Sprint running speed is a fundamental fitness quality in many team sports, such as soccer, rugby, and basketball, and improving this fitness quality is one of the main goals of athletes' physical preparation in those sports (20). In particular, the ability to rapidly increase sprint velocity over short distances, which is often termed sprint acceleration ability, is considered to be of major importance because sprint efforts are typically of short duration (e.g., 10–20 m or 2–3 seconds) during team-sport competition (20,21).
Although actual practice of sprint skill is the most specific and essential method for enhancing sprint performance, other types of training such as weight training, plyometric training, flexibility training, and assisted sprint training are also used supplementary to the development of sprint acceleration ability (7,9,23). Resisted sprint training is one such supplemental training method, in which athletes sprint with added loads such as uphill running, weighted sled towing, and weighted vest sprinting (5,12). Weighted sled towing is a commonly used form of resisted sprint training, which uses mass (inertia) of a sled and the friction between the sled and the ground surface as external resistance to sprint movement (5,22). It is assumed that directly adding an external load to sprint movement in the form of a weighted sled increases the demand for horizontal ground reaction force (GRF) impulse production at each ground contact. Repetitively applying such a stimulus over time is likely to induce specific adaptations within the neuromuscular system allowing larger horizontal GRF impulse production, which would theoretically lead to increased step length, thereby improving sprint performance under unresisted conditions (5,15,22).
Despite the popularity of weighted sled towing and seemingly logical assumption behind its mechanisms, little experimental evidence exists to support its effectiveness. To the best of our knowledge, only 3 peer-reviewed studies have reported the effects of training with weighted sled towing on sprint acceleration ability. Zafeiridis et al. (24) compared the effects of 8 weeks of weighted sled towing with 5 kg and unresisted sprint training and found that the sled towing group significantly improved sprint acceleration performance (0–20 m), whereas the unresisted group did not. Spinks et al. (22) conducted an 8-week training study comparing the effects of weighted sled towing with 12.6% of body mass vs. unresisted sprint training. They reported that the weighted sled towing significantly improved sprint acceleration performance (0–15 m) compared with a nontraining control group, but the improvement was not significantly different from that of unresisted sprint training. Harrison and Bourke (11) examined the effect of adding weighted sled towing with 13% of body mass to normal rugby training, which included (unresisted) speed training drills, over 6 weeks. They found that the addition of weighted sled towing to the rugby training improved sprint acceleration performance (0–5 m) to a greater extent than normal rugby training alone. Viewed collectively, the limited existing literature indicates that short-term training (≤8 weeks) with weighted sled towing is more effective than or at least as effective as unresisted sprint training for improving sprint acceleration performance.
Controversy exists as to the optimal training load of weighted sled towing for enhancing sprint ability (5). Coaches and sports scientists generally recommend that athletes perform weighted sled towing with relatively light external resistance, which is ≤10% of body mass, or that which reduces sprint velocity by ≤10% (1,12,14,15,22). The rationale behind this so-called “10% rule” is the belief that applying heavy external resistance (>10%) to sprint movement would alter sprint mechanics excessively, and the long-term use of such heavy resistance results in chronic deterioration of sprint technique and performance under unresisted conditions (1,15). It is also argued that the use of light training loads is necessary to ensure the specificity of weighted sled towing (1,12,15). However, this line of reasoning against the use of a heavy training load for weighted sled towing is largely based on coaches' subjective opinions, and no research has provided evidence to support or refute it (12,14,15). Moreover, it could be argued that performing weighted sled towing with a light load may not provide sufficient contrast to practicing sprint skill under unresisted conditions, questioning the use of external resistance at all.
No previous research has compared the training effects of weighted sled towing with different magnitudes of external resistance in a single study and specifically tested the “10% rule.” Therefore, the purpose of this study was to compare the training effects of weighted sled towing with 2 different external loads that reduced sprint velocity by approximately 10 and 30% and to determine the effect of the magnitude of a training load in weighted sled towing on the development of sprint acceleration ability. We hypothesized that the use of heavier load would provide greater training overload and improve sprint acceleration performance to a greater extent than the light load. Information derived from this study should help validate or dismiss the “10% rule” for weighted sled towing and would allow coaches to better prescribe weighted sled towing for their athletes.
