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The Longitudinal Effects of Resisted Sprint Training Using Weighted Sleds vs. Weighted Vests

Clark, Kenneth P1; Stearne, David J1; Walts, Cory T2; Miller, Anthony D1

Journal of Strength and Conditioning Research: December 2010 - Volume 24 - Issue 12 - p 3287-3295
doi: 10.1519/JSC.0b013e3181b62c0a
Original Research
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Clark, KP, Stearne, DJ, Walts, CT, and Miller, AD. The longitudinal effects of resisted sprint training using weighted sleds vs. weighted vests. J Strength Cond Res 24(12): 3287-3295, 2010-The purpose of this study was to determine the longitudinal effects of weighted sled (WS) and weighted vest (WV) sprint training on maximum velocity sprint performance and kinematics. Twenty male collegiate lacrosse players were randomly assigned to a WS group (n = 7) towing 10% body mass, a WV group (n = 6) loaded with 18.5% body mass, or an unresisted (UR) active control group (n = 7). All subjects completed 13 training sessions over 7 weeks. Pre- and post-test measures of sprint time and average velocity across the distance interval of 18.3 to 54.9 m were used to assess sprint performance, whereas high-speed video (300 Hz) and motion-analysis software were used to analyze stride length, stride rate, ground contact time, and flight time. A 3 × 2 repeated measures analysis of variance was performed for each dependent variable and revealed no significant between-group differences for any of the sprint performance or kinematic stride cycle measures. Effect size statistics suggested small improvements in 18.3- to 54.9-m sprint time and average velocity for the UR group but only trivial improvements for the WS and WV groups. With regard to sprint performance, the results indicate that WS and WV training had no beneficial effect compared with UR training. In fact, for the loads used by WS and WV in this study, UR training may actually be superior for improving sprint performance in the 18.3- to 54.9-m interval.

1Human Performance Laboratory, Department of Kinesiology, West Chester University of Pennsylvania, West Chester, Pennsylvania; and 2Athletic Department, Haverford College, Haverford, Pennsylvania

Address correspondence to Kenneth P. Clark, KC652700@wcupa.edu.

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Introduction

Linear sprinting speed is critical to performance in many sports and is widely considered a primary factor in athletic performance evaluation (27,30). Although certain genetically determined physical qualities (such as proportion of fast twitch muscle fibers) may influence sprinting ability (19), emerging evidence suggests that sprint training techniques may be effective. Linear sprinting is composed of acceleration and maximum velocity phases, and performance in each phase may be evaluated discretely and therefore may require specific training methods (1,30). Various longitudinal sprint-specific training protocols have been implemented with the goal of increasing sprinting speed (14,15,21,22,24,28,33). However, coaches and athletes continue to seek the most effective means for enhancing sprinting performance.

Recent coaching literature has suggested performing resisted sprints while towing a weighted sled (WS) or wearing a weighted vest (WV) (8,30). When loaded appropriately and performed correctly, these modalities should overload the athlete while still retaining a high degree of sprint training specificity. Despite the theoretical benefits associated with weighted sled or weighted vest sprint training, research validating these training methods for improving maximum velocity sprinting performance is limited. Several studies have examined the effects of weighted sled towing, but most of these have focused on acceleration performance (17,18,20,28). Only a few studies have investigated the use of weighted sleds on maximum velocity sprint performance (1,2,14,33), and neither the longitudinal effects of this training modality nor the ideal training load has been clearly determined for this phase of sprinting (1,2). Although several prior investigations have analyzed the longitudinal effects of wearing weighted vests on athletic performance (3-5,26), subjects in all of these 3-week “hypergravity” studies wore the vests for all waking hours of the day (including training sessions), and thus applications to more typical training settings are problematic. The few studies examining the effects of weighted vest/belt sprinting in a conventional training environment have investigated acute kinematic changes (1,9), and empirical evidence supporting the longitudinal efficacy of this training modality for improving maximum velocity sprinting is needed (8).

