Introduction
A variety of training approaches such as Olympic weightlifting, powerlifting, plyometrics, and resisted sprinting have been used to improve lower-body strength, power, and sprinting performance (7,23,36). Each of these approaches would appear to have its own advantages and disadvantages. For example, Olympic weightlifting and powerlifting exercises allow very high forces to be developed, with very high power outputs also seen in the weightlifting exercises (11,14). However, these exercises are typically bilateral in nature; predominantly involve vertical movement and the production of vertical forces; and, especially for powerlifting exercises, are performed at relatively low velocities. Plyometrics and resisted sprinting can be done in the horizontal direction; they also can performed unilaterally and in a fashion that involves the relevant muscles at velocities, postures, and ranges of motions that are similar to unresisted sprinting (22,23,33,37). The magnitude of the forces found in plyometric and resisted sprinting exercises are generally much less than that seen in Olympic weightlifting and powerlifting (17,26) and in contact situations such as rugby union scrummaging (31). Perhaps the primary reason for this is that plyometric and resisted sprinting are generally done with relatively low loads (e.g., bodyweight for plyometrics and up to 30% of body mass for resisted sprinting).
Although all of these training approaches are commonly used to improve overall athletic ability (e.g., sprinting and jumping), the results of a recent review by Cronin et al. (8) in some way question the efficacy of a number of these approaches. Specifically, they found that to improve sprinting speed by 2%, an increase in lower-limb strength of 23% was required. Such results suggest that strength and conditioning coaches may need to explore other resistance training approaches to better improve athletic ability in their athletes.
Very heavy sprint-style sled pulls (e.g., with loads substantially greater than body mass) may prove to be 1 such option. Heavy sled pulls have been proposed to be a form of training that may bridge the gap between gym- and track-based conditioning for track and field sprinters (19). This type of training may also prove beneficial for those who compete in American football, rugby, and strongman because all of these events require very high levels of horizontal total body momentum to be generated in contact situations (e.g., making or breaking tackles and scrimmaging or when towing heavy objects such as trucks) (2,9). In contrast, other coaches and researchers (27,29) have suggested that resisted sprinting with loads greater than 20% body mass may cause detrimental changes in sprint technique. This may reflect the fact that when performing resisted sprints, the athlete experiences a decrement in sprinting speed as a result of a reduction in step length and step rate and increased ground contact time, with these acute effects becoming more pronounced as the loads exceed 20% body mass (1,21,22,24). These coaches and researchers may therefore by fearful that these acute changes in sprint technique when performing resisted sprints may become ingrained long term, thus causing negative adaptations in sprinting technique and performance. We would argue that this is unlikely to occur for several reasons. Contemporary resistance training science is based on the premise that only by overloading the muscles in movements similar to that found in the sporting context will performance improve. On this basis, the acute negative effects of resisted sprinting may actually act as the stimulus for long-term benefits. This is supported by the results of many resisted sprint training studies that have shown chronic improvements in sprinting (especially acceleration phase) performance and no long-term negative changes in technique as a result of a variety of resisted sprinting approaches (16,30,34,37). For example, Myer et al. (30), who used resisted sprint loads that were considerably greater than the maximum 20% body mass suggested by some authors (27,29), reported significant training-related increases in unresisted acceleration-phase sprint speed and step rate and no negative chronic changes in any other biomechanical technique-type variables.
Based on the overload principle and the positive chronic (training) effects reported for resisted sprinting in several studies (16,30,34,37), the potential exists that even heavier (greater than body mass) sprint-type sled pull exercises may prove beneficial for a variety of athlete groups. Although such heavy sprint-style sled pulls are undoubtedly already used by some conditioning coaches, additional research into the acute and chronic effects of such training needs to be conducted so that conditioning coaches have a greater understanding of the key technical points for optimal performance, potential applications, expected training response, and the possible injury risks inherent to this form of training. A similar approach is now being taken with the potential applications of other strongman exercises for general conditioning practice. For example, McGill et al. (25) examined the lower back and hip loads in 3 athletes who performed exercises like the tyre flip, Atlas stones, log lift, farmers walk, and yoke walk. Similarly, Berning et al. (3) used 6 subjects to examine the metabolic costs associated with pushing and pulling a heavy car a distance of 400 m, whereas Keogh et al. (20) examined the temporal determinants of tyre flip performance in 5 athletes.