Experimental Approach to the Problem
This study used a 2-group, randomized, longitudinal experimental design to compare the training effects of weighted sled towing between heavy and light loads. All subjects attended a baseline testing session consisting of a 10-m sprint test before the initiation of the training intervention. Subsequently, the subjects were matched for 10-m sprint time in the pretest and allocated to either a heavy-load weighted sled towing group (heavy group; n = 10) or a light-load weighted sled towing group (light group; n = 11). Each group had similar sprint acceleration ability at baseline (Table 1). As the independent variable was the “magnitude of external load,” we did not include any control group (either unresisted sprint training group or no sprint training group) in this study. Within 7 days of the completion of their pretraining test, all subjects commenced a supervised 8-week training program, in which the subjects in the heavy and light groups performed weighted sled towing twice a week using external loads that reduced 10-m sprint velocity by approximately 30 and 10%, respectively. Within 10 days of the completion of the 8-week training intervention, the subjects repeated the same test as performed before the training and, then, changes in the dependent variables before and after training were compared between the groups.
Twenty-one physically active men who had experience in team sports such as soccer, basketball, field hockey, and Australian Rules football were recruited for this study. Their performance standards ranged from recreational performers to regional-level athletes. When this study was conducted, the subjects were either in their off-season or preseason preparation period, and none of them were in the competition period. The sample size was determined based on the previous studies that examined the effects of weighted sled towing on sprint acceleration performance (11,22,24). The subjects' characteristics are shown in Table 1. The subjects were instructed to maintain their normal diet and training pattern for the duration of the study.
This project was reviewed and approved by the institutional ethics committee. All the subjects were informed of the study requirements, benefits, and possible risks and gave their written informed consent before the study began. From two of the subjects who were under 18 years of age, informed consent was obtained also from their parents/guardians.
The subjects participated in 2 supervised sprint training sessions per week (on inconsecutive days) for 8 weeks, which were performed on an outside grass field. Each training session began with a thorough warm-up consisting of light jogging (5 minutes), dynamic stretching exercises (approximately 10 minutes), and submaximal unresisted 10-m sprints at 50, 75, and 90% of maximal intensity, each performed for 1 repetition. Subjects then performed a series of maximal-effort weighted sled towing over 5–15 m with 1–2 minutes of rest between repetitions (Table 2). The sprint distances of 5 and 15 m were included in the training program in addition to 10 m to add variety to the program, so that the subjects could work technically on the early acceleration (5 m) and the longer acceleration (15 m). The sprint training program followed a periodized plan to allow progression and to prevent overtraining. The total number of sprints and the total sprint distance covered were matched between the groups. However, the external loads added were different between the groups such that the heavy group used a weighted sled that reduced 10-m sprint velocity by approximately 30% relative to the unresisted condition, whereas the light group used a weighted sled that reduced 10-m sprint velocity by approximately 10%. The selection of this training load for the light group was based on what coaches and sports scientists generally recommend as the optimal training load of weighted sled towing (14,15). On the other hand, the training load for the heavy group was chosen arbitrarily to examine the training effect of weighted sled towing with an external load that is greater than what is typically recommended. Note that we use the terms “light” and “heavy” in this study to describe the magnitudes of the 2 added loads in a relative (rather than absolute) sense. During weighted sled towing, the participants were connected to the sled by means of a shoulder harness that was in turn connected to the sled by a connecting lead.
