The attribute of speed is widely considered to be an essential prerequisite for athletic success across a wide array of popular sporting activities (2,8,12). The capacity to move rapidly presents a considerable competitive advantage across almost all land-based sports such as football, basketball, and track and field. Maximum running speed is often the decisive factor in determining the outcome of many sporting contests (1,2,8). Coaches and athletes alike seek techniques that effectively develop the biomotor ability of speed. In the mid-20th century, an increasing cohort of coaches believed the attribute of speed to be an inherent quality-a characteristic that was genetically predetermined and, therefore, unable to be effectively trained (3,6). Further research in neuromuscular physiology has scientifically disproved this assertion, forever discrediting the once accepted axiom that “sprinters are born and not made” (13).
Numerous methods have been advocated and employed by coaches to effectively develop athletes' speed; each has met with varying degrees of success. The methodology of resisted sprint training (sprinting with the application of various forms of external resistance) has come to the forefront as one of the most commonly accepted methods for training speed and its various subcomponents (2,7,15,16). In particular, the resisted sprint training modality of additional mass training has been touted as an effective means through which to facilitate the development of athletes' sprint-specific strength-a position seemingly well justified by its adherence with currently accepted knowledge in the fields of sport and exercise science.
The adverse impact of adding resistance to sprint training is that sprint kinematics (stride length, stride frequency, interval velocity, and lower limb ranges of movement) are often compromised during resisted sprint training interventions, which impact on a biomechanically efficient sprint technique and, perhaps more importantly, predispose athletes to an increased risk of injury (2,9). Currently, considerable debate exists among members of the scientific and coaching fraternity with respect to the underlying validity of the existing contingent of resisted sprint training protocols. This lack of knowledge on the influence of resistance training on functional performance is common across many training modalities and represents a clear deficiency in the scientific literature. This research hypothesized that the addition of specific lower limb loads will not adversely affect sprint kinematics.
The aim of this research was to examine the efficacy of lower extremity added mass training for the purposes of developing athletes' sprint-specific strength and, consequently, overall sprint performance. The specific objectives identified for the research project were to examine the impact and effects of lower extremity added mass training on key lower extremity kinematic parameters of the sprint cycle and to examine the immediate postintervention impact of lower extremity added mass training on subsequent sprint performance.
Experimental Approach to the Problem
To address the research objectives, a 3-dimensional (3D) analysis of sprint performance was conducted during the acceleration phase of straight-line 40-m sprints. Two separate 40-m sprint testing sessions were conducted on separate days, approximately 1 week apart to allow for full neuromuscular recovery. The first session (baseline) consisted of 6 sprints and was used to develop “normal” data for all test variables for each athlete. The subsequent test session was subdivided into 3 sections (a) pretest (2 trials), (b) resisted sprint (2 trials), and (c) posttest (3 trials).
Eight highly trained (national representative) male competitive beach sprinters participated in the research; the number of years of sprint-specific resistance training as well as the athletes' best time for the 100 m (which is 88% of the current world record) are listed in Table 1. Human research ethics approval was obtained prior to the commencement of the study. A “preparticipation medical screening questionnaire” determined the suitability of athletes prior to commencement. At the time of testing, all athletes were in the general preparatory phase of their training cycles.
Basic anthropometric data were compiled and collated for each participant immediately following the conclusion of the orientation sessions preceding initial baseline testing. Total body mass was measured to the nearest 0.01 kg with calibrated electronic scales (Wedderburn Pty Ltd., Willawong, Australia), and standing height of individual subjects was measured to the nearest 1 mm with a portable stadiometer (Mentone International, Moorabbin, Australia). Infrared timing gates (Fusion Sport, Brisbane, Australia) were positioned at the 0-, 10-, 20-, 30-, and 40-m zones. Kinematic data were collected over a complete stride (between the 25 and 30 m marks of each trial) using 6 infrared cameras (Qualisys Medical AB, Gothenburg, Sweden) to track low mass, retroreflective markers located on key head, trunk, and lower limb landmarks. Data were collected at 200 Hz. The Qualisys software uses standard methods to develop 3D coordinates for each marker before standard biomechanical modeling software (Visual3D; C-Motion, Inc., Germantown, MD) was used to construct a 3D model of the subjects' head, thorax, pelvis, and lower limbs. The raw data were smoothed with a 10-Hz low-pass filter, and the following spatiotemporal variables were calculated. Temporal variables included total stride time (t(Tot)), stride length, stride frequency, flight time (t(FT)), ground contact time (t(GC)), stride velocity (SVel), and the various time and average velocity variables for each 10-m interval over the 40 m. Spatial variables included sagittal plane hip and knee joint angular displacement variables such as hip angle at toe off, peak hip flexion during recovery, hip angle at foot strike and peak knee flexion during recovery, peak knee extension prior to foot strike, knee angle at foot strike and peak knee flexion during stance, and knee angle at toe off. Saggital plane hip and knee joint angular velocity variables included hip extension velocity at toe off, peak hip flexion velocity during recovery, peak hip extension velocity prior to foot strike, hip extension velocity at foot strike, peak hip extension velocity during propulsive phase, peak knee extension velocity during recovery, peak knee extension velocity prior to foot strike, knee extension velocity at foot strike, and peak knee extension velocity during the propulsive phase.
