The warm-up (WU) is widely used by trainers and physical therapists before a range of exercises and athletic performance. WU can be divided into 2 main categories: active warm-up (AWU), consisting of aerobic exercise of low and mild intensity of 5-10 minutes (7), and passive warm-up (PWU), requiring a heat source to raise muscle or core temperature. Most WU effects were ascribed to temperature-related mechanisms, i.e., decreased muscle stiffness, increased nerve-conduction velocity, increased anaerobic energy yield, and thermoregulatory strain (5,6). Other mechanisms, not related to temperature changes, were proposed, such as an increased resting oxygen uptake and postactivation potentiation (14). It was also hypothesized that WU may involve some psychological effects such as increased preparedness (5,6).
Stretching is often employed in addition to WU. The acute effects of stretching on muscle performance, in particular, passive (static) stretching, are still under debate. Some authors suggested a decrease of force and power production after a stretching protocol. The reasons for such a decrease are both structural and neuromuscular. Lieber (18) in 1991 found an increase of sarcomeral length after passive stretching on single fibers of frog semitendinous muscle in vitro. Guissard and Duchateau (14), Magnusson et al. (19), and Taylor et al. (24) observed a decrease in stiffness of the musculo-tendinous unit and an increase in muscular compliance of the plantar flexor with a consequential decrease of force measured under isometric conditions. Moreover, Purslow (22) observed a change in the reorientation of the intramuscular connective tissue after passive stretching. From the neuromuscular point of view, Avela et al. (1) and Guissard et al. (15) found a decreased activity of muscle spindles of the plantar flexors after a bout of static stretching. The stretching-induced effects on muscle were investigated also under dynamic conditions, especially using jumping tests as a mean to evaluate the maximal muscular power (Ẇpmax ). Church et al. (8), Cornwell et al. (9), and Young et al. (30) found a decrease in Ẇpmax after a bout of stretching during squat jump (SJ) and countermovement jump (CMJ) tests. The authors suggested a decrease of musculo-tendinous unit stiffness and a decreased spinal reflex excitability. However, Hunter and Marshall (16) found no differences in the Ẇpmax developed during both CMJ and drop jump after static stretching. Witvrouw et al. (27) suggested that stretching maneuvers could improve the performance of athletic tasks requiring high-intensity short-stretching cycles, due to the storing of elastic energy during the stretching itself, and to the subsequent release from muscles and tendons during the concentric phase.
There are few studies in the literature dealing with the combined effects of WU and stretching on Ẇpmax. Church et al. (8) in 2001 found a significant worsening of SJ performance after AWU plus stretching versus AWU alone. Behm et al. (2,3) in 2004 found an increased reaction time after stretching in respect to AWU alone, concluding that an acute bout of stretching might have impaired the WU effects. Young et al. (29) reported that the addition of two 4-minute periods of static stretching to a run caused an impairment to fast stretch-shortening-cycle muscle performance. Yamaguchi et al. (28) found that static stretching applied for 30 seconds neither improved nor reduced muscular performance, whereas McMillian et al. (20) found that the subjects scored better after static stretching on the 5-step jumps then after no WU. Other authors focused their studies on the relationship between PWU and stretching (12,17,21). They confirmed that superficial heat elicited by the application of moist heat packs induced an increase in muscle compliance similar or higher than that evoked by stretching alone and leading in turn to an improvement of the range of motion.
These conflicting results and the lack of studies, as far as we know, evaluating the effect of PWU+S on the muscular performance prompted the present study. Our final aim is to provide practical suggestions to trainers, coaches, and sports medicine physicians on the use of stretching combined with WU to increase muscular performance. This study will show that AWU alone is the best procedure to be adopted to enhance the maximal anaerobic power, and, when combined with stretching, the positive effect is slightly reduced.
Experimental Approach to the Problem
Maximal anaerobic power was measured with 2 modes of vertical jump test: squat jump (SJ) and countermovement jump (CMJ). For the first test the subjects started the jump with a knee angle fixed to 90° and the muscles were activated for concentric contraction only. For the second, the subjects were asked to start in orthostatism and, after a quick flexion/extension of the knees, to jump, reaching the maximal height.
Fifteen male subjects (age 23 ± 0.2 years, height 177 ± 2 cm, body mass 74 ± 2 kg, thigh skin-fold 7.7 ± 1.7 mm; (mean ± SE)) moderately active (5.5 ± 2 hours per week of aerobic training) volunteered in this study. They were fully informed about aims and procedures and signed a consent form before enrollment. Approval was obtained by the ethical committee of the University of Milan.
All the tests were performed in the same room at constant temperature (21 ± 1°C) and relative humidity (50 ± 2%), at the same time of day. The jump tests consisted of two different random series of jumps, each of them in sequence of 5 SJ or 5 CMJ. To avoid muscle fatigue, a pause of 40 seconds between each jump and a resting period of 5 minutes were observed. The subjects performed the jumps in different conditions (Figure 1): C, control condition (jumps alone, no stretching and no warm up; Figure 1A); S, jumps preceded by stretching (Figure 1B); AWU and PWU, jumps preceded by active warm-up or passive warm-up (Figure 1C); and AWU+S and PWU+S, jumps preceded by S and AWU (AWU+S) or PWU and S (PWU+S) (Figure 1D).