The purpose of this study, therefore, was to gain more insight into the kinematic characteristics of a strongman-type event: the heavy sprint-style sled pull. In particular, this study sought to (a) compare the acceleration and maximum velocity phase kinematics of the heavy sled pull, and (b) gain some insight into the kinematic determinants of this task by comparing the within- and between-subject fastest and slowest sled pulls. It was hypothesized that (a) the maximum velocity phase would involve significantly longer step lengths and step rates, shorter ground contact times, and a more vertical trunk posture than the acceleration phase, and (b) the faster trials in both the within- and between-subject analyses would be characterized by significantly greater step lengths and step rates and shorter ground contact times than the slower trials.
Methods
Experimental Approach to the Problem
The present study used a cross-sectional approach to examine the kinematic characteristics of a strongman-type event-the heavy sprint-style sled pull (hereafter known as the heavy sled pull)-with a load many times greater than examined in any other previous peer-reviewed study. The dependent variables were average velocity; step length; step rate; ground contact time; swing time; and the trunk, thigh, and knee angles at foot contact and toe-off. These variables were selected because all have some relevance to sprint performance (13,15) and all can be measured from a sagittal plane video analysis. Specifically, this study sought to determine how the kinematics of the sled pull (examined with 1 absolute load) may differ across what we have termed the “acceleration” and “maximum velocity” phases for (a) all the subjects' trials; (b) the fastest and slowest trial for each subject (to examine the within-subject effect of fatigue); and (c) the 6 fastest and 6 slowest of all trials irrespective of subject (to examine the between-subject differences in the best and worst trials). The probability of statistical significance was examined using 2-tailed paired T-tests for the within-subject comparisons and with independent 2-tailed T-tests for the between-subject comparisons. Cohen effect sizes were also used in all comparisons to determine the magnitude of effect.
Subjects
Eight male subjects initially participated in this study. All subjects were experienced in resistance training and commonly utilized exercises such as squats, deadlifts, and power cleans in their training programs. These subjects also all had at least some background in resisted sprint-style sled pulls, with 4 of the subjects having previously competed in at least 1 strongman competition that included the truck pull and other forms of sled pulls. The remaining 4 subjects were actively competing in powerlifting, bodybuilding, or American football. However, 2 of the 8 subjects were unable to complete all 3 trials of the sled pull and as such their data were not included in this study. The remaining 6 subjects who completed all trials were 27 ± 4 years old with a mass of 101 ± 12 kg and height of 184 ± 6 cm. Although the use of 6 subjects in this study compares favorably to the scientific literature on strongman training where 3 to 6 subjects have been used in each study (3,20,25), we acknowledge that our sample may still not be representative of how the heavy sled pull is performed by other athletes, particularly those of a more elite standard. Testing was conducted in early autumn with the majority of these subjects in their late pre-season or early in-season phase. Written informed consent was obtained from the subjects prior to their participation in this investigation. The investigation was approved by an Institutional Review Board for the use of human subjects.
Procedures
Subjects completed a gym-based warm-up consisting of 5 minutes of cycling and several submaximal sets of front squats, back squats, or power cleans for approximately 10 minutes. This was followed by 2 submaximal sets of the sled pull with loads of between 80 and 120 kg. In between sets of their warm-up, selected anatomical landmarks were marked on each subjects' body. These landmarks were the acromion process on the shoulder, the greater trochanter of the hip, the mid-point of the lateral joint line of the tibiofemoral (knee) joint, and the lateral malleolus of the ankle. After completing their warm-up and marker placement, all subjects performed 3 sets of heavy sled pulls over 25 m with the goal being to perform each set as quickly as possible.
