This study falls within the field of physical activity and sport, specifically the flexibility warm-up program (flexibility work to improve performance during the warm-up) in healthy sportspeople.
Flexibility (1) refers to the range of motion (ROM) and depends on both the extensibility of the muscle, tendon, ligament, and joint capsule (passive extensibility) and on the muscle force required to generate the movement (active flexibility) (25). The aim of a flexibility warm-up program is to prepare the muscle for action, without increasing the existing ROMs, by enhancing joint amplitudes related to the sporting movement. Furthermore, the choice of stretching exercise is important to ensure that the muscle and tendon respond to the demands of the sport, because the exercise chosen should not affect the mechanical and sensory properties of the muscle. Thus, in mechanical terms, contractile elements interact with conjunctive elements surrounded by the sarcoplasm. This heterogeneity leads to a viscoelastic dynamic behavior (25) that is noted as an internal resistance to stretching (passive tension [PT]), which is directly proportional to the speed of stretching and inversely proportional to the temperature (15). These concepts are related to the stiffness and compliance (40) and effects on the stretch-shortening cycle (SSC) (33,20). In sensory terms, there is a response from the muscle spindles and the Golgi tendon organs ([1,11,32]; see Table 1).
The acute or immediate effect of static stretching is a change in the stiffness and viscoelasticity (15,27). It should be noted at this point that this effect is particularly noticeable after lengthy and prolonged stretching (creeping effect). This type of stretching is not suitable during warming up for various reasons, including the fact that it reduces the ability of the tendon to store energy (1,33,36) and because it produces changes in the stiffness, thereby affecting the speed with which the force is generated and transmitted to the articulated bone levers. The ability to generate active force (active tension [AT]) can also be affected by changes to the optimal sarcomere length and the optimal joint angle. Finally, such stretching can alter the PT, which is the force that opposes the lengthening produced upon stretching a muscle at rest. This lengthening force extends the elastic structures (desmin and titin) to the limit of their elasticity (10,15,33). This study was intended to evaluate the differences between different types of static stretching, which differ in terms of both their extension times and muscle activation or relaxation during their execution (Figure 1), and their effects on muscle–tendon function; therefore, a series of test was performed to determine the short-term or acute effects of different stretching exercises on explosive force (vertical jump) during warm-up.
The hypothesis to be tested is that static active stretching in AT produces better results in terms of explosive force (vertical jump) and is therefore the most suitable for warming up.
Experimental Approach to the Problem
A controlled, crossover clinical study with random assignment of the stretching type was used to evaluate the short-term effects of stretching on explosive force during warm-up. It was hypothesized that static active stretching in AT produces better results in terms of explosive force and is therefore the most suitable for warming up.
The sample consisted of 49 student volunteers (14 women and 35 men) enrolled in the “physical and sporting activities monitor” advanced vocational training program at the Joaquim Blume institute for higher education. All subjects were physically active (between 15 and 20 hours of physical activity per week) and aged between 18 and 30 years (mean: 20.4 years). As far as the prior level of flexibility is concerned, because this was a crossover study with a very homogeneous group, the minor variations in flexibility were found to accumulate.
Inclusion and Exclusion Criteria
Healthy, physically active volunteers, except for those with a sedentary lifestyle or with prior injuries incompatible with stretching or jumping, injuries concomitant with the study or unexpected situations (pregnancy, illness), were included.
This study was approved by the Clinical Research Ethics Committee of the Universitat Internacional de Catalunya and by the Catalan Sports Council. All volunteers signed the corresponding informed consent.
The dependent variables measured during the jump test were squat jump (SJ), countermovement jump (CMJ), drop jump (DJ) (distance: m), and elasticity index (EI, %). The independent variables were warm-up and type of stretching.
The test protocol involved 3 informational sessions to confirm the sample (project presentation and signing of informed consent, jump test practice and stretching practice). Pre– and post–warm-up and intervention data were collected for a jump test with the after randomly assigned stretching exercises: no stretching, “NS”; and stretching exercises: static passive stretching “P”; proprioceptive neuromuscular facilitation (PNF) techniques; static active stretching in PT; static active stretching in “AT” (Figure 2), of the quadriceps muscle, hamstrings, and triceps surae complex.