Training Monitoring and Sled Mass Adjustment
In the first training session of the 8-week training period, the absolute sled mass for each subject was selected based on his body mass, referring to the method of Lockie et al. (15). In the subsequent training sessions, the sled mass was adjusted individually using the following procedure: (a) At the end of a warm-up, the subjects performed 3 maximal unresisted 10-m sprints without towing a sled, which were timed using timing lights (Swift Performance Equipment, Lismore, Australia); (b) The subjects completed the prescribed weighted sled towing program as shown in Table 2, during which the 10-m sprint time was recorded under resisted conditions using timing lights; and (c) After the training session, the 10-m sprint velocity under unresisted and resisted conditions were compared, and the absolute sled mass was adjusted individually for the subsequent training session, so that the subjects always sprinted at the prescribed percentages of maximal unresisted sprint velocity (i.e., approximately 70 and 90% for the heavy and light groups, respectively). This adjustment procedure was performed once a week for a total of 8 times over the 8-week training intervention period.
Before and after the 8-week training period, the subjects performed a 10-m sprint test. The time of day for testing was standardized (±2 hours) on each testing occasion, and the subjects were instructed to refrain from strenuous exercise for 24 hours before each testing session. The subjects were also requested to be rested, normally hydrated, and abstain from eating 2 hours before testing. During testing and training, strong verbal encouragement was provided to increase motivation levels.
10-m Sprint Test
This test was used to quantify sprint acceleration ability. Although “acceleration” is physically defined as the rate of change in velocity, we used 10-m sprint time as a working/practical definition of sprint acceleration ability in this study (18). The test distance of 10 m was chosen because (a) it is often used to assess sprint acceleration ability of team-sport athletes (2,10), (b) mean sprint distances during team-sport competitions are typically 10–20 m (21), and (c) the initial 10 m of sprint running has been shown to be a specific component representing initial acceleration ability (8).
The 10-m sprint test was conducted indoors on a hard flat surface. Before the sprint trials, the subjects undertook a standardized warm-up that involved light jogging, dynamic stretching exercises, and familiarization with the sprint protocol at submaximal intensity. Subjects then performed 6 maximal-effort sprints over 10 m from a parallel standing start (6); the subjects started in a standing position with the toes of both feet parallel at 0.3 m behind the initial timing gate and with the first step moving forward (no step backward was allowed). Rest periods of approximately 2 minutes were allowed between trials to ensure a maximal effort and minimum fatigue for each sprint (3). The split time at 5 and 10 m from the start line was measured using a dual-beam electronic timing system (Swift Performance Equipment), with an accuracy of 0.01 seconds. The mean sprint time of the 3 best trials was used for statistical analyses.
Ground Reaction Force Analyses
During the 10-m sprint tests, GRFs were collected at a sampling frequency of 1,000 Hz, separately at the first ground contact and at 8 m from the start using 3 force plates (Type 9287BA; Kistler Instrument Corp., Winterthur, Switzerland) recessed in the floor, giving a total length of 2.7 m of measurement surface. Because it was not possible for us to simultaneously record GRF at the 2 different points in the same trial, GRF at the first ground contact after the start was collected in the first 3 trials, then we changed the start line and the placement of the timing gates, and GRF at 8 m from the start was recorded in the subsequent 3 trials. The recorded GRF data were filtered using a fourth-order recursive, zero phase-shift, Butterworth low-pass filter with a cutoff frequency of 100 Hz. Then, we calculated resultant, vertical (effective), net horizontal, propulsive, and braking impulses, using methods described by Kawamori et al. (13), and all the GRF impulses were normalized to body mass (17). The GRF variables from the 3 trials were averaged separately for each recording point to be used for statistical analyses.