Although inspection of the resisted sprinting literature indicates the use of both relative (i.e., a percentage of body mass) (24,29) and absolute loads (37), no studies have used loads that even approached body mass for this type of activity. Although this may reflect the at this point unsubstantiated recommendations of several authors who recommend loads no greater than 20% body mass (27,29), the focus of our study was on the use of heavy sled pulls that by virtue of their force-velocity characteristics would be better described as a specific strength/power rather than speed exercise. Another challenge in determining appropriate loads for this study was that the resistance for a horizontal exercise such as the sled pull is a product of the vertical (gravitational) force of the weighted sled and the coefficient of friction between the sled and ground. In the context of the present study, we felt that the use of the same absolute load for all athletes was more applicable than the use of relative (body mass-normalized) loads for 2 reasons: (a) strongman competitions use an absolute load for all athletes within a weight class when performing the truck or sled pull; and (b) in contact sports like American football and the rugby codes where very high levels of body momentum need to be generated when scrummaging and breaking or making tackles and so on, the resistance forces encountered will be absolute in nature, reflecting the mass of the opposition players with whom physical contact is made. Prior to conducting the study, the majority of potential subjects attended at least 1 familiarization (training) session on the surface where testing was to be conducted. These session(s) allowed us to determine a challenging yet realistic resistance that these subjects were likely to be able to pull for 3 sets of 25 m, each set separated by a 3-minute rest period. As a result of this process, a combined sled mass (including the sled, harness, chain, and weight plates) of 171.2 kg was used by all subjects for their 3 sets.
Because of the mass of the sled, all subjects were instructed to start in a 4-point power stance with each foot and hand on the ground and the hands positioned just inside the start line (Figure 1). The subjects were connected to the sled via a shoulder harness that was in turn connected to the sled by a sturdy chain. Once the sled started to move forward, the subjects were allowed to adopt a more vertical trunk posture and were encouraged to swing their arms as they would do in the acceleration phase of a sprint start. The subjects initiated the start of the sled pull on their own accord, with timing commencing when the first forward movement of the athlete was detected by the timer. The time for each set was recorded via stopwatch, with timing terminated when the subject reached the 25-m mark.
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Figure 1: Subject in the 4-point stance position about to start the sled pull. The subject is connected to the sled via the chest-mounted harness and the chain.
Figure 2 shows a schematic of the data collection procedures. The 2 cameras were set at approximate hip height for each subject at a distance of 11 m perpendicular to the intended direction of the sled pull. Each of the 2 cameras had a field of view of just more than 5 m. The first camera captured the first 5 m (0-5 m), which was considered the “acceleration phase,” and the second camera captured the last 5 m (20-25 m), which was referred to as the “maximum velocity” phase. Markers were placed along the course at 0, 5, 20, and 25 m to identify the acceleration and maximum velocity phases and to allow calibration of the step length distance.
Figure 2: Schematic of data collection procedures showing the location of the cameras, calibration markers, acceleration and maximum velocity phases, and the direction of the sled pull. Note: This diagram is not necessarily drawn to scale. ● = calibration markers,
Symbol = cameras.
Caption not available.
Data Analyses
The video footage collected from the 2 cameras (Sony, Tokyo, Japan, Pal, 50 Hz, 1/1,000 seconds) was captured, and a kinematic analysis was undertaken with Silicon Coach Pro video analysis software (Dunedin, New Zealand). Two linear kinematic (average velocity and step length), 3 temporal (step rate, ground contact time, and swing time), and 3 segment/joint-angle (trunk, thigh, and knee angles) variables were calculated for all full steps within the acceleration and maximum velocity phases of each trial. All 3 angles were measured at toe-off and foot contact. These 2 points of the gait cycle were defined as follows:
Toe-off
The first frame in which the foot lost contact with the ground, thus starting the swing phase.
Foot contact
The first frame in which the foot regained contact with the ground, thus starting the ground contact phase.
The 2 linear kinematic variables measured in this study were defined as follows:
Average velocity
This was calculated according to first principles by dividing the distance travelled in meters by the time taken in seconds.
Step length
The horizontal distance from the anterior-most point of the toes at foot (ground) contact to the corresponding ground contact of the contralateral (opposite) foot.
The 3 temporal variables that were measured in this study were defined as follows:
Step rate
The inverse of the step time, where step time is the sum of the ground contact and swing times.
