The exercises were performed with both the sides, always following the same order (right pectoral, left pectoral, right triceps, and left triceps). Each static stretching exercise was performed in a single set lasting 30 seconds and a rest interval of 10 seconds between the movements. The subjects executed the stretching until the muscular discomfort point. In the CC, the subjects remained at rest for 150 seconds on a chair.
The subjects arrived at the laboratory 2 hours after having a light meal and were instructed to avoid any caffeine- and alcohol-containing beverages 48 hours before the tests. Thirty seconds after the SC or CC, the subjects performed the fatigue protocol. The protocol consisted of 4 sets of BP at 80% of 1RM until voluntary exhaustion, with a rest interval of 2 minutes between the sets. The subjects were asked to perform the maximum number of repetitions in each set. The rate of force decline between the first and the fourth sets was used as fatigue index (FI) as proposed by Sforzo and Touey (29):
where FI is fatigue index and TF is total force (load lifted up × number of repetitions performed).
The data are presented as mean ± standard error. Normality was checked using Shapiro-Wilk’s test, the homogeneity of variances was verified using Levene's test, and the sphericity was assessed by Mauchly's test. The total number of repetitions and the FI between conditions and across days were compared using the dependent Student's t-test. Two-way analysis of variance for repeated measures with main effects for conditions (SC and CC) and sets (1–4) was used to compare changes in repetitions during sets between the groups. When the F ratio was significant, Tukey's post hoc test was applied to identify the differences. The data were stored and analyzed using the statistical package for the social sciences (SPSS for Windows Version 17.0; SPSS, Inc., Chicago, IL, USA). Statistical significance was set at p ≤ 0.05.
No significant difference was found between the 2 experimental sessions (fatigue protocol) for the total number of repetitions (day 1, 20.5 ± 0.9; day 2, 20.3 ± 1.1; p = 0.982) and FI (day 1, 75.5 ± 3.3; day 2, 80.7 ± 3.2; p = 0.145).
The main finding of our study was that the short-duration static stretching performed before resistance exercise does not affect the FI and the total number of repetitions performed in multiple sets of BP.
Investigations about the acute effects of static stretching on fatigue and the number of repetitions performed in multiple sets in resistance exercises are limited, unlike the great evidence observed in maximal and explosive strength (14,32). Our results are in agreement with some of these few previous investigations that have not found a negative effect of static stretching on strength endurance (9,10). In this sense, the novel and most significant feature of our study was the inclusion of multiple sets in a very popular exercise (BP). Nevertheless, other researches have shown that static stretching performed before resistance exercise reduces the number of repetitions in a single and multiple set (2,4,7,20). This difference in results may be related to some important methodological procedures, mainly, the duration of stretching, in which the negative influence on muscular endurance was observed in protocols with a larger static stretching volume (7,9,17,24). It is worth noting that these investigations, which have observed a decrease in performance, applied a stretching protocol with a larger volume than what is commonly used in practice (24). In this regard, we chose 30 seconds of stretch duration for each movement because this short time is sufficient to exert influence on the structural properties of soft tissues (12) and negatively influence muscle performance (5,36). Taken together, these results highlight that longer stretch times may negatively affect the structural properties of soft tissues, muscle fiber length-tension relationship, and a decrease in the activation of motors unit, which may explain, at least partially, the effect of static stretching on performance decrements (8,16,35).
The type of exercise used in studies also could contribute to the conflicting results among investigations because fatigue is dependent on the task (13). Another possibility may be associated to the difference in subject training status among studies. The movement velocity in which the resistance exercise was performed also may affect the results (19). In our investigation, the participants were oriented to perform the exercise at a fast velocity, although the movement velocity has not been controlled. Additionally, other variables, such as the test used to evaluate fatigue and the lack of standardization of the muscle groups stretched, make difficult a more detailed comparison between the studies. Another critical point observed in previous studies that focus on static stretching is the intensity standardization. The relative subjectivity to the articular discomfort adopted in most studies may bias the comparisons.
The present study analyzed the outcomes in detrained subjects. Considering that the status of resistance training plays an important role in motor units recruitment (11), and one of the potential mechanisms suggested to performance decrements is the depression of motor neuron excitability particularly in high-threshold motor units (8), it would be reasonable to expect the absence of effects in subjects who were detrained in resistance training exercise because detrained individuals might not be able to recruit their fastest motor units efficiently. However, this hypothesis may be not suitable because previous investigation have found no static stretching effect even in trained subjects. For example, Franco et al. (9) studied 19 men who were performing resistance training for at least 6 months and observed that previous static stretching (1 × 20 seconds, 2 × 20 seconds, and 3 × 30 seconds) did not influence the number of repetitions performed in BP. Gomes et al. (10) investigated 15 men who had at least 2 years of resistance training practice, thus highly familiarized with the exercises performed in their study (BP and knee extension), and yet they did not find influence of previous static stretching (3 × 30 seconds) on the number of repetitions. Our findings extend these reports to detrained subjects. Taking into account that in practical and research settings, the subjects who have practiced resistance training in the past may be interested in restarting their exercise program, our results may be useful to understand how previous static stretching exerts influence in detrained subjects.
Recently, previous studies have shown that for adequate evaluation of maximal strength, familiarization sessions for the 1RM test should be conducted until the stabilization of the load lifted is achieved (23,33). Otherwise, there might be a strong possibility of underestimating maximal strength. Given that we believe that the three 1RM trial applied in our experiment is a strong point of our study.
The present investigation has some important limitations such as these data cannot be generalized neither to other exercises and populations such as more trained subjects nor for complete resistance training sessions. Furthermore, we did not perform a direct measurement of flexibility after the static stretching exercises. However, to our knowledge, this was the first study to investigate the effect of prior static stretching that uses a more practical approach, such as the duration of applied stretching, and the number of sets used in a very popular exercise (BP), chiefly, in men.
The previous findings of a decrease in muscle performance have led a number of coaches and exercise professionals to recommend the avoidance of stretching before strength activities. However, these recommendations may be questioned, as these protocols are prolonged and do not represent the common routines applied in the field. Our results may be useful for those individuals who are afraid of practicing stretching during warm-up procedures. Thus, we do not recommend against static stretching during warm-up before resistance training exercise because such conduct does not seem to influence positively or negatively the performance in multiple sets at least in the BP exercise.
The authors thank all the participants for their engagement in this study and also the Coordination of Improvement of Higher Education Personnel (CAPES/Brazil) for the master scholarship conceded to A. S. Ribeiro, and the National Council of Technological and Scientific Development (CNPq/Brazil) for the grants conceded to E. S. Cyrino.
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