Fifty healthy, trained athletes (age, 20.5 ± 1.4 years; age range, 19.1–22.0 years; height, 1.81 ± 0.2 m; weight, 77.2 ± 2.6 kg; body fat, 8.2 ± 2.6%) volunteered to participate in this study (Table 2). Inclusion criteria included: (a) absence of musculoskeletal injuries for at least 6 months before the study, (b) active participation in sport training (≥4–6 times per week), (c) lack of any physical activity/exercise for at least 48 hours before each trial, and (d) high speed (10 m time <1.94 seconds; 20 m time <3.1 seconds) and agility (T-test time <10 seconds) performance according to National Strength and Conditioning Association standards and previous studies that used elite athletes (3,5,16,17). After receiving a detailed verbal and written explanation of the study's benefits and risks, each participant signed an informed consent. The study has the approval of university's institutional review board and ethical committee, and procedures were in accordance with the Declaration of Helsinki.
Body mass was measured to the nearest 0.5 kg (Seca 710; Seca, Birmingham, United Kingdom) with subjects wearing the underclothes and barefooted. Standing height was evaluated to the nearest 0.5 cm (Seca stadiometer 208). Percent body density and fat were calculated from 7 skinfold measures using a Harpenden caliper (John Bull, Bedfordshire, United Kingdom). Each skinfold measured at the right body side and the average of 2 measurements was recorded.
Sprint and Agility Performance Evaluation
At baseline (control trial) and at the end of each experimental trial, participants completed speed and agility testing. A familiarization period on sprint and agility testing was used 3 weeks before the study to avoid a learning effect on performance. Infrared light sensors (Newtest, Oulu, Finland) were used to measure time to complete a 20-m sprint (with split at 10 m) and a T-test (16). Sprint and agility testing was performed from a standing start position. To control the effect of possible metabolic fatigue, each participant performed each test twice with a 4-minute rest in between. Speed and agility testing order was randomly selected, that is, speed and agility testing was performed according to 1 of 4 sequence schemes: (a) speed-speed-agility-agility, (b) speed-agility-speed-agility, (c) agility-speed-agility-speed, and (d) agility-agility-speed-speed. The best time recorded was used for statistical analysis. Interclass coefficient (ICC) between measurements speed testing ranged between 0.90 and 0.95 for MP and 0.92 and 0.95 for HP, whereas ICC for agility testing ranged between 0.91 and 0.96 for MP and 0.93 and 0.95 for HP.
A 1-sample Kolmogorov-Smirnoff test verified data normality, and thus, the use of nonparametric tests was not necessary. A 1-way repeated-measures analysis of variance (ANOVA) was used to establish the effect of SS duration on speed and agility testing. To estimate the effect of speed/agility performance level (HP vs. MP), a 2-way (group × time) repeated-measures ANOVA with planned contrasts on different time points was used for data analysis. A Bonferonni test was used for post hoc analysis when a significant effect was detected. Significance was accepted at p ≤ 0.05. For effect size determination, generalized eta-squared values (
) for repeated measures were calculated (2). Data are presented as mean ± SE.
Data analysis revealed that sprint performance of 10 and 20 m was enhanced by 2.8–3.2% (p < 0.001;
= 0.12 for 10 m,
= 0.11 for 20 m) in response to 15- and 20-second SS trials as compared with the control (Figures 2A, B). Sprint performance remained unchanged in all other trials. Agility performance remained unaffected in all trials (Figure 2C).
The second objective of the study was to determine whether the effect of SS duration depends on the level of performance (speed and agility) of participants. Thus, the statistic median was used to classify participants as either HP or MP. The statistic median was 1.82 seconds for 10-m speed testing, 3.01 seconds for 20-m speed testing, and 9.86 for T-test agility testing. The HP participants demonstrated a better performance in speed (10 and 20 m) and agility testing in all trials (p < 0.001;
= 0.97–0.98 for speed testing and
= 0.9 for agility testing) than MP. Post hoc analysis revealed that MP improved (p < 0.001;
= 0.25 for 10-m testing,
= 0.35 for 20 m testing) their performance in both speed tests in the 15- and 20-second trials by 4.2 and 4.1%, respectively (Figures 3A, B). Speed performance in HP remained unaltered in all trials. In respect to agility performance, MP demonstrated an improved (p ≤ 0.05;
= 0.36) performance in T-test in response to the 10-second (by 3.3%) and 15-second trials (by 3.6%). In contrast, agility performance in HP remained unchanged in all trials (Figure 3C).
Results of the present investigation suggest that the effects of SS exercises on speed and agility performance may be duration dependent. Shorter stretch durations (i.e., ≤20 seconds) may result in acute improvement of the time to complete the speed and agility tests. Furthermore, it seems that the effects of SS on speed and agility performance are evident only in MP participants. Highly conditioned (in speed and agility) individuals may not benefit from this type of stretching routine.
Previous research has produced contradictory results in regards to the effects of SS and other forms of stretching on speed performance. Specifically, speed has been found to increase (23), decrease (6,12,13,15,17,29,37), or remain unaffected (5,10,34). This discrepancy may be attributed to differences in SS durations or differences in conditioning levels of participants or muscle groups stretched in these studies.