Ground contact time
The time from foot contact to toe-off of the ipsilateral (same) foot.
Swing time
The time taken from toe-off to foot contact of the ipsilateral (same) foot.
The 3 segment/joint angles analyzed in this study (Figure 3) were defined as follows:
Figure 3: Schematic of the segment and joint angles calculated. An increase in trunk angle (A) was indicative of the trunk becoming more vertical. A positive thigh angle (B) meant that the thigh was anterior to the hip (related to hip flexion), and a negative thigh angle meant that it was posterior to the hip (related to hip extension). An increase in the knee angle (C) indicated extension of this joint.
Trunk angle (A)
The angle subtended from the shoulder, hip, and horizontal axis.
Thigh angle (B)
The angle subtended from the knee, hip, and vertical axis.
Knee angle (C)
The angle subtended from the hip, knee, and ankle markers.
Statistical Analyses
The dependent variables were calculated for all complete steps that were observed in the acceleration and maximum velocity phase of each trial. Standard descriptive statistics (means and standard deviations) were reported for the acceleration and maximum velocity phase for all statistical comparisons. The initial statistical comparison involved examining potential differences in the acceleration and maximal velocity phase for all 18 trials performed in this study (3 trials for each of the 6 subjects). This involved the use of 2-tailed paired T-tests and Cohen effect sizes (ES) to determine the probability of significance and the magnitude of effect, respectively. Further insight into the determinants of heavy sled pull performance was obtained by comparing (a) the fastest and slowest trial for each subject (within-subject effect); and (b) the 6 fastest and 6 slowest of all trials, irrespective of subject from the entire group (between-subject effect). These analyses were performed separately for the acceleration and maximum velocity phases and were conducted with ES and 2-tailed paired and independent T-tests, respectively. All statistical analyses were conducted using Excel with significance set at p ≤ 0.01 in an attempt to control for the relatively large number of statistical comparisons. In accordance with the updated effect size magnitudes for sport science research proposed by Drinkwater et al. (10), the magnitude of the effect given by the ES was defined as trivial for ES <0.2, small for ES = 0.2 to 0.6, moderate for ES = 0.6 to 1.2, and large for ES >1.2.
A power analysis was performed using data from Murphy et al. (28), who compared the slowest and fastest subjects' unresisted 15-m sprints. Using these data, our study had more than 80% power with a risk of Type I error of <5% for detecting significant differences in many of our dependent variables. Intratester reliability of all temporal measures was high (ICC = 0.95-0.99, coefficient of variation [CV] = 1.1-5.6%).
Results
The majority of the 25-m heavy sled pulls trials took ∼12 to 18 seconds to complete, with the range being 10 to 40 seconds. The overall group data (involving all 3 trials for all 6 subjects) for the acceleration and maximum velocity phases of the heavy sled pull are presented in Table 1. The acceleration phase was characterized by a significantly lower average velocity, step length, and swing time than the maximum velocity phase. Several significant differences in segment/joint angles were also observed with the maximum velocity phase tending to have a significantly more vertical trunk and greater thigh angle and knee extension than the acceleration phase at both foot contact and toe-off. The only exception to this was for the thigh angle at toe-off, where no significant difference was observed. Effect size analysis revealed that average velocity was the only variable in which the difference between the acceleration and maximum velocity phases was large.
Table 1: A comparison of the kinematics of the acceleration and maximum velocity phases of the heavy sled pull for the entire group.
Tables 2 and 3 present the within-subject comparisons, which examined the potential differences in the fastest and slowest trial for each subject in the acceleration and maximum velocity phases, respectively. During the acceleration phase, there was only 1 significant difference with the thigh angle at toe-off being greater in the faster trials than in the slower trials. In contrast, many significant differences were observed between each subject's slowest and fastest trial in the maximum velocity phase. The faster trials had a significantly greater average velocity, step length, step rate, and knee extension at toe-off than the slower trials. No large ES were observed for any of these comparisons.
Table 2: Within-subject analysis comparing the slowest vs. fastest trial for each subject in the acceleration phase.