A circuit was designed, and the study group was subdivided into groups of 4, with each stage of the test being supervised by the same researcher, to optimize data collection. Thus, the first researcher was responsible for the pre– and post–warm-up jump, the second for controlling the time and execution of the continual running and the third for supervising and demonstrating the stretching exercise proposed for that particular session. Furthermore, because it was impossible to have 5 researchers controlling different stretching exercises in the same session, it was decided that all groups should undertake the same stretching exercise, although neither the researcher nor the participants were aware of which stretching exercise they were to perform until they had finished warming up. The sessions were held on different days at the same time (11 am), with a period of 72 hours between tests to prevent any after-effects from the previous session and without having performed any physical activity beforehand. All participants were allowed to drink water (200 ml) immediately after completing the warm-up.
Description of the Interventions and Equipment
An SJ, which assesses the concentric force, a CMJ, which assesses the explosive force and reuse of the elastic energy during inversion of eccentric to concentric movement, and a DJ, which assesses the reactive-ballistic explosive force (a jump from a height of 40 cm was chosen to assess the quadriceps and triceps surae complex) were performed following the Bosco methodology (5). The EI, which compares the SJ with the CMJ and provides information regarding the viscoelastic and neuromuscular capacities ([SJ − CMJ/CMJ] × 100), was also determined. Each jump was repeated 3 times, with a 20-second rest between each repeat and a 1-minute rest between each jump type; the best value was chosen for each jump.
The Bosco platform was used together with the ERGO-JUMP® Bosco System® (Byomedic 2008, Barcelona, Spain). This system allows the jump time and the distance covered by the subject to be recorded and can receive and store this information and subsequently visualize and analyze it by way of an RS232 connection. Likewise, the system allows the information to be downloaded in Excel format for subsequent storage and manipulation. The accuracy was 0.00001 seconds. This test is reliable and has been validated (24,34).
A conventional (continual running), time-limited (10 minutes), low-intensity warm-up was performed (26). A Geonaute TRT'L 100 chronometer was used for timekeeping.
There is currently no single, internationally accepted classification for stretching exercises (1,25,26). Herein, we analyzed the effect of the following static stretching exercises on the quadriceps, hamstrings, and triceps surae complex.
Static passive stretching (P) places a muscle group under progressive, slow tension with assistance from an external force, in this case the person's own weight, in a comfortable and relaxed position. The stretch is held for 30 seconds and repeated twice for each muscle group (1,2).
The PNF techniques, in this case contract relax stretching, isometrically contract (4 seconds) the muscle that is to be stretched in the preelongation state. The muscle is then allowed to relax (4 seconds). The following period takes advantage of the postisometric inhibition state to stretch passively (15 seconds). This process is repeated twice for each muscular group, and the execution position is the same as that for static passive stretching (1,11,35).
Static active stretching in passive tension produces tension by contracting the antagonist muscle. This stretch is maintained for 6 seconds and is repeated twice for each muscle group (6,11).
Static active stretching in active tension involves simultaneous contraction and stretching of the previously lengthened muscle (put under tension by a prior eccentric activation). The stretch is performed in a position similar to that taken during the sporting movement, usually in standing position. Stretching is maintained for 4 seconds and repeated twice for each muscle group (6,11).
Sample homogeneity was checked using the Kolmogorov–Smirnov test and the Levene test to check that the sample variance was constant. The sample was found to be homogeneous in terms of all the jump test variables; therefore, the data were analyzed using parametric tests. The descriptive analysis was performed on the basis of variable type (Table 2).
The post and premeasurements for each intervention group (NS, P, PT, PNF, and AT) were analyzed and any significant changes determined. Student's t-test was applied to compare the pre and postmeasurements for paired samples.
The variables were analyzed by performing pairwise comparisons of the means of the differences (post − pre) for each pair of intervention groups (NS, P, PT, PNF, and AT) focusing successively on the pre/post difference and the absolute value of this difference. A 1-way analysis of variance and Tukey's post hoc tests were used to perform this analysis.
The type 1 error (α) was set to 5% (95% confidence interval). All analyses were performed using the R statistical software package (the R Foundation for Statistical Computing) (18).
