The relationship between stretching and fatigue during resistance training is not clear, especially in sessions including several exercises and sets. Available studies about the potential effects of stretching on strength generally applied maximal tests, as for repetition maximum tests (1RM) (24), isokinetic peak torque (PT) (25), or power tests (jumps) (7). A recent meta-analysis (23) indicated that possible negative effects of stretching before exercise would be more likely to occur on maximal muscle strength (especially maximal isometric tests) and power, regardless of the training status. Resistance training sessions usually apply many exercises and sets, performed with loads lower than 1RM or maximal power (4,11,14). It is at least unclear whether stretching before resistance training sessions could negatively affect the performance in terms of the number of repetitions in multiple sets of strength exercises.
Another issue refers to the kind of stretching techniques. Probably, the most applied methods are the static and proprioceptive neuromuscular facilitation (PNF). In the static stretching, the muscle is gradually lengthened to the point of discomfort, which is held for some seconds. The PNF combines passive stretching and isometric contractions to facilitate neuromuscular inhibition, aiming to reach a greater range of motion (21). Most studies about the influence of stretching on the number of repetitions within resistance training applied static stretching (4,11,16,17). Two of these studies investigated the effects of PNF on muscle endurance (4,11). Franco et al. (11) compared the effects of static and PNF stretching on the number of repetitions performed in just 1 set of the bench press. The PNF was found to influence the fatigue rate, but no information was provided about the PNF protocol. More recently, Barroso et al. (4) investigated the impact of static, ballistic, and PNF stretching on the maximal strength (load corresponding to 1RM) and number of repetitions with submaximal load (3 sets at 80% 1RM) of the leg press performed by strength trained individuals. The stretching routines included 3 exercises performed with 3 sets of approximately 30 seconds each. The PNF was shown to decrease both 1RM load and number of repetitions, but evidently additional research would be necessary to extend these results to other muscle groups, subjects’ training status, and nature of stretching protocols.
For instance, some evidence suggests that the deleterious effects on strength seems to be related to the total stretching duration, being more significant after stimuli >45 seconds (23). Stretching routines performed before resistance exercises are frequently longer than this threshold (>60 seconds) (1,2,20). Given that stretching effects on muscle strength and endurance are transient and depend on the nature of the stretching stimuli (9,16), a possible dose-response relationship between stretching and fatigue during resistance training cannot be discarded—longer stretching may induce higher fatigue in comparison with shorter stretching protocols.
In brief, previous studies have examined the effect of stretching on maximal strength (6,10,13), but fewer have examined the effect of PNF stretching on fatigue in terms of the number of repetitions completed during a multiple set resistance exercise bout. However, the amount of fatigue that might be induced by different doses of PNF before resistance training has not been sufficiently addressed. This information would be useful for the practitioner when designing exercise programs and combining in the same training session stretching and strength exercises. Therefore, this study aimed to investigate the influence of a short PNF stretching protocol on the number of repetitions performed in multiple sets of the leg curl exercise. It has been hypothesized that short PNF stretching performed before training would have little or no influence on fatigue, as represented by the total number of repetitions across the multiple sets.
Experimental Approach to the Problem
To test the hypothesis that the total number of repetitions within multiple sets of a resistance exercise would be little affected by prior PNF stretching performed with low volume, data were assessed on 5 nonconsecutive days. On the first day, the participants underwent anthropometric measures and familiarization with the strength and stretching exercises. On the second day, the load corresponding to 10RM was determined for the leg curl, being retested on the third day. The leg curl was used for specificity reasons, because this exercise recruits the stretched muscles (hamstrings). By doing so, we aimed to control for the specificity of prior stretching effects on the muscle group activated during the strength exercise. On the fourth and fifth days, the strength exercise protocol was performed either after PNF stretching or without stretching (control session), in a counterbalanced design. All visits to the laboratory were separated by 48- to 72-hour intervals, and the subjects were instructed not to engage in any physical activity in the 24 hours before and after the experimental sessions.
Three sets of 2 stretching exercises for the hamstrings were performed according to the hold-relax PNF protocol. One minute after the stretching protocol, the strength exercise began. First, a specific warm-up with 12 repetitions at 40% of the 10RM load was performed. After 2 minutes, 4 sets to failure of the bilateral leg curl using 10RM load with 2-minute rest intervals between sets were performed. During the exercise sessions, the subjects were encouraged to perform all sets to concentric failure, and the same range of motion used during the 10RM testing was used to define the completion of a successful repetition.