Table 3: Within-subject analysis comparing the slowest vs. fastest trial for each subject in the maximum velocity phase.
Tables 4 and 5 report the data for the between-subject analyses, in which the 6 slowest and 6 fastest trials of the entire group are presented for the acceleration and maximum velocity phases, respectively. During the acceleration phase, the fastest trials had significantly greater step length and swing time than the slower trials. At both foot contact and toe-off, the fastest trials were also characterized by a significantly more vertical trunk and greater knee extension. Similar results were observed for the maximum velocity phase, where the fastest trials had significantly greater average velocity, step length, and step rate but less ground contact time than the slowest trials. The majority of the segment/joint-angle differences between the fastest and slowest trials were observed at foot contact, where the fastest trials had a significantly more vertical trunk and greater thigh angle and knee extension than the slowest trials. The only segment/joint-angle difference at toe-off was observed for the trunk angle where the fastest trials had a significantly more vertical trunk position. Effect size analysis revealed that for virtually all of the variables in which significant between-subject differences were detected for the acceleration and maximum velocity phases, the magnitude of these effects were large.
Table 4: Between-subject analysis comparing the 6 slowest and 6 fastest trials from the entire group in the acceleration phase.
Table 5: Between-subject analysis comparing the 6 slowest and 6 fastest trials from the entire group in the acceleration phase.
Discussion
To the authors' knowledge, this is the first biomechanical study of a sprint-style sled pull with loads greater than 32% body mass. Results of the present study were generally consistent with the initial hypotheses, whereby a number of significant differences in the sled pull kinematics were observed between the acceleration and maximum velocity phases and when comparing the within- and between-subject slowest and fastest trials.
When examining the overall groups' results for all 18 trials, the acceleration phase was characterized by a significantly lower average velocity and a shorter step length and swing time than the maximum velocity phase. The acceleration phase also tended to have a significantly more horizontal trunk, a smaller thigh angle, and less knee extension than the maximum velocity phase at both foot contact and toe-off. The majority of these differences are consistent with previous comparisons of the acceleration and maximum velocity phases of unresisted (7,13) and resisted sprinting (6,7,21,24).
The overall group data revealed no significant differences in the step rates and ground contact times between the acceleration and maximum velocity phase; if anything, the data revealed that the acceleration phase tended to have greater step rates and shorter ground contact times. Because such a result was in contrast to the literature for unresisted sprinting (12,13,15), we inspected each individual's data and found that the slowest 2 individuals' average velocity was greater in the acceleration than maximum velocity phase. This result appeared to reflect their greater ground contact time and resulting slower step rate for the maximum velocity than acceleration phase. We therefore felt that a better understanding of the kinematic determinants of heavy sled pull performance and hence its potential use in conditioning might be obtained by examining the within- and between-subject effects.
It is somewhat surprising that there appear to be no studies that have performed a within-subject analysis (for the same experimental condition) on either unresisted or resisted sprinting. By performing such an analysis, we could gain some insight into the effects of fatigue on heavy sled pull performance and how the prescription of such exercise may affect performance over multiple sets. When this comparison was conducted for the acceleration phase, the only significant within-subject difference was that their thigh angle at toe-off was greater in their fastest than slowest trial. This suggested that each subjects' fastest trial was characterized by a greater hip extension range of motion than their slowest trial. This supports the view that the hip extensors play an important role in generating propulsive anteroposterior forces during the acceleration phase of sprinting (4,36). During the maximum velocity phase, each subjects' fastest trial had a significantly greater average velocity, step length, step rate, and knee extension at toe-off than their slowest trial.
Another method we used to obtain greater insight into the determinants of heavy sled pull performance was a between-subject analysis that compared the slowest 6 and fastest 6 (of 18) trials performed, irrespective of subject. Although this type of approach has been performed in several unresisted sprint studies (13,28), no such studies have yet been conducted during any form of resisted sprinting.