Additionally, it has been suggested that training with weighted sleds and weighted vests may elicit longitudinal changes in sprinting performance through different kinematic mechanisms. Because of the acute increases in muscular force output required to overcome the drag of a weighted sled, it has been suggested that a longitudinal effect of weighted sled towing may be an increase in propulsive forces generated by the leg musculature and thus an increase in stride length (2,17). Longitudinal training with weighted vests has been suggested to increase the eccentric strength of the leg extensor muscles during the braking phase of ground contact and to increase muscle and leg spring stiffness, potentially decreasing ground-contact time and thus increasing stride rate (8,30). However, these proposed kinematic adaptations are theoretical only at this point and require empirical validation.

Therefore, the purpose of this study was to determine the longitudinal effects of resisted sprint training using weighted sleds or weighted vests and to compare the relative effectiveness of each of these training methods for improving maximum velocity sprint performance. Furthermore, this study aimed to investigate the longitudinal kinematic adaptations elicited by these resisted sprint training modalities. The primary hypothesis was that subjects training with the weighted sleds and weighted vests would demonstrate significantly greater improvements in maximum velocity sprint performance than subjects completing sprint training without resistance. Two corollary hypotheses were that subjects training with weighted sleds would demonstrate increases in stride length that were significantly greater than the other experimental groups, whereas subjects training with weighted vests would demonstrate increases in stride rate (achieved through decreases in ground contact time) that were significantly greater than the other experimental groups. This study was designed to provide coaches and athletes with valuable insight into the most effective methods for improving maximum velocity sprint performance.

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Methods

Experimental Approach to the Problem

A pre-test, post-test randomized group design utilizing 3 training groups was employed to examine the longitudinal effects of training with weighted sleds and weighted vests in trained collegiate male lacrosse players. After pre-testing, subjects were randomly assigned to 1 of 3 treatment conditions: (a) sprint training while towing weighted sleds (WS), (b) sprint training while wearing weighted vests (WV), and (c) an active control group that completed all sprint training while unresisted (UR). All training groups completed 13, 60-minute training sessions spaced over 7 weeks. The effects of the experimental training protocol were determined by pre- and post-tests of sprint time and average velocity for the distance interval of 18.3 to 54.9 m (20-60 yards) and by the kinematic parameters of stride length, stride rate, ground contact time, and flight time.

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Subjects

Twenty-five male National Collegiate Athletic Association (NCAA) Division III lacrosse players originally volunteered for the study. During the course of the experiment, 2 subjects incurred muscle strains during training and could not complete the experimental protocol, and 3 subjects had to drop out for reasons unrelated to the training protocol; thus, 20 subjects were used in the final statistical analysis (subject characteristics are presented in Table 1). This subject pool was selected for several reasons. First, these varsity-level collegiate athletes participated in the study shortly after the conclusion of their 6-week NCAA nontraditional fall lacrosse season and were therefore in a trained state of physical fitness. Second, subjects of this age, training experience, and physical profile were considered most likely to possess the base levels of strength and anaerobic power necessary for the safe and effective completion of the experimental training protocol. Finally, none of the subjects had previously completed any resisted sprint training.

Table 1

Table 1

This study was approved by the college's Institutional Review Board. Prior to participation, each subject signed a written informed consent document in accordance with the Institutional Review Board's policies for use of human subjects in research. None of the subjects had any physiological or orthopedic limitations that could have affected performance as determined by completion of a health history questionnaire before beginning the study.

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Procedures

Data Collection and Data Analysis

The effects of the experimental training protocol were determined by pre- and post-tests of maximum velocity sprinting speed. Previous literature has indicated that elite sprinters typically accelerate for 30 to 50 m, reach maximum velocity somewhere between 30 and 60 m, and decelerate after 60 m (6,19). However, it has also been noted that the acceleration phase may be shorter than 30 m depending on an athlete's physical profile and sprinting ability (32). Therefore, maximum velocity sprinting performance for subjects in the present study was assessed by measuring their sprint times and average velocity across the distance interval of 18.3 to 54.9 m.

