Sprint performance is of great importance in many sports, with elite soccer players spending approximately 11% of the game sprinting, which equates to a 10- to 15-m sprint every 90 seconds (3,20), with similar findings reported in rugby league (5,13), rugby union (7–9), and field hockey (16). The outcome of the game tends to be determined during these periods of sprinting.
Sprinting requires high levels of acceleration and as such strength to overcome the inertia of the body mass (BM). A number of studies have investigated the relationship between strength and sprint performance, demonstrating that, in general, stronger athletes perform better during sprint performances (2,6,14,19). This may be explained by the fact that peak ground reaction forces and impulse are strong determinants of sprint performance (11,17,18,21).
Studies have used various methods to assess strength, including isokinetics (2,4), machine squats (10), and free weight squats (2,4,6,14,19), when investigating the relationship between strength and sprint performance. The strongest correlations were achieved with free weight squats (14,19). Others have also looked at multiple tests, usually including assessment of power, to develop a model to predict sprint performance (4,15).
There appears to be a clear relationship between maximal free weight squat strength and sprint performance (2,19) in well-trained individuals. Wisloff et al. (19) found a strong correlation (r = −0.94) between 1 repetition maximum (1RM) squat performance and 10-m sprint time; however, in contrast, Harris et al. (10) found a weak and nonsignificant correlation between Smith machine squats and 10- and 40-m sprint times in athletes. More recently, McBride et al. (14) demonstrated a stronger relationship (r = −0.605, p = 0.01) between relative strength and sprint performance (40 yd) in individuals with a high relative strength (≥2.1 kg/kg) compared to individuals with a lower relative strength (<1.9 kg/kg). However, in contrast, 5-yd sprint times and relative strength showed a nonsignificant correlation (r = 0.45, p = 0.0698). Each of these studies has used well-trained athletes, who tend to have highly developed strength and sprint performance; however, no investigation has been identified if these relationships occur in recreationally trained individuals. The aim of this study, therefore, was to compare maximal back squat strength and 5-, 10-, and 20-m sprint performances and their relationships in well-trained athletes and recreationally trained individuals. It was hypothesized that the relationships between maximal squat strength and sprint performance in recreationally trained individuals would be weaker because of a lower level of conditioning.
Experimental Approach to the Problem
This study was designed to describe the maximal back squat strength and sprint times (dependant variables) over 5, 10, and 20 m and then to determine the relationship between relative strength and sprint performance in well-trained athletes and recreationally trained (≥3 × week for ≥2 years) individuals (independent variables). The sprint distances were selected because these are representative of sprint distances covered during competition in multiple sprint sports (3,5,7–9,12,15,19).
Twenty-four, professional, rugby league players (height = 1.83 ± 0.06 m, BM = 96.5 ± 11.14 kg, 1RM = 170.6 ± 21.4 kg, 1RM/BM = 1.78 ± 0.27) and 20 recreationally trained individuals (height = 1.75 ± 0.07 m, BM = 78.72 ± 10.68 kg, 1RM = 138.6.6 ± 27.9 kg, 1RM/BM = 1.77 ± 0.33) participated in this investigation. The rugby league players had just completed preseason training (including a 4-week strength mesocycle and a 4-week power mesocycle), and all recreationally trained subjects had just completed a power mesocycle (4–8 weeks). All participants provided written informed consent, which was approved by the University Ethics Committee. All procedures conformed to the Declaration of Helsinki.
Order of Testing
Participants attended the human performance laboratory on 2 separate occasions, at the same time of day, to undertake the maximal strength testing and the sprint performances, separated by 48–72 hours. Participants had abstained from training on the day before testing and were asked to maintain a consistent dietary intake on each day of testing.
Maximal Strength Testing
One repetition maximum back squat was assessed via a standardized protocol (1), with warm-up loads approximated via individual training loads. During all attempts, participants were required to squat to a depth where a 90° knee angle was achieved. Before the start of the warm-up, this knee angle was assessed using a goniometer while the participant was squatting. A bungee was placed at the appropriate height so that the participants' buttocks touched the bungee once the squat depth was achieved (Figure 1); this was also reinforced with verbal commands. All participants achieved their 1RM within 5 attempts.
After a standardized warm-up, participants performed three 20-m sprints on an indoor track (Mondo, SportsFlex, 10 mm; Mondo America Inc., Mondo, Summit, NJ, USA), wearing standard training shoes. Sprints were interspersed with a 1-minute rest period in accordance with McBride et al. (14). Time to 5, 10, and 20 m was assessed using infrared timing gates (Brower, Speed Trap 2, Wireless Timing System, Draper, UT, USA.). All subjects began with their front foot positioned 0.5 m behind the start line and were instructed to perform all sprints with a maximal effort.
Intraclass correlation coefficients were performed to assess repeatability between each of the 3 sprints. Independent t-tests were performed to determine differences in strength and sprint times between well-trained athletes and recreationally trained individuals. Pearson's correlations were also performed to determine relationships between measurements of strength and sprint times using SPSS software (Version 16.0; IBM, Armonk, NY, USA). An a priori level of significance was set at p ≤ 0.05. Where no significant differences were found between well-trained and recreationally trained individuals, the data were pooled for correlation analysis. Power calculations, for correlations, were calculated using G*Power (Version 3.1, University of Deusseldorf, Germany) (12).