The sample consisted of 49 volunteers, 38 of whom provided full data sets. The remaining 11 provided partial data sets either because of injury or nonattendance at 1 of the sessions.
The results of the intragroup analysis show that the difference between the post and prejump measurements for each stretching exercise was positive. Thus, the jump performed after stretching was significantly higher than the jump performed beforehand. Statistically significant differences (p < 0.05) were found for static passive stretching, PNF, and static active stretching in AT, with the latter showing the greatest difference. For static active stretching in PT, the results were statistically significant for SJ and CMJ (p < 0.05; see Table 2 and Figure 3).
The EI showed little statistically significant relevance (p > 0.05), except in static passive stretching (p = 0.046), where the (post − pre) difference was negative (−34.8%), thus indicating that the EI after the P stretching exercise was significantly worse than before stretching.
In the intergroup analysis, the effect of the intervention was assessed from 2 different angles. Thus, focusing on the actual post–pre difference (dif) allowed us to assess the direction of the change—a positive difference indicates an increase in the score after the intervention, whereas a negative difference indicates a decrease in the score—whereas focusing on the absolute difference (|dif|) allowed us to assess the magnitude of this change (Table 2 and Figure 3).
The CMJ jump showed statistically significant differences (p < 0.05) between the values for the different stretching exercises (“no stretching” vs. “static active stretching in PT” [p = 0.040], “no stretching” vs. “PNF” [p = 0.028], and “no stretching” vs. “static active stretching in AT” [p < 0.001]), thereby indicating that (a) A comparison of all the possible stretching combinations showed that a higher poststretching CMJ was obtained for “static active stretching in AT,” “static active stretching in PT,” and “PNF” with respect to no stretching (“NS”). (b) The difference between the post and prevalues for the same jump showed a greater difference between these values for “static active stretching in AT” than for “no stretching.”
Stretching exercises are widely performed in the field of sport, although the decision as to which stretching exercise is most suitable has changed frequently as different proposals, some of which are based on empirical observations and have little scientific basis, have appeared in the literature (26). Recent scientific studies in this field have analyzed different aspects of such exercises and have subsequently indicated or contraindicated (27,40) stretching during warm-up. These studies have involved different techniques, ranging from the classical static passive stretching (31,39) to dynamic forms (12), or both (3,4,13,16,17,22,30,38), and some have included PNF (8,9,14,23). The methods for evaluating the effect of stretching, if any, have also differed. Thus, several clinical studies have assessed different manifestations of force, including maximum force (3,23,37), resistance force (14), or explosive force (7-9,12,21,22,28,30,38,39). Finally, such studies have tended to assess the effect of stretching in either the short or long term (19,41). In this context, this study has assessed the short-term effect of different stretching exercises on explosive force, specifically the vertical jump.
The majority of studies designed to assess “explosive force” have been based on the sprint (7,13,31,36,38,39) and the vertical jump test (8,9,12,21,22,28,30,36,38), and we decided to use the latter in our study. Thus, we used the Bosco test (5) to assess the explosive force because of our need to quantify the change in explosive force before and after stretching in a controlled and objective manner. The Bosco test assesses the different manifestations of explosive force indirectly and the majority of previous studies that have used this test have assessed the CMJ (8,22,28,30,36,38). This is probably because the CMJ assesses the SSC in terms of harnessing of the elastic energy, which is the key to assessing the acute effects of stretching on the explosive force studied, and possibly because it is the most sensitive for detecting changes in pre and posttest values.
We analyzed the behavior of the SJ, CMJ, and DJ in combination with the different stretching exercises and found that static passive stretching, PNF, and static active stretching in AT all produced improvements, with the latter showing the greatest difference for the CMJ.
The EI is an indication of the viscoelastic and neuromuscular capacities of the muscle (5). However, despite the fact that these capacities are undoubtedly the key to the immediate effects of stretching on explosive force, several of the aforementioned studies have not assessed them fully. Somewhat surprisingly, we have been unable to find any previous study that has assessed the EI. In our study, the EI was only significant for static passive stretching. Indeed, the negative value obtained for this parameter suggests that this stretching exercise could reduce the muscle's viscoelasticity. Thus, although the result for the jump test was positive, this aspect should be taken into account when using it during warm-up. This finding is in accordance with those of previous studies, which found that stretching exercises can affect the explosive force (3,7,9,14,17,38,39).