Pre-testing was conducted when subjects returned from their college fall vacation, which allowed them to have a 6-day rest interval between the end of their NCAA nontraditional fall lacrosse season and the pre-test. First, all subjects were measured for standing height and body mass. Next, the subjects completed a standardized 10-minute warm-up, including jogging and a variety of movement drills and dynamic stretches designed to prepare them for maximum-effort sprinting. Following a 5-minute recovery period after the warm-up, the subjects completed 3 maximum-effort 54.9 m sprints. The subjects were permitted a 5-minute rest period in between sprints to allow for recovery. The fastest trial for each participant was used in statistical analysis. All pre- and post-test sprinting trials were completed on a hardwood floor in the school's indoor gymnasium.

A laser timing device (Brower Timing Systems Speed Trap II, Salt Lake City, Utah, USA) was used to measure sprinting time in the 18.3- to 54.9-m interval. The laser timing gates were raised to a height of 0.9 m and placed at a width of 1.0 m apart. The first timing gate was placed at the starting line and subjects were positioned in a self-selected 2-point upright starting stance with the front foot 0.9 m behind the starting line (to avoid accidentally triggering the first timing gate with a forward body lean in the stance). Subjects were required to begin the sprint from a completely motionless stance but could start at their own initiative. Interval times were recorded as each subject crossed the timing gates at 18.3 m and 54.9 m to produce maximum velocity data.

To allow for kinematic analysis, a video camera (Casio Exilim EX-F1 camera, Tokyo, Japan) was placed 40.7 m from the first timing gate. The camera was positioned 9.1 m from the proximal edge of the sprinting lane and aligned directly perpendicular to the subjects' line of movement, providing a field of view from the 37.5-m to the 43.9-m mark of the sprint. A complete diagram of the experimental set-up is presented in Figure 1. Subjects were filmed at 300 Hz, and the first complete stride cycle (2 steps) in the camera's field of view was used in the data analysis.

Figure 1

Figure 1

Video from the data collection was processed and analyzed using OpenTrack Motion Analysis Software version 0.2.7. The stride cycle was defined as 2 complete steps, beginning with the instant of ground contact on 1 foot and ending with the next ground contact of the same foot. Stride length was defined as the horizontal distance between successive ground contacts of the same foot. Stride rate was defined as the number of complete strides taken per second, calculated as the inverse of the time required to complete a full stride cycle (1/total stride cycle time). Ground contact time was counted from the instant of initial ground contact until the instant of takeoff on the same foot and thus was calculated as an average for the 2 steps composing the stride cycle. Flight time was counted from the instant of takeoff on 1 foot until the instant of initial ground contact on the contralateral foot and thus was also calculated as an average for the 2 steps composing the stride cycle. Intrarater reliability for the kinematic analyses was calculated with the intraclass correlation coefficient (ICC) (11). ICC values ranged from 0.93 to 0.99, indicating that the kinematic data showed acceptable reliability (23).

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Determination of Resistance Loads for Use in Training

With regard to the resistance loads for the WS and WV groups, previous literature has not yet determined the ideal load to maximize training benefits with weighted sled towing, and various loads have been used in prior studies on WS towing (1,2,14,16-18,20,28,33). Furthermore, there has been very little prior research on appropriate loading schemes or protocols for weighted vests in conventional training settings (1,9). However, to achieve an overload effect while not negatively affecting sprinting biomechanics, it has been suggested that resistances should be loaded to elicit decreases in horizontal sprinting velocity that approach but do not exceed 10% (1,2,13,17,18).

Because the resistance provided by a weighted sled and the resulting decrease in sprinting velocity may depend on the coefficient of friction of the training surface (2,8) and the physical characteristics of the subjects (2,17,18), the resistance load necessary to cause a 10% decrease in sprinting velocity may be specific to the training equipment and surface and to each individual group of subjects. Furthermore, decreases in sprinting velocity while wearing weighted vests may also be specific to the physical characteristics of the subjects (1). In light of these considerations, pilot testing was necessary to determine the appropriate resistance loads for the experimental subjects in the WS and WV conditions.

