For many sporting activities, such as tennis, squash, basketball, and soccer, the athletes never attain maximum speed during sprinting. In fact, sprints most frequently occur over very short distances (0–10 m) from both standing and rolling starts. Consequently, the speed over the first steps and the ability to accelerate quickly would be considered of greater importance (14,20). On this concern, research has found that the first few ground contact phases of a short sprint are dominated by propulsive forces and by concentric muscle actions (2,3,16,17). Despite the attention given to performance assessment by sports scientists, there is a paucity of research examining the relationships between various motor skills, such as sprinting and jumping (15).
Research has reported ambiguous results in the relations observed between distinct strength measurements and sprint performance (8,11,13,23). Although some studies have claimed significant correlations between lower-body muscle strength measures and sprint performance (22), others have not (6). These conflicting results may be the result of the fact that sprinting involves multiple-joint motions with precise coordination between various muscle groups, which is not adequately assessed by single joint tests that isolate muscles. Thus, the relative importance of various lower-body muscle groups to sprinting performance is not totally clear (8,10,13), especially when short sprint is considered. Nevertheless, because explosive muscle actions are of major importance to short sprint acceleration (19), it seems logical that similar resistance training exercises might be suitable for testing and training these neuromuscular capacities. Yet, few studies have examined the relationships between short sprint (<40 m) performance in trained subjects with force parameters, mechanical impulse, and mechanical power during muscle contractions of the lower extremity during countermovement jumps (CMJ). In fact, understanding and developing sports speed would seem essential, given the importance of first-step quickness and acceleration to many sports (20). These findings emphasize the importance of the concentric phase during initial acceleration, and the role of propulsive force developed during the first foot contacts of the sprint in maximizing initial running velocity.
None of the previous studies examined short sprinting time (10 m) with dynamic force performance together with power output, mechanical impulse, displacement, time, and bar velocity measured with a liner transducer in a large sample of trained athletes. To the best of our knowledge, only Marques et al. (14) examined short sprinting (5 m) with distinct strength metrics measured with a liner transducer, but in a small sample of physical students. An important relationship was found between 5-m sprint and maximal lower-body strength, as assessed by the force, power, and bar velocity displacement. It is suggested that sprinting time performance would benefit from training regimens aimed to improve these performance qualities. In fact, there is a paucity of research that has examined the relationships between short sprint (<20 m) performance in a sample of trained subjects and force parameters, mechanical impulse, mechanical power, and bar velocity outputs during muscle contractions of the lower extremity in CMJ. Most of investigations have used isokinetic and isometric tests as indices of strength, but single joint actions are not specific assessment strategies (1). The CMJ exercise was chosen because it seems to mimic short sprinting technique (15,20). Thus, using a multi-joint exercise such as a CMJ test should be advantageous when exploring relationships with a dynamic movement such as sprinting. None of the previous studies examined a 10-m sprint time with different strength metrics together with power output and bar velocity during a loaded CMJ in a large sample of trained male sportsmen.
A force platform would seem to be one of the most commonly used measuring devices in sport biomechanics to access lower-body strength (4). However, some problems of using a force platform are cost and portability due to weight, which makes it difficult to use in field tests. To avoid these problems, a linear transducer could be used because this device can directly measure the position over time. Furthermore, the linear transducer has shown high validity and reliability in measurements of force compared with a force platform (7).
Because explosive concentric muscle actions dominate sprint starts, it seems logical that similar resistance training movements might be suitable for testing and training these neuromuscular qualities. Consequently, the relationship was examined between the kinetics of a weighted explosive vertical jump exercise and sprint start performance, with a view to assessing whether or not such exercise should be recommended for individuals wishing to improve sprint acceleration.
Therefore, the aim of this research was to examine the associations between 10-m time and distinct strength metrics of the CMJ using a linear transducer in a large sample of trained athletes. Examination of these relationships could be of great importance for the optimal development of resistance training programs to improve short sprint performance in athletes. It was hypothesized that power and force would be significantly related to sprint time and also high bar velocity would show significant relationships to better sprint performance.