Previous research has used SS of less than 30 seconds (short duration), between 30 and 60 seconds (moderate duration), and more than 60 seconds (long duration). Most studies suggest that longer durations (90 seconds to 20 minutes) of SS seem to induce speed or strength impairment for as long as 10–60 minutes poststretching (8,9,26,30) while 1 study (20) showed that long-duration SS may not affect power performance. This study showed that longer duration (60 seconds) leave speed unimpaired. Shorter SS durations have been shown to improve (23), deteriorate (12,13), or leave speed or sport-specific performance unimpaired (19,27). Our results indicate that short SS durations (15 and 20 seconds) may actually improve speed acutely. These results coincide with previous findings, suggesting that short SS durations may not disrupt the viscoelastic properties, sarcomeric cross-bridge kinetics, and stiffness of MTU (9,24,25,36). In contrast, longer durations of SS may reduce optimal cross-bridge overlap that, according to length-tension relationship, could diminish muscle force output (14). In addition, prolonged static MTU elongation may reduce its passive or active stiffness (21) and thus its force-generating capacity for as long as 15 minutes poststretching (14). However, SS durations >60 seconds although used to increase muscle's stretching potential, they are not used before competition or training which require maximal force generation. Long SS durations in this study were of smaller magnitude than those used in previous studies reaching several minutes. A plausible explanation for the improved speed performance in response to the short SS protocols is that 10- to 20-second SS may not interfere with MTU properties. Moderate SS durations (30–60 seconds) did not affect speed performance in this study, which is in contrast with previous studies that reported an impairment of 10- to 30-m sprint performance in response to 40- to 50-second SS (6,15,29). However, others also reported no effect of moderate SS duration on sprint, reaction time, and other power-related performance, such as explosive force and vertical jump performance (1,5,34,35).
Differences in speed performance may also obscure observed changes in response to SS protocols of various lengths. Elite sprinters seem to be more sensitive to changes in the MTU's viscoelastic properties and stiffness in response to SS (14,32), and thus, the rate of force transmission, essential in sprinting, may be more susceptible to impairment in HP participants. In fact, leg stiffness has been associated with maximum sprint velocity (7,22). However, Knudson et al. (18) reported that SS-induced decrements in power performance (i.e., vertical jumping) may be related to neuromuscular inhibition rather than changes in muscle stiffness. Flexibility level may also contribute to differences in speed responses to SS protocols because it has been reported that those with higher sit-and-reach performance tend to demonstrate a greater degree of impairment to SS (10). Discrepancies among studies may also be attributed to the different population used by various studies. This study and others used trained individuals (11,15,17,23), whereas other investigators have used adolescent athletes (27,29).
The effects of SS on agility performance have received less attention in the literature. Some studies have reported deterioration (13,15) or no effect (5,23,33) of short-to-moderate duration SS on agility performance. However, these studies examined effects of only 1 SS duration on agility performance. Our results indicate that all agility performance remained unaffected by SS independent of total duration. Gelen (15) reported a deterioration effect of SS (50 seconds) by using a dribbling test, suggesting that the movement pattern may affect the final outcome. Chaouachi et al. (5) also showed that agility remains unaltered by SS but they tested speed and agility on different days, whereas we and others (15) tested them on the same day. However, this factor may have minimal effect because the order speed and agility testing were randomized in this study. A factor that may be critical for the relationship between SS and agility performance is agility level because MP participants did demonstrate an improvement at short durations (10–15 seconds), whereas HP participants did not, independent of duration length. In accordance with our results, numerous studies have suggested that power performance of elite athletes, such as agility, may not be affected (5,10,33) or even deteriorated by SS (12,13,15,17). Studies that used highly trained athletes showed that SS deteriorated speed performance (28,37). Nevertheless, the significance and the repeatability of this finding remain to be explored by future investigations.
This study showed that SS duration of 10–20 seconds may actually improve speed and agility performance, whereas longer stretch durations do not affect, negatively or positively, the performance outcome. This effect may be related to the level of speed and agility performance because those at moderate level demonstrated the greater gains, whereas those at higher conditioning level did not.
Information produced by this study is critical for athletes and practitioners because stretching is an integral part of athlete's preparation for training and competition. It seems that SS of short-to-moderate duration (<60 seconds) may not be detrimental for power performance (i.e., speed and agility) as studies that examined longer durations (>60 seconds) have previously suggested. It must be noted that, according to these results, athletes with high initial speed (<1.82 seconds for 10-m testing and <3.01 seconds in this study) and agility (<9.86 seconds in this study) performance may not be benefited by short-duration SS. In contrast, our results indicate that athletes of lower initial speed (>1.82 seconds for 10 m testing and >3.01 seconds in this study) and agility (>9.86 seconds in this study) performance may be benefited by short SS duration protocols. However, this information must be evaluated by future investigations.
The authors thank all participants for their contribution and commitment to this study. This study was supported by departmental funding, a grant received by Bodosakis Foundation (Greece) for instrument purchase and Grant funding CE-80739.
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Keywords:Copyright © 2016 by the National Strength & Conditioning Association.
warm-up; T-test; flexibility; sprinting; athletes