Five pilot test subjects (age = 22.4 ± 2.1 years; mass = 77.1 ± 4.1 kg; 176.8 ± 8.4 cm) completed 54.9-m sprints both unresisted and with weighted sleds and weighted vests, using loads ranging between 5% and 20% body mass for both types of resisted sprints. Kinematic data for maximum velocity sprinting were collected for the 3 experimental conditions with the varying loads, and these data were used in a regression analysis similar to that used by Lockie, Murphy, and Spinks (17). Results indicated that a load of approximately 10% body mass was appropriate for the WS group and a load of approximately 18.5% body mass was appropriate for the WV group. Resistance loads for the experimental training protocol were based on the subjects' pre-test body mass and were not adjusted during the course of the training. Actual loads used were 10.24 ± 0.23% body mass for the WS group and 18.52 ± 0.18% body mass for the WV group.

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Training Protocol

All training groups completed 13, 60-minute training sessions spaced over 7 weeks (2 sessions per week except for Week 6 because of a school holiday). Although logistical factors such as the periodization phase of the off-season and the length of the school semester limited the number of training sessions, several previous longitudinal sprint training studies have shown this training frequency and duration sufficient for eliciting significant improvements in sprinting performance (14,21,31). In addition to the experimental sprint training protocol, all subjects completed an identical periodized strength training program 3 days per week (on nonsprint training days) for the duration of the study as part of their regular collegiate lacrosse off-season training program.

Each training session began with a 20-minute warm-up where all subjects performed dynamic stretches, neuromuscular coordination exercises specific to sprinting, and various footwork and agility drills. After warm-up, subjects in all 3 groups completed the sprint training protocol listed in Table 2. This sprint training protocol was roughly based on the volume and frequency outlined in a previous sprint training study (14) that used subjects with similar physical characteristics to the subjects in the present study. The WS and WV groups completed all sprints while resisted except for the last 2 sprints of each training session (to reinforce proper sprinting technique when not resisted), whereas the UR completed all sprints without resistance. Subjects in the WS condition towed a sled (Power Systems model Speed Sled, Knoxville, Tennessee, U.S.A.) with weighted plates loaded on top of the sled. Subjects in the WV group wore a weighted vest (Xvest model X4040, Houston, Texas, USA) with 0.45-kg weights evenly distributed across the front/back and top/bottom of the vest. A certified strength and conditioning specialist directed all training sessions to ensure that all warm-up activities and sprints were completed with correct technique and with maximum effort. After the conclusion of the experimental training protocol, all subjects were required to rest for 3 days without any organized physical activity. All post-testing measurements were completed in an identical manner to the pre-testing procedures outlined previously.

Table 2

Table 2

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Statistical Analyses

Descriptive statistics (mean ± SD) are reported for the dependent variables. A 3 (training mode) × 2 (time) repeated-measures analysis of variance (ANOVA) was used to analyze potential between-group differences for each dependent variable. Statistical significance was set at p ≤ 0.05. Additionally, percent change and effect size statistics were calculated for each dependent variable (7,25). Statistical analyses were performed using SPSS 15.0 (SPSS Inc., Chicago, Illinois, USA).

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Results

Sprint performance measures are listed in Table 3. ANOVAs revealed no significant group × time interaction effects. The main effect of time for the entire subject population was significant for decreases in sprint time (F(1,20) = 8.10, p ≤ 0.05) and increases in average velocity (F(1,20) = 8.30, p = 0.01). However, there were no significant between-group differences for either 18.3- to 54.9-m sprint time or average velocity. To assess the practical implications of the adaptations to training for each group, percent change and effect size statistics were calculated; interpretations of effect size were based on the scale proposed by Rhea (25) for statistical analysis specific to strength and conditioning research. For the variables of sprint time and average velocity, effect size (ES) statistics suggested small post-training improvements for the UR group (ES = 0.61 and 0.63, respectively) but only trivial improvements for the WS group (ES = 0.03 and 0.02, respectively) and the WV group (ES = 0.25 and 0.24, respectively).