The system was calibrated prior to the onset of each testing session, with the acceptable SD of the measured length of the calibration wand set at a maximum of 0.0015 m to ensure validity of the captured trials. To facilitate efficient neuromuscular functioning and minimize any injury risks associated with inadequate pretrial preparation, subjects were instructed to follow their established, personal warm-up routines prior to the initiation of each testing session.
The resisted sprint trials were identical to the pretest and posttest trials with the exception that they involved the addition of individually tailored weights to participants' thigh and shank segments. Weight was added using custom-made stretch fabric weight pockets filled with small packets of lead shot. Loadings approximating 10% of individual segment mass were calculated for each individual participant based on published anthropometric data (11). To avoid the placement of the weights hindering the athletes' normal sprint stride, the external loadings were applied anterio-posteriorly on the thigh and medio-laterally on the shank at the approximate radius of gyration (11).
The descriptive statistics (means and SDs) and analysis of variance were computed using the Statistical Package for the Social Sciences (version 14.0 for Windows; SPSS Inc., Chicago, IL). Levene's test was applied to determine the homogeneity of the variances. Scheffe's post hoc comparisons and an alpha level of p ≤ 0.05 were set for all analyses. Typical error of measurement (TEM) and coefficient of variation (CV) values were ascertained to determine intrasubject variance. Based on previous research conducted with a sample of elite level sprint athletes (4), a CV of >1% was selected as the minimum indicator for probable change in athletic performance.
Measures for baseline, pretest and posttest conditions were remarkably consistent across the temporal variables of stride time, flight time, ground contact time, and stride frequency (Table 2). The baseline measures were used to quantify the reliability of the data using CV expressed as a percentage and the TEM (4). These ranged from 1.2 to 2.3% (0.01-0.04 seconds) for the sprint interval data, and 0.8-1.3% for the spatiotemporal stride data, which demonstrates that the test measures were stable and reliable.
While the increase in total 40-m sprint time in the resisted sprint condition was not found to differ significantly between conditions, differences approaching a level of significance were identified between the resisted sprint (5.251 ± 0.133 seconds) condition and baseline (5.091 ± 0.082 seconds, p = 0.059), pretest (5.087 ± 0.144 seconds, p = 0.052), and posttest (5.104 ± 0.16 seconds, p = 0.095) conditions. No significant differences were identified to have occurred in total 40-m sprint time between baseline and pretest, baseline and posttest, and pretest and posttest conditions, respectively.
There were no significant differences across the entire range of spatiotemporal and kinematic variables for the 3 baseline trial groupings. Stride velocity was significantly reduced in the resisted sprint condition (p = 0.001). Scheffe's post hoc analysis determined that average stride velocity in the resisted sprint condition was significantly slower (8.78 ± 0.23 seconds) than in the baseline (9.23 ± 0.20 seconds, p = 0.004), pretest (9.21 ± 0.23 seconds, p = 0.009) and posttest (9.14 ± 0.23 seconds, p = 0.028) conditions.
The addition of mass had a significant increase on the total 40-m sprint time (p = 0.017). Significant differences were also found in the sprint times between the 10-m to 20-m (p < 0.001) and the 30-m to 40-m (p < 0.001) zones. The 10-m to 20-m sprint time in the resisted sprint condition was significantly slower (1.24 ± 0.03 seconds) than that in baseline (1.19 ± 0.03 seconds, p = 0.008), pretest (1.17 ± 0.03 seconds, p < 0.001), and posttest (1.20 ± 0.03 seconds, p = 0.027) conditions. No significant difference was identified for 10-m to 20-m sprint time between baseline and pretest, baseline and posttest, and pretest and posttest conditions, respectively. The 30-m to 40-m sprint times revealed that times in the resisted sprint condition were significantly slower (1.16 ± 0.03 seconds) than those in both baseline (1.08 ± 0.04 seconds, p = 0.001) and pretest (1.10 ± 0.02 seconds, p = 0.005) conditions.
No significant differences in angular displacement values for the hip joint were identified, indicating that hip range of movement (ROM) throughout stride remained consistent across all conditions (Figure 1). While hip displacement values at toe off and foot strike were analogous among conditions, a slight reduction in peak hip flexion during the recovery phase of the stride cycle was evident throughout the resisted sprint (89.9 ± 6.0°) condition compared with pretest (94.0 ± 7.2°) and posttest (92.8 ± 7.6°) conditions, respectively (Figure 2).
Similarly, knee ROM remained congruous across all conditions investigated in the current study. No significant differences were observed in angular displacement values for the knee joint at either toe off or foot strike. Mean peak flexion values for the knee joint during both recovery and stance phases remained uniform throughout the various phases of testing. Additionally, no significant variance in peak knee extension prior to foot strike was found to have occurred across conditions. No significant changes in the hip or knee velocity values were found. Trial-to-trial variability across the range of spatiotemporal and time/velocity variables observed throughout baseline testing was relatively low, with CV values of <2.0% being recorded for total stride time.