Those studies that assessed a jump did so with respect to a certain type of stretching exercise or by comparing different types. Thus, in light of their results, some authors advise against static passive stretching (3,7,17,31,38,39), whereas others advise against PNF (9,14), and yet others recommend dynamic stretching (12,17,22,23,36). The most similar study to ours is that undertaken by Rey et al. (28), who compared static passive stretching with static active stretching in AT over an 8-second time period. This study involved a slow yet constant backward and forward movement, and the authors obtained similar results to ours for the CMJ, thus indicating that active stretching is more effective than passive stretching during warm-up.
A comparison of our short-term results with those of other studies that assessed any type of force manifestation as regards the acute effects of stretching shows that the best results in our study were obtained with static active stretching in AT. This static stretching exercise involves a short and low-amplitude activating stretching after prior contraction of the muscle group. It is adapted and can be specifically designed, for application during warm-up, and prepares the muscle both mechanically and sensorially for an immediate effort. Furthermore, its short duration increases the internal muscle temperature by increasing the vascularization (6,11). However, it requires that both the posture to be adopted and the sequence of movements involved in the setup be carefully learned so that its effectiveness is not reduced. This complexity can nevertheless be considered to be an additional benefit because it promotes concentration and an understanding and preparation of posture tone and control.
Some authors recommend avoiding stretching during warm-up for speed disciplines (27) other than those in which wide amplitudes of movement are required. In this case, the use of short, low-amplitude stretching exercises, with few repetitions and in muscle activation, is recommended. This proposal can be combined with static active stretching in AT (11,26).
The studies referred to herein have tended to involve long-term stretching exercises (creeping effect), which reduce the ability of the tendon to store energy (15), or techniques that activate those neuromuscular mechanisms that inhibit muscle tone (32). The longest-lasting stretching exercise in our study, namely, static passive stretching, showed significant differences in terms of EI, which decreased after the intervention. We believe that this type of stretching, together with PNF, is indicated after physical activity to normalize the ROM, during flexibility training to increase the ROM, but is not indicated during warm-up.
Referring to the stretch in the warm-up concludes (a) Stretching before physical activity does not affect explosive force. (b) Static active stretching in AT showed the most significant differences for the jumps tested. Furthermore, it is the most indicated during the warm-up for explosive force disciplines. (c) Static passive stretching can affect the EI in such a way that the mechanical properties of the muscle–tendon group change negatively; therefore, this type of stretching is not recommended during warm-up.
The results of this study suggest that static active stretching in AT can be recommended during the warm-up for explosive force disciplines.
The recommended warm-up in this context therefore consists of initial general work (low-intensity training), followed by static stretching with AT, then a series of dynamic exercises for the various muscle groups, and finally explosive elastic force exercises appropriate for the sporting discipline concerned (29).
Although it was not one of the primary objectives of this work, it should be noted that static passive stretching and PNF would be more indicated during posteffort recovery, in other words during the return to basal state because they aid recovery of amplitudes and the drainage of metabolic waste products (primary recovery) and the recovery and normalization of tone 2 hours postexercise (deep recovery). Second, the regular performance of stretching exercises has positive long-term effects and can maintain or increase the flexibility as a result of “stretch tolerance” and the so-called “Goldspink” effect, without affecting the explosiveness, and can also have a positive effect on the elastic energy restitution capacity (19,41). Such stretching exercises fall within the scope of a flexibility training program, and their performance means that the organism subsequently initiates adaptation and recovery mechanisms. Those stretching exercises that are contraindicated for the warm-up may therefore provide long-term benefits in terms of flexibility training. Finally, although not assessed herein, dynamic stretching is often used by trained athletes and should therefore be included in future studies in this field.
We would like to dedicate this study to the memory of our colleague Asun Estruch Massana, MD. We would also like to thank the IES J. Blume, Byomedics, the Universitat Internacional de Catalunya and the Catalan Sports Council (Consell Català de l'Esport) for their collaboration. The results of this study do not constitute endorsement of the product by the authors. Competing interests: none.
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