Table 3

Table 3

Stride cycle kinematic measures are listed in Table 4. ANOVAs revealed no significant group × time interaction effects. The main effect of time for the entire subject population was significant for decreases in stride length (F(1,20) = 8.68, p ≤ 0.01), increases in stride rate (F(1,20) = 9.42, p ≤ 0.01), and decreases in ground contact time (F(1,20) = 7.15, p ≤ 0.05). However, there were no significant between-group differences for any of the kinematic measures. For the UR group, effect size statistics suggested small increases in stride rate (ES = 0.42) and small decreases in ground contact time (ES = 0.38). For the WS group, effect size statistics suggested small decreases in stride length (ES = 0.42). For the WV group, effect size statistics suggested small decreases in stride length (ES = 0.37), small increases in stride rate (ES = 0.78), and moderate decreases in flight time (ES = 1.00).

Table 4

Table 4

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Discussion

ANOVA revealed that the main effect of time for the entire subject population was significant for improvements in sprint time and average velocity in the 18.3- to 54.9-m interval. These generic improvements indicate that the experimental sprint training protocol, independent of the mode of training, had a beneficial impact on maximum velocity sprinting performance. This positive adaptation to training is not entirely unexpected because prior research has demonstrated that both resisted and unresisted sprint training protocols may elicit significant sprint performance improvements, even in trained subject populations (14,28).

The lack of between-group differences for sprint time or average velocity after training indicates that resisted sprint training with either the WS or WV offered no ergogenic effect when compared to UR training. Furthermore, effect size calculations suggest that the UR group demonstrated small improvements in sprint time (ES = 0.61) and average velocity (ES = 0.63) from pre- to post-test, whereas only trivial improvements in sprint time and average velocity were demonstrated by the WS group (ES = 0.03 and 0.02, respectively) and the WV group (ES = 0.25 and 0.24, respectively). Therefore, the results did not support the primary experimental hypothesis that subjects training with WS and WV would demonstrate significantly greater improvements in maximum velocity sprint performance than subjects completing UR sprint training. In fact, based on the loads used by WS and WV in this study, the results indicate that UR training may actually be superior to WS or WV training for improving maximum velocity sprint performance from 18.3 to 54.9 m. The mean decrease in sprint time of 0.086 seconds demonstrated by the UR group is certainly of practical value for subjects with this training status.

Only a few prior studies have examined the longitudinal training effects of WS on maximum velocity sprinting performance. Zafeiridis et al. (33) examined the effects of an 8-week training protocol using WS (towing an absolute load of 5 kg) vs. UR sprint training on 50-m sprint performance in 22 recreationally trained male collegiate students. The WS group significantly improved running velocity in the acceleration phases (0-10 m and 0-20 m) but did not significantly improve in any of the maximum velocity phases (20-40, 40-50, or 20-50 m), whereas the UR group did not significantly improve in any of the acceleration phase intervals but did significantly improve in all of the maximum velocity phases. Thus, the results from Zafeiridis et al. (33) generally agree with the present study in that UR may be superior to WS for improving sprint performance at certain intervals of maximum velocity sprinting.

Kafer and Adamson (14), however, produced evidence that somewhat conflicts with the findings of the present study, although there are certain differences between the 2 studies. They examined the longitudinal effects of a 6-week training program with WS (loaded with 15% body mass), assisted (overspeed), combined assisted/WS, or UR sprint training on 20-, 40-, and 60-m sprint times in competitive male rugby players. Post-test results showed that all experimental groups demonstrated significant improvements in 60-m times. However, the WS group made greater mean improvements in sprint time in the 20- to 60-m interval than did the UR group, and an analysis of the results led the authors to conclude that training with either WS or a combination of assisted/WS methods may be superior to UR for producing improvements across all of the 20-, 40-, and 60-m distance intervals (14).

With respect to the results for the WV group, to our knowledge this is the first longitudinal study examining the effects of WV on sprinting speed when the weighted vests were worn only during training. As previously mentioned, several hypergravity training studies have examined the use of WV for improving athletic performance when worn for all waking hours in a 3-week period (3-5,26), but results from these studies are not applicable to conventional training settings. In the present study, although the mean decrease in sprint time of 0.053 seconds is still of some practical value for this population, the effect size statistics for WV training illustrate that WV training does not appear as beneficial as UR training.

