflexion was significantly lower in the normal trials compared with the waist (p = 0.034) and shoulder (p = 0.002) attachment conditions. There was no significant difference between the ST trials (p = 0.248).
The aim of this investigation was to examine the kinematics and kinetics of ST when different harness attachment points were used (shoulder and waist). Sleds were loaded to cause a 10% reduction in sprint velocity over a 6-m distance. To the authors' knowledge, this is the first study to use a motion capture system to measure the sagittal plane kinematics of ST. This study will have practical implications to strength and conditioning coaches looking to improve acceleration performance.
Results show that there were significant kinetic differences between the ST conditions and the normal sprint trials, supporting the rejection of the first hypothesis. These findings are contradictory to those of Kawamori et al. (17) who measured various GRF variables with a similar 10% body mass (BM) sled loading. Both ST conditions were significantly different from the normal condition in numerous parameters: net horizontal mean force, net horizontal impulse, and propulsive impulse. Again, in contrast to Kawamori et al. (17), the ST conditions in this study resulted in longer ground contact times and propulsive phase contact times compared with the normal sprint trials. The increased propulsive contact times were not surprising as more propulsive force was required to overcome the extra resistance provided by the ST. However, the increased net horizontal force and propulsive impulse measures could be explained by longer ground contact times thus allowing more time to push in a horizontal direction.
Previous studies have reported that a 10% sled loading (BM or velocity reduction) had no significant acute impact on sprint kinematics (27,28). However, we hypothesized that sprint kinematics during ST would be different from the normal sprint condition. The results of this study supported our hypothesis. There were significant differences between normal sprint trials and both ST conditions in the sagittal plane at the hip, knee, and ankle joints. Peak hip flexion, flexion at footstrike, and flexion at toe-off were greater during the ST trials. Similarly, knee joint flexion was significantly greater for the ST conditions. Dorsiflexion was significantly greater in the ST conditions at footstrike as were the peak angles recorded. These findings contradict the theory that the 10% loading is the ideal because kinematics are not significantly altered (26,28). It is beyond the scope of this study to suggest what the longer term implications of these alterations might be.
Finally, the third hypothesis was also accepted. Both harness attachment points altered kinematics differently. During ST, the harness attachment points affected the athletes differently to those reported previously in heavy walking sled pulls (21). Trunk ROM was significantly lower during the shoulder attachment condition compared with the other conditions (Table 3). In contrast, trunk relative ROM was only significantly greater in the shoulder condition compared with the normal trials. The shoulder attachment leads to significantly greater peak knee flexion when compared with the waist harness (Table 5). The knee joint ROM in the shoulder condition was significantly greater than the normal condition, whereas differences between the waist condition and the other conditions were negligible (Table 5).
Unexpectedly, the ST harness attachment points also impacted stance phase kinetics differently. The waist harness led to significantly greater net horizontal impulse compared with the shoulder attachment condition. Furthermore, the waist condition resulted in significantly greater propulsive mean GRF when compared with the normal sprint condition. Importantly, none of the ST contact time measures were significantly different. Previous researchers (18) have highlighted net horizontal impulses and propulsive force as being key to achieving high acceleration, as such it would be seen that the waist harness is more suitable when training for the acceleration phase of sprinting. It seems apparent that the kinematic alterations caused by the waist harness made the line of action more horizontal, resulting in greater net horizontal impulse.
Our results highlighted differences in trunk angle between ST conditions. Previous investigations have also discussed the importance of trunk lean during ST. Alcaraz et al. (2) suggested that shoulder attachments would increase trunk lean to a greater extent than a waist harness attachment point. They reported that, due to the applied load being higher than the hips (pivot point), the athletes would have to compensate and increase trunk lean. It was proposed that the greater trunk lean would impact on the athletes force vector so that more propulsive GRF was applied compared with vertical GRF. Conversely, when sleds were attached through waist belts, the load passed through the hips, as such these attachments did not promote an increased trunk lean (2). As such, the authors suggested that shoulder harness attachments would be more beneficial when training for the acceleration phase, and waist attachments could be more suited to the maximum velocity phase (2). In contrast, results from this study indicated that negligible differences in peak flexion, angle at footstrike, and toe-off exist between ST conditions at the trunk. The only differences were that trunk ROM was significantly lower during the shoulder attachment condition when compared with the other conditions. Interestingly, the trunk-relative ROM was only significantly greater in the shoulder condition compared with the normal trials. Importantly, kinematic differences between the waist and normal sprint conditions were negligible. Therefore, our findings suggest that when the ST harness attachment is further away from the hips, it alters trunk kinematics to a greater extent, thus reducing net horizontal impulse.
The all-male resistance-trained testing population is a limitation. Previous investigations have demonstrated that women exhibit distinct lower-body kinematics when compared with men (31). As such, the results are limited to this population and may not be applicable to female athletes. Additionally, this study only looked at the harness attachment implications at a set sled loading (10% reduction in sprint velocity). Numerous investigations have highlighted that the kinetic and kinematic alterations differ greatly dependant on sled loading (8,17,26,28). Thus, the findings from this study will not be transferable to different sled loading strategies or the other phases of sprinting.
The current investigation provides new information regarding the influence of different harness attachment configurations on the kinetics and kinematics of ST. The results indicate that ST, with the commonly prescribed loading to cause a 10% decrement in sprint velocity, will alter kinematics at the trunk, hip, knee, and ankle joints. Similarly, both ST conditions led to significant GRF alterations when compared with normal sprinting. The kinematic and kinetic alterations observed in this study differ between the waist and shoulder attachment points. Our results suggest that the waist attachment point seems to be the most suitable when training for the acceleration phase of sprinting. Sled towing with this attachment led to fewer kinematic alterations and greater net horizontal impulses when compared with the shoulder attachment trials. Future research is necessary to explore how the observed harness attachment alterations impact on sprint performance or sprint kinematics and kinetics after prolonged ST training interventions.
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