All the subjects performed the tests and exercise sessions at the same time of the day (9:00–11:00 AM). They were also instructed to keep their dietary habits and to report any changes in physical activity or sleep patterns within the period of the study.
Nineteen men (age, 25 ± 1 years; body mass, 75.8 ± 4.2 kg; height, 178.1 ± 3.8 cm) with at least 6 months of recreational resistance training experience participated in the study. All the subjects answered the Physical Activity Readiness Questionnaire (PAR-Q) (22). Additional exclusion criteria were (a) use of drugs that could affect strength performance; (b) blood pressure abnormalities, heart disease, pulmonary function limitation, locomotion impairment as any bone, joint, or muscle problems that could limit the performance of the exercises; (c) positive PAR-Q. The experimental approach had institutional ethical board approval, and all the subjects signed an informed consent form before participation in the study according to the Declaration of Helsinki.
The PNF stretching was applied according to the “hold-relax” technique, including 3 steps: (a) first step, passive stretching until the maximal range of motion, which was sustained for 10 seconds; (b) second step, at the maximal range of motion, the subjects performed a maximal static contraction for 6 seconds, against a resistance applied by the evaluator; (c) third step, complete relaxation and additional passive stretching until a further maximal range of motion, which was also sustained for another 10 seconds. The following exercises were performed with the subjects laid in a supine position: hip flexion with knee extension and hip flexion with knee flexion. The stretching protocol was focused on the hamstring muscles and was repeated 3 times by alternating the stretched limb. The overall duration of the stretching exercises was approximately 2.5 minutes (150 seconds).
Ten Repetition Maximum Testing
The mass of all weight plates and bars used for measuring the 10RM was determined with a precision scale. The load corresponding to 10RM for the bilateral knee flexion (leg curl machine—Technogym, Italy) was assessed on 2 days separated by 48- to 72-hour intervals to establish its reliability. To minimize errors, all the subjects received standard instructions on the exercise technique before testing: (a) initial position, subjects sit on the chair with the knees extended and ankles in dorsiflexion; (b) final position, the subjects sit on the chair with the knees flexed at 90°.
A warm-up of 10–15 repetitions with a comfortable load was performed before the 10RM testing. The subjects had a maximum of five 10RM attempts of each exercise with 10-minute rest intervals between successive attempts. The standard exercise technique was followed, and verbal encouragement was given during the test. No pause was allowed between the eccentric and concentric phases of a repetition or between repetitions. For a repetition to be successful, a complete range of motion had to be completed. All the tests were performed at the same time of the day between 09:00 and 11:00 AM. The intraclass correlation coefficient (ICC) showed excellent day-to-day 10RM reliability (ICC = 0.96, p < 0.001).
Data normality was confirmed by univariate analysis. Therefore, the total number of repetitions (work volume) within the control and experimental situations was compared by means of a 2-way analysis of variance with repeated measures, followed by Scheffé post hoc verification in the event of significant F ratios. Two-tailed statistical significance for all analyses was accepted as p ≤ 0.05. All statistical analyses were performed using Statistica 7.0 for Windows (Statsoft, Tulsa, OK, USA).
The average 10RM load was 78.3 ± 6.9 kg. Intraclass correlation for the workload corresponding to 10RM was 0.92 (p < 0.001). There was a significant decrease in the number of repetitions performed across the sets (total decrease of 38.6 and 41.0% in control and PNF sessions, respectively; p < 0.001). However, no significant differences between control and PNF sessions were detected in the number of repetitions in each set (Figure 1).
This study aimed to investigate the acute influence of PNF stretching on the total number of repetitions in multiple sets of the leg curl performed with load corresponding to 10RM. Our findings suggested that PNF stretching had no influence whatsoever on the fatigue as reflected by the decrease in the number of repetitions performed along the sets.