During the 10-m sprint tests, sagittal plane video data were recorded at the first ground contact and at 8 m from the start using a digital video camera (MV920; Canon, Inc., Ohta-ku, Japan) that was placed perpendicular to the sagittal plane of motion at a distance of 6 m from the midline of the running lane. The shutter speed of the video camera was 1/500 seconds, and the camera view was calibrated using a frame of known dimension. As already described, video at the first ground contact after the start was recorded in the first 3 trials, and then video at 8 m from the start was recorded in the subsequent 3 trials. To analyze the recorded sagittal plane video data, video analysis software (siliconCOACH Pro; siliconCOACH Ltd., Dunedin, New Zealand) was used. Step length was the horizontal distance between the point of touchdown of 1 foot and that of the following touchdown of the opposite foot, and step frequency was the reciprocal of step time (i.e., time taken to complete a step). Only 1 step was analyzed for each point (i.e., the first ground contact and 8 m). It should be noted that although step cycles analyzed are those “around” the first ground contact and 8 m rather than exactly “at” the first ground contact and 8 m, the expression “at” will be used for simplicity. Similarly, the expression “GRF recorded at 8 m” will be used although it is “GRF recorded around 8 m” to be exact. Video-derived variables from the 3 trials were averaged separately for each recording point and used for statistical analyses.
The reliability of measures derived from the 10-m sprint test (time, GRF) was determined in our previous study (13) using a similar group of subjects and the same investigator. The intraclass correlation coefficient for 10-m sprint time and GRF ranged between 0.56 and 0.74, and 0.57 and 0.83, respectively.
An independent t-test was performed on the baseline measures to determine if the 2 groups were evenly matched before the training intervention. A 2-way repeated measures ANOVA with “group” (heavy and light) as the between-subject factor and “time” (pretest and posttest) as the within-subject factor was used on each dependent variable to examine difference in the change over time between heavy and light groups. When a significant F value was found, least significant difference post hoc tests were performed. The level of statistical significance was set at P < 0.05. Statistical analyses were performed using the software package SPSS version 16.0 (SPSS, Inc., Chicago, IL, USA). All raw data are shown as mean ± SD.
No significant differences between the groups were evident for any variables before the training (Tables 3–5).
There was no difference in the number of training sessions attended by each subject between the groups over the course of the study (heavy, 84% and light, 84%). Missed training sessions were rescheduled, but complete compliance could not be achieved because of the reasons such as minor injuries (not related to the experiment), illness, weather (rain), travel, and competition commitments.
Training Monitoring and Sled Mass Adjustment
Over the 8-week training period, the subjects in the heavy and light groups used sleds weighing 33.1 ± 5.9 and 10.8 ± 2.3 kg, corresponding to 43.0 ± 7.0 and 13.1 ± 2.1% of their body mass, respectively. The use of such sled mass resulted in decrements in 10-m sprint velocity by 28.1 ± 1.5% (heavy) and 9.7 ± 1.3% (light). How some of these variables changed over the course of the 8-week training period is presented in Figure 1.
Effects of Weighted Sled Towing
10-m Sprint Test
The 10-m sprint time significantly decreased from pre- to posttraining by 5.0 ± 3.5% in the heavy group (p < 0.001) and by 3.0 ± 3.5% in the light group (p = 0.008) (Table 3). No significant group × time interaction was found for 10-m sprint time. Over the first 5 m, sprint time significantly decreased from pre- to posttraining by 5.7 ± 5.7% in the heavy group (p = 0.004), but the 2.8 ± 4.9% change in the light group did not reach statistical significance. No significant group × time interaction was found for 5-m sprint time.
Ground Reaction Force Analyses
At the first ground contact after the start, no significant group × time interaction was observed for relative impulses (Table 4). Similarly, there were no significant changes from pre- to posttraining in relative impulses.
At 8 m from the start, significant group × time interactions were found for resultant impulse and vertical impulse but not for the other impulse measures (Table 5). The heavy group significantly decreased resultant impulse by 4.3 ± 6.7% (p = 0.019) and vertical impulse by 11.5 ± 13.7% (p = 0.005) from pre- to posttraining, whereas the light group did not. The other GRF-related measures did not change significantly from pre- to posttraining in either group, with no significant group × time interactions.
At the first ground contact after the start, no significant group × time interaction was found for step frequency and step length (Table 4). The heavy group significantly increased step length by 8.1 ± 10.8% from pre- to posttraining, whereas the light group showed no significant change. Step frequency did not change significantly from pre- to posttraining in either group.