Experimental Approach to the Problem
After a standard warm-up, participants performed 3 maximal CMJ trials in a Smith machine. The bar of this apparatus had a linear transducer attached (T-FORCE, Murcia, Spain). The rotary encoder of the linear transducer recorded the position and direction of the bar (weighted 17 kg) to within an accuracy of 0.0002 m. Peak instantaneous power was calculated by the product of velocity taken with the linear transducer. Only the concentric portion of the CMJ was taken for analysis. Vertical instantaneous velocity (v) was directly measured by the device and sampled at a frequency of 1 kHz. The linear transducer was interfaced with a personal computer by means of a 14-bit resolution analog-to-digital data acquisition board, where a specialized software (T-FORCE Dynamic Measurement System) application automatically calculates the relevant kinematic and kinetic parameters of every jump, provides real-time information on screen, and registers all data on disk for subsequent analysis. The derived mechanical variables were calculated by the software as follows: displacement was obtained by integration of v data with respect to time; instantaneous acceleration (a) was obtained from differentiation of v with respect to time; instantaneous force (F) was calculated as F = m (a + g), where m is the moving mass (in kg) that must be manually entered into the software for each set, and g is the acceleration due to gravity (being 9.81 m·s−2); instantaneous power output resulted from the product of the vertical applied force and bar velocity (P = F·v). Eccentric (negative v) and concentric (positive v) phases of the movement were automatically detected. In the CMJ, the concentric phase was defined from the moment following the end of the eccentric phase to the point where peak velocity is reached (which takes place some milliseconds before takeoff from the ground). Because the effect of friction force was negligible in pilot testing, it was not taken into consideration in the calculations. Similarly excluded from consideration was the constant downward force exerted by the cable because it was minimal compared with the weight being lifted. The validity and reliability of this system have been previously established (9). The coefficient of variation (CV) ranged from 3.6 to 17%, with the values being greater for the rate of force development (RFD) measures, whereas the intraclass coefficient correlation (ICC) values ranged from 0.93 to 0.98 with the lowest value being recorded for RFD at peak force.
A group of 32 male trained subjects volunteered to participate in the study (mean ± SD: age 21.4 ± 1.5 years, body mass 67.5 ± 4.8 kg, body height 1.74 ± 0.02 m). All participants were sports science students who were previously familiarized with all test procedures 4 weeks before the measurements were applied. All were trained amateur athletes of different sports (e.g., soccer, futsal, track and field, and team handball). Consequently, all the participants were well conditioned once they could squat 2 times their body mass. Subjects were also familiar with all of the testing procedures and exercises, as they had been performing them as part of their regular training routine. Before commencing the study, subjects had a physical examination, and each was cleared of any medical disorders that might limit full participation in the investigation. Subjects were required to sign an informed consent form before the study. The study was conducted according to the Declaration of Helsinki and was approved by the institutional review boards of the University of Beira Interior and Research Center in Sports, Health and Human Development, Portugal.
Each participant initiated the CMJ from a standing position, performed a crouching action followed immediately by a jump for maximal height. Hands remained on the bar for the entire movement to maintain contact between the bar and the shoulders. Three minutes of rest was provided between each trial to minimize fatigue. The trial-to-trial reliability of the CMJ measured by the linear transducer gave an ICC of 0.91–0.96 for concentric force, maximum power, and maximum RFD. The mean and the peak bar velocity ICCs were 0.91 and 0.93, respectively. The CV were 4–10% with the linear transducer. Only the best attempt was taken for analysis. For sprint testing, subjects were required to perform 3 maximum effort sprints of 10 m. Times were recorded using Brower equipment (Wireless Sprint System, Draper, UT, USA). Subjects performed the sprints with 3-minute rest periods. Only the best attempt was considered. The sprints reported an ICC of 0.92–0.98 and CV of 1.6%.
Mean (±SD) values were calculated for each variable. The normality and homoscedasticity assumptions were checked, respectively, with the Shapiro-Wilk and the Levene tests. The ICC was used to determine between-subject reliability of jumping tests. Within-subject variations for all tests were determined by calculating the CV as outlined by Hopkins (12). Pearson product-moment correlation coefficient was used to verify the association between variables. The level of significance was set at p ≤ 0.05.
Sprint performance variables and CMJ mechanical parameters are presented in Table 1. Pearson product-moment correlation coefficients between 10-m sprint performance and strength metrics of the CMJ are presented in Table 2. A moderate relationship was observed between several kinetic and kinematic jumping parameters and 10-m sprint time (range, r = −0.450 to −636; p ≤ 0.05). More noticeable was the significant predictive value of peak bar velocity and sprint performance (r = −0.630; p < 0.01) to sprint performance. Nonsignificant relationships were observed between mechanical impulse, RFD, and sprint time.