Intraclass correlations demonstrated a high level of reliability between each of the 3 trials for the 5-, 10-, and 20-m sprints (r = 0.96, p < 0.001; r = 0.97, p ≤ 0.001; r = 0.97, p < 0.001, respectively).
There were no significant differences (p > 0.05) in 5-m sprint times between the well-trained group (1.05 ± 0.06 seconds) and recreationally trained group (1.08 ± 0.06 seconds), although the well-trained group demonstrated the lowest mean 5-m sprint times. In contrast, the well-trained group generated significantly (p = 0.004; p = 0.002) lower 10- and 20-m (1.78 ± 0.06 seconds; 3.03 ± 0.09 seconds) sprint times compared with the recreationally trained group (1.84 ± 0.07 seconds; 3.13 ± 0.11 seconds) (Figure 1).
In terms of absolute strength, well-trained individuals (170.63 ± 21.43 kg) were significantly stronger (p = 0.01) than the recreationally trained individuals (135.45 ± 30.07 kg) (Figure 2); however, in contrast, there were no significant differences (p > 0.05) in relative strength between groups (1.78 ± 0.27 kg/kg; 1.78 ± 0.33 kg/kg, respectively).
Statistically significant correlations were found between 5-m sprint time and relative squat strength (r = −0.613, power = 0.96, p = 0.004) for the combined data.
Relative squat strength and both 10- and 20-m sprint times showed significant correlations in the recreationally trained group (r = −0.621, power = 0.51, p = 0.003; r = −0.604, power = 0.53, p = 0.005, respectively) (Figures 3 and 4) but nonsignificant correlations for the well-trained group (r = −0.336, power = 0.52, p = 0.109; r = −0.360, power = 0.52, p = 0.084, respectively).
This investigation found no significant differences (p = 0.068) between well-trained (1.05 ± 0.06 seconds) and recreationally trained (1.08 ± 0.06 seconds) groups for 5-m sprint times, although the well-trained groups were slightly (0.03 seconds) quicker. This may be because of the fact that even though well-trained individuals (170.63 ± 21.43 kg) were significantly stronger (p = 0.01), in terms of absolute strength, when compared with recreationally trained individuals (135.45 ± 30.07 kg), there were no significant differences (p > 0.05) in relative strength between groups (1.78 ± 0.27 kg/kg; 1.78 ± 0.33 kg/kg, respectively). The initial start (0–5 m) during a sprint is likely to be affected more by relative strength than absolute strength because of the requirement of accelerating the individuals' BM. There may also be an upper threshold for strength, beyond which further increases in strength have less effect on sprint performance and acceleration.
In contrast, the well-trained group performed significantly (p = 0.004; p = 0.002) quicker 10- and 20-m (1.78 ± 0.06 seconds; 3.03 ± 0.09 seconds) sprints compared with the recreationally trained group (1.84 ± 0.07 seconds; 3.13 ± 0.11 seconds). This may be partly explained by Baker and Nance (2) who postulated that performance over greater sprint distances may be affected by the stretch shorten cycle, therefore possibly reducing the relationship between maximal strength and sprint performance over longer distances, as can be seen in the results of this investigation. Further possible explanation for the quicker 10- and 20-m sprint performances by the athletes may be explained by better technique and the ability to generate high forces at high velocities possibly because of a higher percentage of type II muscle fibers, although this was not assessed within the study.
Statistically significant correlations were found between 5-m sprint time and relative squat strength (r = −0.613, power = 0.96, p = 0.004) for the combined data, which is stronger than the relationships previously reported by McBride et al. (14), who found a moderate (r = −0.45, power = 0.44, p = 0.0698) although nonsignificant relationship between relative squat strength and 5-yd sprint times.
Relative squat strength and both 10- and 20-m sprint times showed significant correlations in the recreationally trained group (r = −0.621, power = 0.51, p = 0.003; r = −0.604, power = 0.53, p = 0.005, respectively). However, this was not as strong as the relationship previously reported by Wisloff et al. (19) (r = −0.94, p < 0.05) between absolute 1RM squat performance and 10-m sprint time; in contrast, these findings are similar to those previously reported by McBride et al. (14) who found a strong relationship (r = −0.605, p = 0.01) between relative strength and sprint performances (10 and 40 yd). As aforementioned, Baker and Nance (2) explain that differences in the relationships between strength and sprint performance may be partly explained by a greater involvement of the stretch-shorten cycle over greater distances or other factors such as reduced muscle cocontraction, with increased sprint skill. The assessment, therefore, of peak force or peak power during squat jumps or countermovement jumps may be a better predictor of sprint performances over distances of >10 m in well-trained athletes.
It is suggested that future research should determine if increasing squat strength results in a concomitant increase in sprint performances and also determine if muscle tissue distribution rather than relative strength explains the differences in sprint performances more clearly.
Because of the relationship between squat strength and sprint performance, it is suggested that increasing squat strength is likely to improve sprint performance at short distances (∼5 m). Moreover, increasing squat strength may improve sprint performance (decrease sprint time) in recreationally trained individuals to a greater extent than in well-trained athletes, especially at distances up to 20 m. These results, indicating that relative strength, are important for initial sprint acceleration (0–5 m) in all athletes but more strongly related to sprint performance over greater distances (10 and 20 m) in recreationally trained individuals.
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