The addition of training with 10% individual segment weight to the lower limbs led to a significant reduction in sprint time for both 10-m to 20-m and 30-m to 40-m splits and the total 40 m measure. Resisted sprint training is a sprint-specific training protocol employed commonly by athletes and coaches for the purposes of enhancing sprint-specific strength and, therefore, increasing sprint performance. Previous research in the field has focused on investigating the impact and effects of a range of resisted sprint training modalities on sprint kinematics, including hill sprints (9), sprinting on sand (10), sprinting with specially designed “parachutes” (14), and sprinting while towing weighted sleds (16). There were no significant differences observed across the entire range of angular displacement and velocity variables in this current study. Similarly, no effect was recorded between the added mass condition and the sprint performance in the postintervention sprint trials, with only 1 of the 8 subjects experiencing a positive pretest to posttest improvement in total 40-m sprint time. This is in contrast to other forms of resisted sprint training, which found significant changes in sprint kinematics (2,16). Consequently, such findings have led many researchers questioning the specificity of this form of sprint-specific resistance training and, therefore, its underlying validity as efficacious training protocol. Furthermore, some authors have suggested that because of the observed range of kinematic adaptations to the various forms of resisted sprint training; this form of training could ultimately prove to be detrimental to athletic performance, in that athletes' technique may be comprised if the protocol is employed inappropriately or on too frequent a basis (5).
The stride velocity throughout the 20-m to 30-m phase of the sprint trials was also shown to be significantly reduced in the added mass condition. Importantly, however, no significant differences were observed across the remaining spatiotemporal variables selected for analysis including t(Tot), stride length, stride frequency, t(GC), and t(FT). Additional mass training is one form of resisted sprint training that, on the surface, would appear to be free of the limitations of the various alternative resisted sprint training methods. In particular, the fact that no extraneous horizontal forces are exerted on the sprint athlete's torso (as observed across a range of commercially available resisted sprint training devices including weighted sleds, resistance bands, and speed “chutes”) represents a considerable boost to the protocol's claim to specificity and, consequently, its validity as a functional sprint-specific resistance training intervention.
The primary strength of added mass training lies in the logic underpinning its rationale “same action, same motor program, slightly increased mass” and its adherence to currently accepted methodologies of sport training. Proponents of this form of resisted sprint training claim that, through a carefully considered approach to the implementation of an added mass training intervention, it is possible to cater for the principles not only of specificity but additionally individualization and progressive overload. Yet, little empirical research has been conducted to assess its efficacy as a valid sprint-specific training protocol. While the current protocols regarding the implementation of added mass training are guided by anecdotal evidence and extrapolation from a range of alternate resisted sprint training methodologies, no clear scientific guidelines in relation to the calculation and distribution of applied loadings presently exist. Throughout this study, the applied loads were calculated at approximately 10% of individual segment weight, with the distribution of the loadings being evenly applied about the radius of gyration of the respective lower extremity segments (thigh and shank). To the best of the researchers' knowledge, no other published studies in the field of added mass training have sought to examine the impact and effects of such a load configuration on sprint kinematics. Additionally, very few studies pertaining to added mass training (and, moreover, resisted sprint training protocols in general) have examined the immediate postintervention effects of added mass on resultant sprint performance. This research provides the first known descriptions of both the acute and the immediate postintervention impact and effects of this specific form of lower extremity added mass training on sprint kinematics and ensuing sprint performance.
The slight decrease in stride velocity during the resisted sprint condition observed was found to be a product of minimal increases in both flight time (0.362 ± 0.023 seconds) and ground contact time (0.098 ± 0.016 seconds) coupled with a slight decrease in stride length (4.03 ± 0.19 m) and stride frequency (2.19 ± 0.14 seconds). Moreover, although not found to be statistically significant, total stride time was observed to have slightly increased during the resisted sprint condition (0.460 ± 0.031 seconds), which could reflect an increase in the stride cycle variability.
Although considerable differences in mean values were evident across a number of variables examined in the current study, high intrasubject and trial variability-particularly in regard to sprint kinematics-led to only a modest number of measures achieving a level of statistical significance (p < 0.05) between conditions. It is also recognized that the sample size (N = 8) is small but is typically of the elite sprinting population. The emergence of clear “positive respondents,” “neutral respondents,” and “negative respondents” was also noted when analyzing individual results across the range of dependent variables included in the present analysis.
The combination of a reduction in sprint time and stride velocity with no statistically significant impact on resultant sprint kinematics would suggest that this form of additional mass training could prove beneficial for the purposes of developing athletes' sprint-specific strength and, ultimately, sprint performance. Importantly, it would appear that lower extremity mass and inertial manipulation (at 10% individual segment weight) will not adversely affect the technique of highly trained elite sprinters or predispose athletes to an increased risk of “overstride” related hamstring injury. The high level of intrasubject variance observed in individual responses throughout the project would indicate that caution be exercised when implementing an added mass training intervention with individual athletes.
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Keywords:© 2009 National Strength and Conditioning Association
resistance training; kinematics; performance enhancement