Stretching consisted of 2 maneuvers currently used by athletes. In the first, the hamstring and the calf complex were passively stretched by a trainer; the subjects lay supine with the stretched leg completely extended and the ankle in dorsiflexion. In the second maneuver, the subjects were in the orthostatic position, and the hip flexor muscles and the quadriceps were stretched, while the foot of the stretched leg leaned on an external support. The knee was flexed and the hip was completely extended. For both conditions, the elongation amplitude was defined as the onset of soreness, and then maintained for 30 seconds, 4 times for each leg, at 1-minute intervals. During the exercises, the subjects were instructed to keep the stretched muscle completely relaxed.
Active Warm-Up and Passive Warm-Up
In AWU, the 2 series of jumps were preceded by 8 minutes running on a treadmill (Technogym Runrace, Gambettoio, Italy). The running intensity corresponded to 60-65% of the theoretical maximal heart rate calculated following Tanaka's equation (23). Between running and jumps, a pause of 17 minutes was observed. In PWU, the external surfaces of both thighs and legs were passively heated by means of an electric blanket (Textherm, Milan, Italy), which allowed the skin to reach a skin temperature (Tsk) of 40°C within 15 minutes. After PWU, a pause of 17 minutes was observed, thus enabling subjects to start jump test with a Tsk comparable to that during the AWU condition.
In AWU+S and PWU+S, the stretching exercises used were preceded by WU (active or passive in accordance with the protocol) after a pause of 5 minutes. Protocols were carried out on separate days, when subjects were invited to avoid training.
Skin Temperature and Subcutaneous Fat Temperature of the Thigh
Skin temperature was detected in both lower limbs, in the medial part of the vastus lateralis and vastus medialis muscles and over the medial gastrocnemius by means of thermocouples (Thermonetics Corporation, San Diego, CA; r = 3.5 mm, accuracy 0.1°C). Skin temperature was continuously monitored throughout all experiments and averaged every 15 seconds. Before tests, all thermocouples were calibrated against a mercury thermometer in a thermostatic bath.
On the basis of previous studies (26), subcutaneous fat temperature (Tsf) is linearly related to Tsk, when thigh skin-fold values range from 2-10 mm. All subjects involved in the study fulfilled this parameter. Subcutaneous fat temperature was then indirectly calculated, following the equation: Tsf = Tsk × 0.579 + 14.705.
Jumps were performed on a force platform operating at 500 Hz (4 Jump, Kistler, Switzerland). The degree of flexion during CMJ and SJ was constantly monitored by a biaxial electrogoniometer (model AH140, Biopac Systems, Inc., Goleta, CA), applied on the femoris and tibia external faces. The best 3 jumps of each series were chosen, on the basis of the following criteria: i) absence of a countermovement at the beginning of the SJ (defined as a decrease of at least 3% of the force corresponding to the subject's weight on the force platform); and ii) the best value of maximal power output achieved. The flight time (Ft), the peak force (Pf), and the maximal power (Ẇpmax ) were assessed for each SJ and CMJ.
Data are presented as mean ± SE. All parameters were normally distributed (Kolmogorov-Smirnov test). Fifteen subjects was selected to ensure a statistical power higher than 0.70. Since no main effect between SJ and CMJ was evidenced, the system collapsed: a one-way analysis of variance (ANOVA) for repeated measures was used to analyze performance parameters of SJ and CMJ. The same test was also used to calculate differences between Tsf calculated at rest and Tsf calculated just before the first jump series, after the 6 experimental protocols. Once a statistical significance was found, a Student-Newman-Keuls post-hoc test was applied. A paired t-test was applied to evident differences between the same protocols during different jumps. The significance level was set at p ≤ 0.05. Statistical analyses were performed using a commercially available Sigma Stat for Windows (Version 3.11, Systat Software Inc., San Jose, CA).
Table 1 shows the average Tsf calculated in the different experimental conditions just before the first jump test. Before jumps, Tsf was significantly higher in AWU, AWU+S, PWU, and PWU+S versus baseline Tsf and the pretest Tsf in C and S. No differences were found among pretest Tsf in AWU, AWU+S, PWU, and PWU+S.
Figure 2 shows the values of Ft (A), Pf (B), and Ẇpmax (C) for all protocols in the 2 different conditions, CMJ (white bars) and SJ (black bars). Statistical significance from Student-Newman-Keuls post-hoc analysis is separately reported in Tables 2, 3 and 4. All the considered parameters were significantly higher after CMJ with respect to SJ regardless of the protocol (p < 0.05).
During CMJ, significant differences in Ft values (Figure 2A) were observed between AWU versus PWU and PWU+S (p < 0.05). The highest Ft was observed after AWU (546 ± 13 ms), whereas the lowest value was found after PWU+S (472 ± 15 ms).