Examining the effects of the training intervention on the stride cycle kinematic measures helps elucidate the mechanisms underlying the post-training changes in sprinting performance. ANOVA revealed that the main effect of time for the entire subject population was significant for 3 of the kinematic measures. Results indicated significant decreases in stride length, significant increases in stride rate, and significant decreases in ground contact time for the population as a whole after training. Prior literature has suggested that short-term maximization of sprinting velocity, whether during an individual sprint, a single testing session, or over an entire season, may occur when the greatest stride rate (and not stride length) is achieved; it has also been demonstrated that there is a negative relationship between stride rate and stride length (12). Thus, it is possible that the experimental training regimen caused each individual subject (independent of experimental group) to seek an optimal combination of stride rate and stride length and that this produced mean increases in stride rate and mean decreases in stride length for the population as a whole after training. The improved mean sprint performance measures demonstrated by the entire subject population after training indicate that the increases in stride rate were proportionately greater than the decreases in stride length and this is confirmed by the percent change statistics for the entire population.

With regard to the kinematic effects of resisted sprint training, previous literature has suggested that WS training may increase stride length over time as a result of the increases in the muscular force production required to overcome the additional load during training (2,17). Therefore, the first corollary hypothesis for this experiment was that subjects in the WS group would demonstrate increases in stride length after training that were significantly greater than those demonstrated in the UR or WV groups. However, this hypothesis was not supported because results showed no between-group differences in stride length after training. As with the UR and WV groups, the WS group actually demonstrated mean decreases in stride length after training. Indeed, despite the theoretical rationale for WS training to cause an increased stride length over time, evidence to support this training effect is lacking. In the only other published study to investigate the longitudinal effects of WS training on maximum velocity stride cycle kinematics, Zafeiridis et al. (33) also found no significant pre- to post-test differences in either stride length or stride rate.

One independent variable that requires further investigation is the selection of load and the resulting biomechanical effects of WS training. Although prior research has identified load selection as a key factor in designing a WS training protocol (2,20), the optimal loading schemes for longitudinal training have not yet been determined, particularly for maximum velocity sprint training (2). Loading schemes that are too light and do not overload the subject may not induce a sufficient training stimulus and therefore may not result in enhanced muscular force production or ground force application after training. However, sprinting with WS has been associated with acute changes in sprint mechanics (1,33), and loads that are too heavy may cause improper technique and minimize any expected increases in stride length and sprinting velocity. Although the loading scheme used by the WS group in the present study was set to elicit decreases in horizontal sprinting velocity that approached but did not exceed 10% (1,2,13,17,18), sprint performance was not significantly enhanced after training, indicating that future investigations should examine how modifying this loading scheme could better optimize the post-training changes in maximum velocity sprinting performance and biomechanics.

With regard to the WV group, changes in stride rate were expected after training. Previous research has demonstrated that there is a relationship between acute increases in stride rate and leg spring stiffness (10). Furthermore, it is thought that running economy can be enhanced by increasing lower-extremity musculotendinous stiffness through training (29). Therefore, it has been suggested that longitudinal increases in stride rate through decreased ground contact times may be developed through increasing leg muscle and joint stiffness, with WV training as 1 mode that could potentially elicit this effect (8,30). Thus, for the present study, the second corollary hypothesis was that subjects in the WV group would demonstrate decreases in ground contact times and increases in stride rate that were significantly greater than the UR or WS groups. Although ANOVA revealed no between-group differences in stride rate after training, effect size statistics did show a small increase in stride rate for the WV group. However, the mechanism for this increase in stride rate was not consistent with the hypothesis because it appears that the changes in stride rate resulted from moderate decreases in flight time rather than from substantial changes in ground contact times. Furthermore, effect size statistics suggest that the WV group demonstrated a small decrease in stride length after training.