It has been demonstrated that the performance in strength exercises may be jeopardized by prior stretching (7,10,13,15,19,24). However, it is worthy to mention that in previous research a high static stretching volume has been applied. Moreover, possible effects on strength were assessed using PT and 1RM tests. Cramer et al. (9) tested the effect of static stretching on the PT of knee extension at 60 and 240°·seg−1 in 14 young active women. The stretching session totalized 16 minutes, including 4 exercises performed with 4 sets of 30 seconds with 20-second intervals. A significant decrease in PT was detected in both speeds. Bacurau et al. (3) evaluated the effect of 20-minute static stretching of quadriceps and hamstrings muscles (6 exercises, 3 sets of 30-second stretching with 30-second intervals) on the load corresponding to 1RM leg press, and reported a decrease of about 15% (213.2 ± 36.1 vs. 184.6 ± 28.9 kg, p < 0.05). Winchester et al. (24) tested how many 30-second stretching sets (1–6 sets) would be necessary to provoke a decrease in the 1RM load in the knee flexion exercise. A single stretching set was capable of decreasing the 1RM by 5.4%, whereas the accumulated effect of the 6 sets induced a decrease of 12.4%.
On the other hand, in this study, the PNF method was applied with a substantial lower volume in comparison with that of these studies. As hypothesized, the number of repetitions decreased along the sets, but no additional effect could be accounted for the prior PNF stretching. It has been suggested that the decrease in strength performance after stretching would be related to a drop off in the activation of motor units, alterations in muscle and tendon viscoelastic properties, and changes in muscle fiber length-tension relationship (8,10,18). However, these alterations seem to be induced by high volume and intensity stretching stimuli (7,10,13), and their effects would be more likely to occur in maximal strength performance situations (13,18,19,24).
This idea has been reinforced by some studies (12,24), which demonstrated that strength performance may be reduced by stretching routines over 60 seconds, whereas durations <30 seconds could be applied without negative effects on strength performance. This could help in explaining the present results, because the strength performance was assessed in terms of the total number of repetitions with submaximal load, and given that the PNF stretching duration was shorter than that usually found in research that reported a significant decrease in strength performance.
This is probably the first study to examine the number of repetitions able to be completed after a relatively short duration PNF stretching protocol in hamstring muscles. However, results from some research investigating the effects of stretching protocols with low volume on strength performance seem to concur with our findings. For instance, Beedle et al. (5,8) failed to identify effects of static stretching (3 sets of 15- and 10-second intervals) on the load corresponding to 1RM of the bench press and leg press. Winke et al. (25) reported similar results for the isokinetic strength at 60 and 210°·s−1, measured in the knee extension performed with or without prior static stretching (2 exercises, 3 sets of 30- and 15-second intervals).
Even studies that have investigated the effects of PNF on muscle endurance and strength applied sensibly longer stimuli compared with those in this study. Barroso et al. (4) applied 3 exercises performed with 3 sets of 30-second PNF, which represents approximately 5 minutes of stretching vs. 2.5 minutes in this study. Our findings indicate that PNF stretching lasting approximately 2.5 minutes did not negatively influence the number of repetitions completed in a multiple set resistance exercise bout. It is therefore tempting to speculate that there might be a threshold of PNF duration to affect the muscle endurance. These results warrant future investigation evaluating the duration of PNF stretching and its relation to the number of repetitions performed in resistance exercises.
A possible limitation of this study was to investigate the effects of PNF on fatigue, with regard to only 1 exercise instead of observing a sequence of resistance exercises. However, the interval between stretching and resistance exercises placed at the end of a greater sequence would be too long, limiting the potential effects of stretching on these exercises. Additional research is needed to address the possible effects of concurrent training, combining flexibility and strength exercises, on the fatigue rate. It would be useful to assess the number of repetitions within a sequence of resistance exercises performed after different types of stretching routines.
In conclusion, prior PNF stretching lasting approximately 2.5 minutes, including 3 sets of 2 exercises aiming the hamstrings, did not reduce the number of repetitions of the leg curl performed with multiple sets until fatigue. Contrarily to some previous research, short duration PNF stretching might be part of flexibility training before a strength bout including multiple sets and submaximal load. Future research should evaluate the possible existence of a threshold in PNF stretching duration to negatively affect muscle endurance.
Our findings suggest that short duration PNF stretching may be performed before resistance training, with no deleterious consequence on the fatigue reflected by the total number of repetitions, in routines including multiple sets and submaximal loads. This is useful information to practitioners who desire to apply flexibility and resistance training in the same session (“concurrent training”), because total training volume (number of repetitions × external load) does not seem to be affected.
This study was partially supported by grants from the Carlos Chagas Filho Foundation for the Research Support in Rio de Janeiro (FAPERJ) and the Brazilian Council for the Research Development (CNPq).
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