In SJ, the Ft observed in AWU (502 ± 17 ms) was significantly higher as compared to all other protocols (p < 0.05). The lowest value during SJ was obtained after the C protocol (458 ± 18 ms).
Pf during CMJ (white bars) and SJ (black bars) for the different protocols are shown in Figure 2B. In CMJ, Pf was significantly higher (p < 0.05) after AWU (1904 ± 48 N), S (1842 ± 58 N), and AWU+S (1877 ± 40 N) versus C (1798 ± 67 N). AWU was also significantly higher with respect to PWU (1722 ± 13 N) and PWU+S (1724 ± 51 N), p < 0.05.
Similarly to CMJ, during SJ, Pf significantly increased after AWU (1801 ± 48 N), S (1714 ± 60 N), and AWU+S (1784 ± 40 N) versus C (1748±55), p < 0.05. A significant increase of 8% and 7.6% (p < 0.05) was also observed between AWU versus PWU (1656 ± 33 N) and PWU+S (1663 ± 56 N), respectively.
Ẇpmax in CMJ (white bars) and SJ (white bars) are shown in Fig 2C. During CMJ, the maximal Ẇmax value was obtained after AWU (7955 ± 173 Nm·s−1) and the lowest values were found after PWU+S (6635 ± 187 Nm·s−1). Ẇmax was significantly higher in AWU in respect to PWU (6953 ± 141 Nm·s−1) and PWU+S, p < 0.05. Even in SJ, Ẇmax was higher in AWU (5358 ± 256 Nm·s−1) than in other protocols. Significant differences were found in AWU as compared to PWU (4175 ± 145 Nm·s−1) and PWU+S (4335 ± 167 Nm·s−1), p < 0.05.
The most important finding of this study was that passive stretching did not negatively affect the maximal anaerobic power, per se, but seems to inhibit the effect of AWU, which is confirmed to induce a significant advantage in power output. Moreover, Ẇpmax measured during the 2 modes of jump did not seem to be positively affected by PWU, per se, even if followed by stretching.
The highest values of Ft, Pf and Ẇpmax were obtained after AWU applications for both jump conditions. Especially during SJ, AWU seemed to particularly improve the Ft values. Again, Pf seems to be positively influenced by S and its combination with AWU.
Our results are in agreement with those of Faigenbaum (10) and Gray (13), who found an increase of Ft during vertical jump after AWU as compared to PWU. These authors suggested that such a difference could be due to: i) an unlike muscular temperature at the onset of the exercise; and ii) a different metabolic activity, i.e., an increase in muscle acetylcarnitine concentration preexercise that could lead to a decrease in blood and muscle lactate concentration before and during intense dynamic tasks (13). Others studies also showed that passive warming affects the Pf, the twitch speed, and the electromechanical delay in maximal isometric contractions of leg extensor muscles (4). The reasons for these changes are mainly related to biochemical changes, i.e. an impaired Ca2+ release and uptake in the muscle (11). On the other hand, the changes in the muscle-tendon complex properties, related to their temperature, are likely to be associated with alterations of various muscle functions, since the force exerted by the muscle fibers stretches the compliant tendon before it is transmitted to the bone (25). Gray (13) found that an equal enhancement of the muscular temperature, as a consequence of PWU or AWU, did not influence muscle metabolism before a dynamic exercise. Our experimental set-up was designed so that no differences in temperature occurred before jump tests (see Table 1). This could suggest that the differences found in Ft and Pf values after AWU and PWU could be mainly ascribed to a different muscular metabolic activity instead of the temperature.
Passive stretching was found not to negatively affect vertical jump performance, per se, but seemed to inhibit the positive effect induced by WU. These findings are in agreement with those of Church (8), who found no difference in vertical velocity during jumps in young adults after stretching. In another study, Behm (2) found an impaired effect of the AWU after a bout of passive stretching of the lower limb muscles. The authors suggested that such impairment could be due to the acute effects of stretching. Stretching would influence muscle fiber properties for a number of reasons, such as the increase of muscle compliance and the decrease of stiffness (19,24), which in turn alter the interaction between the cross-bridge (22); the increase of the sarcomeral length (18); and increases of the muscle spindles threshold (1,15). All of these effects seemed to counteract the advantages of AWU and to emphasize the effects of PWU on the muscle-tendon complex, i.e., an increase of compliance. Thus, the ability of the tendon to transmit the force to the bone appeared to decrease. In our study, the values of Pf obtained after AWU+S and PWU+S were lower than those after AWU and PWU alone, even if not statistically significant.
In conclusion, AWU seemed to be methodologically more appropriate than PWU to improve muscle performance prior to brief and powerful efforts. Stretching did not reduce maximal anaerobic power, per se, but following AWU it seems to inhibit the well-known positive effect of this maneuver. For this reason the employment of passive stretching prior to fast, dynamic tasks remains questionable.
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Keywords:© 2008 National Strength and Conditioning Association
passive stretching; lower limb muscle; vertical jump; squat jump; maximal anaerobic power