The aim of this study was to examine the associations between short sprint performance and multiple kinematics and kinetic variables during a vertical jump in a sample of male trained subjects. To our best knowledge, this is the first study to attempt examination of this issue with so much extent strength metrics measured with both a force platform and a linear transducer that can better explain 10-m sprint performance in a group of trained athletes as the one presented here. The major findings of the current experiment were the significant associations between 10-m sprint time and peak velocity (r = −0.630; p < 0.01) as well as the nonsignificant predictive value of mean force, mechanical impulse, and RFD. It may be suggested that the peak bar velocity is an important factor to consider to develop the short sprint performance in trained athletes.
In a previous study, Sleivert and Taingahue (20) examined the relationships between squat exercise and sprint performance. The authors observed a weak but significant correlation (r = − 0.45; p ≤ 0.05) between bar velocity and 5-m performance. However, the peak bar velocity used by Sleivert and Taingahue (20) corresponded to 30% of 1 repetition maximum during a traditional squat and not a stretch shorting movement such as the one presented here. Similar to the present study, but focused in the upper extremity muscles, Gorostiaga et al. (10) observed a significant relationship between bar velocity during a bench press test using 30% of maximal load and standing ball throwing velocity for elite (r = 0.67) and amateur team handball players (r = 0.71). This value is very similar to the one that was found in the current study. Nevertheless, Marques et al. (14) failed to observe any significant association between 5-m sprint times and mean propulsive velocity and also peak velocity.
Taken together, these data suggest that sprinting performance may be related to the capacity to move light external loads with lower limbs at maximal velocities. The sports science literature reported several studies that claimed significant correlations between force and sprint times (19), although others have failed to report such results (13,15). These discrepancies could be because of the fact that sprinting is a complex ability (2,3,8) that requires proper motor coordination between joints and muscles. Sprinting performance over very short distances (e.g., 10 m) is considered by many to require specific strength qualities and training techniques (2). It is well accepted that shorter sprints require a greater contribution of concentric muscle contractions and knee extensor activity (21). Young et al. (23) investigated the relationship between force measures (Smith Machine squat jump with a 19 kg bar load from a 120° knee angle) and sprinting performance of 20 elite junior track and field athletes. The best predictors of starting performance (time to 2.5 m) included force relative to body weight generated after 100 milliseconds from the start of the concentric jump movement (r = 0.73) and peak force (r = 0.72). Using a similar methodology, Wilson et al. (21) were able to observe that force at 30 milliseconds in a concentric squat jump was significantly correlated to sprint performance (r = 0.62). Other studies (21,23) also indicated that strength qualities such as the RFD or force applied at 100 milliseconds may be more important than maximal strength. Nesser et al. (19) reported significant correlations between 40-m sprint time and peak isokinetic torque at a speed of 7.85 rad·s−1 for the hip and knee extensors and knee flexors (r = −0.54 to −0.61). However, more recently, we (14) failed to show significant correlations between maximum RFD and sprint times but also between times to maximum rate of force with 5-m sprint performance. The present results, therefore, support what little has been reported in the literature and indicate that the development of peak force plays a larger role.
Vertical impulse has been defined as an important determinant factor of sprinting ability. Wilson et al. (21) investigated the relationship between impulse developed in the first 100 milliseconds of a concentric squat jump (unloaded) and sprinting ability over 30 m. Although reported as nonsignificant, they reveal a moderate correlation (r = −0.49) between impulse at 150° and sprinting ability. Interestingly, the relationship between impulse at 110° and sprint ability was low (r = 0.06, NS). Perhaps the influence of the starting knee angle is critical to the relationship between concentric only machine squat jump strength measures and sprint ability. It may be hypothesized that the length-tension relationship of the hip and knee extensors at lower starting knee angles is biomechanically less specific to the actual knee angles encountered in 10-m sprints. Our study corroborates the findings reported by Wilson et al. (21) showing that impulse is not a strong parameter in predicting sprinting time over short sprints. Therefore, a certain discrepancy should be expected between the CMJ impulse measure and the 10-m performance obtained. It should be kept in mind that the sample used by other studies comprised subjects of different sports, levels, and genders, which may account for the variation in results as compared with our study.