Hypothetically, decreasing flight time while holding ground contact time and stride length constant will cause increased sprinting velocity as a result of an increased stride rate. However, in the present study, although subjects in the WV group did demonstrate mean increases in stride rate as a result of mean decreases in flight time, they also demonstrated mean decreases in stride length after training. Although neither ground reaction forces nor center of mass kinematics were measured in this study, it may be speculated that members of the WV group altered the vertical velocity of their center of mass at takeoff after training and that this led to their reductions in stride length. Vertical velocity at takeoff has been shown to have a positive correlation with flight time and step length and a negative correlation with step rate, and it has been cited as a source of the negative interaction between step length and step rate (12). A decreased vertical velocity at takeoff would theoretically explain both the decreased flight time and decreased stride length demonstrated by the WV group after training. Sprinting with WV loaded with 15 to 20% body mass has been shown to acutely elicit significant decreases in flight time (9). It may be that the subjects in the WV group adopted the biomechanics programmed during WV training, which they then transferred to unresisted sprinting, with substantial effects on their post-training kinematics. However, at the present time, this explanation is speculative only, and as with the WS, more research is needed to determine how various loading schemes affect the longitudinal effects of WV training.

There were no corollary hypotheses regarding the UR group, although this group showed the greatest mean improvement in the sprint performance measures after training. Effect size statistics suggest that the UR group demonstrated small decreases in ground contact time and small increases in stride rate after training. As suggested for the entire subject population, it is possible that the experimental training regimen caused the subjects in the UR group to alter their combination of stride rate and stride length in an attempt to maximize velocity, leading to these changes in kinematics. The results from the present study differ from Zafeiridis et al. (33), who found that the UR group significantly increased stride length in the maximum velocity phase after training and that this caused their overall increase in sprint performance in this phase. These authors commented that the kinematic mechanisms underlying the improvements in sprinting performance were surprising because it was expected that the UR group in their experiment might demonstrate improvements in stride rate, rather than stride length, after training. Indeed, with reference to the present study, it is not completely unexpected that subjects in the UR group decreased ground contact times or that they improved sprinting performance through increases in stride rate that were proportionately greater than their decreases in stride length. Sprinting is a skill based on proper technique, and it is possible that subjects in the UR group improved performance and decreased ground contact times simply through programming correct motor patterns (i.e., efficient arm drive, correct foot strike, etc.).

As previously mentioned, an area for future research is the long-term effects of different loading schemes on subjects performing WS and WV training. The loads used in the current study were based on prior recommendations that resistances should be set to elicit decreases in horizontal sprinting velocity that approach but do not exceed 10% (1,2,13,17,18). However, this loading strategy has not been empirically validated as the optimal load for eliciting long-term improvements in performance, and it is possible that subjects performing WS or WV training may receive greater benefit from lighter or heavier loads. Kristensen, van den Tillaar, and Ettema (15) investigated the effects of 6 weeks of resisted, assisted, or UR sprint training and found that subjects improved most from pre- to post-test in the trained form of sprinting. The authors concluded that the principle of velocity specificity applies to sprint training (15). If this principle is true, loads for maximum velocity-resisted sprint training should perhaps be relatively lighter so that the velocity of movement remains closely matched to UR sprinting. This idea has also been endorsed in the literature, where it has been suggested that heavier loads may be more appropriate for resisted sprint training in the acceleration phase but that lighter loads may be more appropriate for the maximum velocity phase (2,8). Alternatively, loads heavier than those used in the present study may be required to present a sufficient training stimulus. In the study by Kafer and Adamson (14), the WS group towed 15% body mass during training and subsequently demonstrated superior post-training results to the WS group in the present study, raising the question of whether the load in the present study was heavy enough. Although there are no similar longitudinal studies on WV training to serve as a point of comparison, the issue of load needs to be addressed in future research for both WS and WV sprint training.

Finally, as previously mentioned, the major limitation of this study was that kinetic and joint angular kinematic data were not collected during testing or training. This limited the interpretation of the results, specifically the biomechanical changes that took place after training. Future investigations in this line of research might benefit from the addition of kinetic analysis and joint-specific angular kinematic analysis.

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Practical Applications

For the loading schemes used in this study, the results suggest that maximum velocity sprint performance might be most effectively enhanced by UR training protocols. Therefore, coaches and athletes may consider performing UR sprint training rather than WS or WV training when attempting to enhance performance in the maximum velocity phase.

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Acknowledgments

This study was funded in part by a Student Research Grant from the National Strength and Conditioning Association.

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Keywords:

maximum velocity sprinting; sprinting kinematics

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