Modalities to improve flexibility (joint range of motion) are divided into static stretching (SS) in which muscles are stretched while holding the joint at a fixed angle, and dynamic stretching (DS) where muscles experience dynamic stretch-shortening within the maximal range of motion (Alter, 2004). Although flexibility can be significantly improved after static stretching, muscle strength and function are often attenuated over 60 seconds per muscle group (Herda et al., 2008; Ryan et al., 2008; Sekir et al., 2010; Mizuno et al., 2013; Behm et al., 2016). The latter outcome has been attributed to the reduction of the neural drive (Fowles et al., 2000; Kay and Blazevich, 2009; Trajano et al., 2017) and a decrease in muscle-tendon unit (MTU) stiffness (Fowles et al., 2000; Morse et al., 2008; Ryan et al., 2008). In contrast, DS generally has a lesser effect on flexibility than static stretching, but it does not attenuate muscle functions (Yamaguchi and Ishii, 2005; Bacurau et al., 2009; Sekir et al., 2010). Possible factors involved in DS include shortening of active muscle contraction induced by DS, which might contribute to muscle functions not being attenuated after DS. This can be due to a retained neural drive and/or a negligible change in MTU stiffness after DS (Sekir et al., 2010; Mizuno and Umemura, 2016).
The DS performs dynamic stretch-shortening of the MTU within the maximal range of motion due to active muscle contraction. Thus, in addition to active muscle contraction, dynamic stretch-shortening of the MTU is also considered as a factor of DS that affects the MTU. Previous animal (Mutungi and Ranatunga, 1998) as well as human (McNair et al., 2001; Avela et al., 2004; Yeh et al., 2007) studies have shown a decrease in MTU or muscle stiffness that underwent repetitive and passive stretch-shortening cycles. In these previous studies, to undergo repetitive and passive stretch-shortening of the plantar flexors, the ankle joint was dorsi-flexed in a large range from the anatomical position or plantar flexion of 20[degrees] to around the maximum dorsiflexion range of motion (DFROM) angle (McNair et al., 2001; Yeh et al., 2007). A previous study has reported that MTU stiffness decreases, even if such modalities are performed in a small range of joint movement (the anatomical position to 10[degrees] of dorsiflexion) (Avela et al., 2004). Thus, a modality that conditions the MTU by SS combined with dynamic length changes of the plantar flexors by a small range of repetitive and passive plantar- and dorsiflexion of the ankle joint (likely passive DS) will be effective in improving muscle stiffness while retaining muscle functions. To the best of our knowledge, no study has examined such a stretching maneuver.
In the present study we developed a novel stretching technique which employs the features of DS (in the form of oscillation) added onto SS, allowing for the advantages of both SS and DS. We named this technique minute oscillation stretching (MOS). The present concept of MOS involves giving a large amplitude oscillation (15 mm; ankle joint angle change of approximately 5[degrees]) to the forefoot so that the major plantar flexors undergo a longitudinal length change, thereby applying repetitive small length changes to the MTU, like DS, under passive stretching.
The altered flexibility is reported to continue for 10-120 min (Fowles et al., 2000; Power et al., 2004; Ryan et al., 2008; Sekir et al., 2010; Mizuno et al., 2013; Taniguchi et al., 2015) after SS, and at least 10 min after DS (Mizuno and Umemura, 2016). A combination of these interventions may further elongate their after effects, but this idea has not yet been tested.
In the present study, we investigated the MTU of the lower leg for the purpose of verifying the effects of MOS on muscle strength, flexibility, and continuance of altered flexibility. It was hypothesized that MOS would not decrease muscle strength but maintain improvements and flexibility, similar to SS.
Experimental design and procedures
The present study aimed to clarify the effects of MOS on muscle strength, flexibility, and continuance of altered flexibility, compared to SS. The right ankle joint was tested in all participants under the following 3 conditions: SS intervention, MOS intervention, and no stretching (control). The participants were tested under these conditions in a random order, with an interval of 3 days or longer, after measuring maximal voluntary plantar flexion torque, to determine the basis for DFROM measurements as described later. The following dependent variables were evaluated before and immediately after intervention: isometric maximal voluntary plantar flexion torque, DFROM, and muscle and tendon elongation during DFROM measurements. DFROM was also measured after SS and MOS interventions (Post), and at 15 min (Post 15), 30 min (Post 30), and 60 min (Post 60) post-stretching to examine the continuance of altered flexibility.
The participants were 10 recreationally active men without apparent neurological, orthopedic, or neuromuscular problems in their lower legs (age, 22 [+ or -] 2 years; body height, 1.70 [+ or -] 0.06 m; body weight, 64.3 [+ or -] 8.9 kg; mean [+ or -] standard deviation [SD]). They all participated in sports activities (ball games [n = 5], running [n = 3], kenjutsu [n = 1], and archery [n = 1]) at least once per week. The details and purpose of this study as well as the benefits and risks associated with participating were explained to each participant in advance, and written consent was obtained thereafter. This study was approved by the Ethics Review Committee on Human Research of Waseda University and performed in accordance with the Declaration of Helsinki.
Measurements of maximal voluntary muscle strength
Isometric maximal voluntary plantar flexion torque was measured using an isokinetic dynamometer (VTF-002, VINE, Japan), with the knee fully extended in a sitting position, and the ankle secured to the foot plate of the dynamometer at 0[degrees] (anatomical position). The signal obtained from the dynamometer was amplified (DPM-711B, Kyowa, Japan), then digitally converted to 1 kHz through an A/D converter (PowerLab/16SP, ADInstruments, Australia), connected to a personal computer (FMV Lifebook, Fujitsu, Japan) and recorded using appropriate software (LabChart ver.7, ADInstruments, Australia). Before measurements were taken, the participants were instructed to warm-up, producing force below the maximal strength (80% of maximal effort), twice. After at least a 1 min rest, participants performed 2 maximal voluntary plantar flexion torque exertions with a 1 min rest between trials. The peak torque was analyzed per measurement, and the third measurement was performed when the values differed by 5% in 2 of the measurements. The higher value of the 2 measurements was taken as the maximal voluntary plantar flexion torque. Maximal voluntary plantar flexion torque was measured before and immediately after the intervention.
Measurement of dorsiflexion range of motion
Flexibility was measured as DFROM using the same isokinetic dynamometer used for...