The RFD has been one of the most important variables to explain performance in activities where great acceleration is required (18). This can be related to the fact that the greater the RFD, the higher the power will be and the force generated against the same load. In most sports activities, the RFD is strongly related to performance abilities, such as sprinting, in which force production time is very small. A previously published report examining the relationship between the RFD and sprint performance have provided equivocal findings, with some studies reporting a significant relationship and others failing to observe a positive association (18). The present study failed to indicate a significant association between different rates of force measurement and 10-m sprint time. It is difficult to compare the results of these studies because they differ markedly in a number of factors, including the method of measurement. Yet, the variations in correlation coefficients may have been explained by the differences in reliability for measuring peak RFD (CV = 6–12%) when compared with measuring peak force (CV = 4–10%).
An important amount of scientific literature focuses their attention on clarifying the relationship between mechanical power output and athletic performance (9). A concern raised by this literature is that the power measurements and protocols used in these studies can vary considerably (5). Along the same line, Carlock et al. (4) stated that making comparisons between various studies is rather difficult because there are different exercises being used to measure peak power output. Despite these limitations, there is a growing body of knowledge on the relationship of power to sprint performance. Most researchers have found moderate to strong correlations between jump height (and/or relative peak power), measured during a vertical jump, and sprinting performance (11). Theoretically, there should be a significant relationship between these parameters, as a rapid stretch shortening cycle occurs both in jumping and sprinting. The present study indicated that peak power could explain approximately 36% of the sprint performance. Sleivert and Taingahue (20) who investigated the relationship between 5-m sprint times and power variables in trained athletes could observe that both mean power and peak power relative to body mass were to a moderate degree negatively correlated with 5-m sprint time (r = −0.64 to 0.68). Unfortunately, the authors chose not to incorporate body mass into the equation of force, asserting that it is not strictly mechanically correct to do so. The authors (20) noted that not using system mass has the effect of markedly reducing power outputs and altering the point on the power. Cronin and Hansen (6) noticed that peak power output measured on a force platform in the squat jump (expressed relative to subject's body mass) was found to be related to the 5-m (r = −0.55; p ≤ 0.05) and 10-m (r = −0.54; p ≤ 0.05) sprint times.
These findings highlight the important relationship between 10-m sprint and maximal lower-body strength, as assessed by the force, power, and bar velocity. This research possessed some limitation that should be considered. First, this study used a sample of well-trained subjects but not elite athletes. Second, one could speculate about testing of the lower body is conducted using a jump squat movement in an apparatus involving a barbell attached to vertical supports (Smith Machine apparatus). As a result, the Smith machine restricts movement of the barbell to the vertical plane and potentially decreases variability in performing the movement. However, this assumption is yet to be investigated. Furthermore, we only assessed lower-body kinetics and not kinetics variables playing an important role in short sprint performance. Given the fact that sprinting is a highly complex motor skill, it would be unlikely to find a single test that accounts for nearly all of the variability in sprinting.
Finally, it should be also noted that correlations can only give insights into associations and not into the cause and effect. Therefore, the practical applications described herewith need to be interpreted with this in mind. In terms of isoinertial assessment or any assessment for that matter, the strength and conditioning practitioner or scientist must be cautious in describing relationships between variables. As observed in this study, the relationship between CMJ mechanical parameters (i.e., velocity, force, and power) and sprint performance was found to differ according to each selected variable. This has important implications for correlational research in that nonsignificant relationships between movements may be reported, when actually it is the measure and not the movement that are unrelated. Based on the current results, it possible that the peak bar velocity is an important factor to consider to develop the short sprint performance in trained athletes. Thus, it is suggested that sprinting time performance would benefit from training regimens aimed to improve these performance qualities. Moreover, the great majority of research uses acyclic vertical type movements (e.g., squat, vertical jumps) to predict an activity that is cyclic and horizontal in nature. Further research may benefit from investigating movements that require greater horizontal force production.
Improvement in short distance sprint ability is a major training goal for many sports, and countermovement jumping is a well-recognized training exercise used to achieve this. In individual sports like basketball, soccer, or team handball, for example, athletes must improve sprint performance over very short distances to achieve better personal specific performances. In fact, a team sport athlete must sprint than his or her opponent. These findings should be interpreted with caution because correlations do not signify causation, so additional research is required to clarify whether improvements in upper-body strength, velocity, or power as a result of resistance and/or plyometric training will indeed improve jumping ability in trained track and field athletes.
Coaches often express the need to have access to an easily administered test that will allow assessment of the athlete without actually measuring the sports performance. This study represents one approach to assessing the physical state of elite team sports athletes that might satisfy this need.
We thank the dedicated group subjects who participated in this study.
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Keywords:Copyright © 2014 by the National Strength & Conditioning Association.
lower extremity; force; power; sprinting; bar velocity