Results of the between-subject analysis revealed many significant differences between the 6 slowest and 6 fastest trials. Within the acceleration phase, the fastest trials had significantly greater step length and swing time and a more vertical trunk and greater knee extension (at both foot contact and toe-off) than the slowest trials. Similar results were observed during the maximum velocity phase, with the fastest trials having a significantly greater average velocity, step length, and step rate and less ground contact time. The fastest trials also had a significantly more vertical trunk and greater thigh angle and knee extension, with these differences more pronounced at foot contact than toe-off.
Many of the significant differences between the fastest and slowest trials reported in both the within- and between-subject analyses were consistent with what would be expected based on the impulse-momentum relationship (18). This relationship states that the change in momentum (product of a system's mass and velocity) is equal to the magnitude of the impulse (product of force and time of force application) applied in the direction of motion. According to Hunter et al. (18), increases in step length and step rate, and ultimately sprinting speed, occur as a result of an increase in the propulsive anteroposterior impulse. Hence, the faster trials would have most likely been characterized by a greater propulsive anteroposterior impulse than the slower trials. Because results of the within- and between-subject analyses also indicated that the ground contact time tended to be significantly less for the fastest than slowest trials, the fastest trials would have had to have had substantially greater peak anteroposterior forces and rates of force development (RFD) than the slowest trials. Because peak force and RFD are 2 physical qualities that appear important in many sports (35) and because maximizing the anteroposterior forces and impulses is vital for sprinting (18), these results further suggest that the heavy sled pull may be a useful conditioning exercise for many athletes, particularly those in sports such as American football, the rugby codes, and strongman.
One of the primary roles of the conditioner is to provide appropriate instructions and feedback to their athletes during training. Research has shown that the type of instructions provided to athletes can influence the RFD and to a lesser extent the peak force produced (32). Because the generation of high levels of force and impulse appear necessary for the development of muscular hypertrophy, strength, and power (5), it would appear imperative that the conditioner provide instructions and feedback that assists the athlete to maximize these kinetic factors during resistance training. It is acknowledged that conditioners are unlikely to be able to routinely monitor the ground reaction forces and impulses during heavy sled pulls or any other exercise. However, they may utilize the naked eye or some simple video analysis of step length, step rate, ground contact times, and segment/joint angles to gauge each athletes' sled pull performance and gain some estimate of any changes/differences in their anteroposterior ground reaction forces and impulses. By keeping track of the athletes' technique and performance time across multiple sets, the conditioner may be in a better position to individualize the exercise prescription in terms of loads used, sets performed, distance of each sled pull, and rest periods utilized.
Practical Applications
Many significant differences were found in the sled pull kinematics between the acceleration and maximum velocity phase. This suggests that conditioners need to think about how the sled pull loads and pulling distance may alter the kinematics and what combinations of these variables might best replicate the demands of the athletes' sport or address some of their relative physical weaknesses. Results of the within- and between-subject analyses revealed that the slowest heavy sled pulls were characterized by significantly shorter step lengths and slower step rates than the fastest trials. Because the reduction in step length and step rate for the slowest trials was associated with a greater increase in the ground contact rather than swing time, it suggests that the ability to produce very large propulsive anteroposterior forces and impulses during relatively short periods of ground contact is critical for successful heavy sled pull performance. Consequently, athletes in sports such as American football, the rugby codes, and strongman who experience many forceful collisions and/or need to tow large resistances may benefit from using heavy sled pulls as a specific strength and power exercise. Because the development of maximal strength and power requires moderate to long rest periods, conditioners may wish to minimize the degree of fatigue across multiple sets of heavy sled pulls. Our results suggest that as long as each sled pull is performed for relatively short durations (i.e., <20 seconds), well-trained subjects should be able to perform 3 sets of heavy sled pulls with a 3-minute rest between sets with minimal detriments in performance and change in overall technique. However, because each athlete is different, conditioners may need to pay attention to some of the main kinematic characteristics (e.g., step length, step rate, and some segment/joint angles) of the heavy sled pull to better understand the acute response and the likely chronic effect of such training on each individual athlete.
Acknowledgments
We wish to thank all of the subjects who volunteered to take part in this study. We have no relationship with any companies that could benefit from the results of the present study. The results of this study do not constitute endorsement of the product by the authors or the NSCA.
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