At 8 m from the start, no significant group × time interaction was observed for step frequency and step length (Table 5). The heavy group significantly increased step frequency from pre- to posttraining by 8.1 ± 9.0%, whereas the light group showed no significant change. Step length increased significantly from pre- to posttraining by 5.1 ± 7.3% in the light group but not in the heavy group.
The main finding of this study is that 8-week training programs of weighted sled towing with heavy or light training loads that reduced sprint velocity by approximately 30 and 10%, respectively, were similarly effective in improving 10-m sprint performance. It is often recommended that weighted sled towing be performed using relatively light external resistance such as a load that reduces sprint velocity by ≤10% (i.e., 10% rule) based on the assumption that adding too heavy external resistance to sprint movement would disrupt running mechanics and eventually lead to decreased sprint performance (15). However, the lack of significant difference observed in this study between heavy and light groups in the pre-post changes in 10-m sprint performance does not support such a contention. In fact, 5-m sprint performance improved only in the heavy group, indicating the potential advantage of using a heavier sled in developing sprint performance in the initial acceleration phase when loads are high and movement speed is relatively lower.
Theoretically, the heavier the added load in weighted sled towing, the greater the training overload, but the less specific the training movement becomes to unresisted sprint running. It may be that the balance between “overload” and “specificity” in a training stimulus will ultimately determine the training outcome, and the training load that optimizes such a balance would maximize the training effect of weighted sled towing. Alternatively, it could be argued that weighted sled towing is effective in improving sprint performance not because it is specific to sprint movement but rather the neuromuscular adaptations it causes are what is required for improving sprint acceleration performance. If this is the case, then the fact that weighted sled towing is movement specific to normal sprint movement is irrelevant, and the fact that adding heavier load in weighted sled towing brings about larger acute changes in sprint kinematics (15,19) should not be a concern in selecting training load for weighted sled towing. Either way, we examined only 2 training loads in this study, and thus, it is unknown whether the use of even lighter or heavier external resistance (e.g., loads that reduce sprint velocity by approximately 5 or 50%) improves sprint acceleration ability to a greater extent. Thus, additional research examining various loads other than those used in this study is required to identify the true optimal training load of weighted sled towing.
Regarding the video and GRF analyses of the 10-m sprint, the most notable finding is that the heavy group significantly decreased resultant and vertical impulses at 8 m from pre- to posttraining and such changes were significantly larger than those in the light group. On the other hand, horizontal components of impulses did not show any significant changes in either group. These findings are unexpected because we originally hypothesized that training with weighted sled towing would improve impulse production especially in a horizontal direction, which would in turn increase step length, and that such changes would be magnified by using heavier resistance. This hypothesis, especially the former part, is common among coaches and sports scientists (15) but not supported by our findings. On the contrary, the improvement in 10-m sprint time by heavy-load weighted sled towing in this study was accompanied by the increased step frequency, which may be attributed to decreased vertical impulse production, both observed at 8 m from the start. Based on such findings, it seems necessary to revise our understanding of potential mechanisms of weighted sled towing to improve sprint performance; i.e., weighted sled towing especially with heavy loads improves sprint acceleration performance by teaching athletes to direct GRF impulse more horizontally, not necessarily by allowing athletes to produce larger horizontal or resultant GRF impulse. In fact, recent biomechanical research shows that the orientation or the angle of GRF applied to the ground during the contact phase is more important to sprint acceleration performance than the total amount of GRF (13,16), indicating that this is a favorable change for sprint acceleration. If this new hypothesis regarding the potential mechanism of weighted sled towing is correct, then weighted sled towing could be regarded as a skill practice exercise more than a strengthening exercise. That is, it does not necessarily strengthen the neuromuscular systems involved in sprinting in a specific manner, as many believe, but rather it teaches a more efficient/effective way to apply GRF impulse while sprinting, which has more to do with motor control than muscular strength improvement per se. Alternatively, it is still possible that weighted sled towing does improve neuromuscular capacities involved in sprinting in a specific manner, and it is these stronger and more powerful muscles that allow athletes to direct GRF impulse more horizontally, which was not possible with weaker muscles.
