Static stretching is usually involved in pre-exercise routines in sport and rehabilitation environments (Behm et al., 2016), in order to enhance range of motion (ROM) around a joint (Young and Behm, 2002). However, a large body of evidence reported that it may acutely impair the subsequent muscular performance, reducing maximal voluntary strength, muscle power or balance (Behm and Chaouachi, 2011). Theses impairments could originate from neural (reduced central drive (Budini et al., 2017; Trajano et al., 2017)) and peripheral (altered intrinsic muscular properties and musculotendinous unit (MTU) stiffness (Opplert et al., 2016)) mechanisms. Alternatively, dynamic stretching, consisting of muscle contractions to move the joint through a full active ROM, is also incorporated as pre-exercise interventions (Opplert and Babault, 2018). In addition, recent studies revealed that dynamic stretching may provide an acute increase in muscular contractility (Yamaguchi et al., 2007), sprints, jumps or even balance (Opplert and Babault, 2018). Therefore, such a modality appears to be more favorable than prolonged static stretching to prepare the musculotendinous system for subsequent exercises.
These two pre-exercise activities might be expected to impact differently the MTU stiffness and thus the state of the muscle-tendon system for subsequent exercises. Indeed, it is known that mechanical effects of stretching could be mediated by a number of stretch characteristics, including amplitude and muscular activation (Taylor et al., 1990; Knudson, 2007), which obviously differ between static and dynamic stretching. For instance, static stretching implies a greater load of MTU than dynamic stretching. It results from a greater ROM and the stress relaxation which occurs when the muscle is kept stretched in a fixed position (Magnusson et al. 1996b). Thus, static stretching would likely involve larger changes in stiffness of different tissues, especially muscle fascicles (Mizuno et al., 2013). Moreover, because muscles are contracting actively and rhythmically, dynamic stretching may increase muscle fibers temperature (Fletcher, 2010) and thus lower viscosity (Mutungi and Ranatunga, 1998; Bishop, 2003a). Indeed, it has been shown that repetitive muscle contractions, such as those produced during warm-up procedures, also influenced MTU mechanical properties (Bishop, 2003b, a). In addition, repetitive movements and contractions may also induce improved muscle contractility, mainly attributable to temperature- and/or potentiation-related mechanisms (Bishop, 2003a; Turki et al., 2011). Consequently, the influence of dynamic stretching on viscoelastic and contractile properties seems to result from both the stretching of muscle-tendon structures and muscle warm-up. However, how these two mechanisms interact and specifically impact the mechanical properties remains unexplored.
We aimed to test the hypothesis that dynamic stretching would result from an interaction between MTU stretching and muscle warm-up. Therefore, the effects of MTU stretching have been studied through the comparison between static and dynamic stretching, while the muscle warm-up effects were investigated comparing dynamic stretching to submaximal isometric muscle activity (of a similar intensity). To test this hypothesis, this study explored mechanical properties of plantar flexor muscles through contractile properties, passive resistive torque and muscle fascicles extensibility.
Thirteen healthy recreationally active men (mean [+ or -] standard deviation: age 24.9 [+ or -] 2.5 years; height 1.81 [+ or -] 0.05 m; body mass 82.3 [+ or -] 11.6 kg; 6.5 [+ or -] 2.5 h of physical activity per week, such as soccer, rugby or handball) without recent injury or illness were recruited for this study (see the statistical analysis section for the statistical power values). They were all volunteers and gave their written consent to participate in the experiment after being informed about the investigation. The study conformed to the standards set by the World Medical Association Declaration of Helsinki "Ethical Principles for Medical Research Involving Human Subjects" (2008) and approval was obtained from the local committee on human research (EAST-1, number: 2017-72).
This study used a randomized and cross-over design with control condition to assess the effects of three conditioning interventions on mechanical properties of plantar flexor muscles. Participants attended the laboratory on five separate occasions. The first session served as familiarization in order to practice the different conditioning activities and the testing procedures. Four 2 x 20-s experimental sessions including (i) static stretching (SS), (ii) dynamic stretching (DS), (iii) submaximal isometric muscle activity (SIMA), and (iv) control (CON) were completed in a random order with at least 48 h between. Testing procedures were conducted immediately before (pre-conditioning tests) and 10 s after (post-conditioning tests) the conditioning activities to quantify changes in evoked contractile properties, maximal voluntary isometric torque, passive resistive torque and muscle fascicles extensibility (Figure 1). Limb dominance was ignored and all experimental procedures were conducted on the right plantar flexor muscles.
Conditioning activities and testing procedures were performed on an isokinetic dynamometer (Biodex System 4, BIODEX Corporation, Shirley NY, USA). To avoid gravitational influences, subjects laid on their left side (Figure 2). The contralateral leg was flexed (about 90[degrees]) for comfort and the right leg fully extended (0[degrees]) to ensure that the plantar flexor muscles were placed under significant stretch (Cresswell et al., 1995). To maintain this position, the right knee was positioned and secured on a dynamometer support. To minimize heel displacements, the foot was positioned and fastened inside a shoe (adapted to participant's size and fixed by the sole to the dynamometer footplate), and firmly attached to the footplate of the dynamometer with straps. The lateral malleolus was aligned to the center of rotation of the dynamometer. From here, subjects were in a standardized position for ~20-min.
The passive maximal range of motion was first determined by the experimenter, stretching slowly the plantar flexor muscles from a maximal plantar flexion until the point of maximal tolerated discomfort ([ROM.sub.end]), and returned immediately to a neutral position (0[degrees]). Then and for all four experimental sessions, subjects were asked to perform 5 repetitions of dynamic stretching (see below for DS procedure). These repetitions aimed to evaluate muscle electromyographic (EMG) activity during DS for reproduction during SIMA conditioning activity (see below for SIMA procedure). Thereafter, while the subjects remained relaxed in the neutral position, the ultrasound probe was positioned and the M-wave recruitment curve was established. Then, a short warm-up of ten incremental submaximal voluntary contractions was performed and followed by a rest period of 5 min
All conditions were time matched for direct comparison. The static stretching (SS) intervention included two sets of static stretching. The subjects' ankle was passively rotated using the isokinetic dynamometer with a standardized 25[degrees] of ROM until individual [ROM.sub.end], in order to match the lengthening duration of the static stretches. A slow angular velocity (5 [degrees].[s.sup.-1]) was set in order to avoid the myotatic reflex (Kay and Blazevich 2010) to obtain a good resolution of the fascicles via ultrasonography. Each set of static stretching took 20 s, including 5 s of lengthening, a 15-s hold before the ankle was released immediately to the start position at 60 [degrees].[s.sup.-1] in readiness for the next stretch (20-s between-stretch rest). The subjects were instructed to relax during the stretching, and not to offer any resistance to the dynamometer, which...