It is important to recognize the fact that the subjects in this study performed not only weighted sled towing but also a considerable volume of unresisted sprints within our training program as a part of warm-up and sled mass adjustment procedure and also outside our program during skill practice sessions and competitions. This perhaps allowed them to practice with their more efficient/effective motor pattern and/or stronger and more powerful muscles (developed by weighted sled towing) under unresisted conditions, thereby potentially facilitating the transfer of training effects. If the subjects performed weighted sled towing only and nothing else, the results might have been different. However, such a training program (i.e., weighted sled towing only) is not realistic for competitive athletes, and this should not be a concern for coaches.
A unique feature of this study was that we prescribed the training load (i.e., sled mass) individually for each subject based on percent decrements in sprint velocity. Such a method is different from those of previous studies in which a training load was chosen as absolute sled mass (e.g., 5 kg) for all the subjects (24) or as percentages of body mass (e.g., 12.6–15% of body mass) (11,22). We believe our method was more appropriate because it allows the training load to be tailored for each athlete regardless of his/her body mass, sprint ability, muscular strength, or any other factors that may influence the degree of decrement in sprint velocity when towing a weighted sled. In addition, the training load selected this way is not affected by factors such as running surface and sled characteristics, which influence the magnitude of friction (force) and change the external resistance applied to an athlete, even when towing a sled of the same absolute mass. Such a sled mass adjustment and monitoring strategy may be perceived as too time-consuming and not practical, but worked well in this study. Moreover, a frequent measurement and feedback of sprint time has an added benefit of motivating athletes, which is worth the time and effort in actual training settings.
Before concluding, we must highlight several limitations/delimitations of this study. First, the results of this study are likely to be specific to the use of weighted sled towing for improving sprint performance in the acceleration phase. Thus, they may not apply to other types of resisted sprint training (e.g., uphill running or weighted vest sprinting) or other phases of sprint running (e.g., maximal speed or speed endurance). In fact, there exist a few research studies indicating that weighted sled towing may not improve maximal speed ability or longer sprint performance as much as unresisted sprint training does (4,24). Second, we did not control the subjects' involvement in other types of training/exercises outside the scope of this study (e.g., sports practice, weight training, and endurance training) during the intervention period, although we asked the subjects to maintain their normal training pattern. It is possible that concurrently performing other types of training affects the weighted sled towing effect either positively or negatively, and therefore this issue needs to be addressed in future studies with tighter control. Third, although the total sprint training distance was matched between the groups, training volume in terms of mechanical work performed was most likely different (i.e., heavy > light) because towing a heavier sled requires greater work output. Therefore, any differences in change scores observed between the groups cannot be attributed solely to a load effect, and may be also because of the difference in the total work performed. Finally, the subjects in this study were either competitive (regional level) or recreational athletes. Thus, the applicability of the findings of this study to athletes of higher competition levels (e.g., elite or high-performance) may be limited.
In conclusion, both heavy- and light-load weighted sled towing were equally effective for improving 10-m sprint performance, but only heavy-load weighted sled towing improved 5-m sprint performance. Thus, our findings did not validate the typical coaching guideline of using relatively light load (i.e., 10% rule) and indicate the potential benefit of using a heavier load for weighted sled towing.
Heavy- and light-load weighted sled towing were equally effective in improving sprint acceleration ability over 10 m, but only the use of heavy load improved 5-m sprint performance. Therefore, it is conclusive that coaches and athletes should abandon the myth regarding the optimal training load of weighted sled towing (i.e., 10% rule) and should explore the use of heavier external resistance for weighted sled towing.
We thank David Bell for his help with data collection. We also thank Jonathon Green for his technical assistance. There was no financial assistance with the project.
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