Exercise involving eccentric muscle contractions is well known to impair force production capacities (Janecki et al., 2011; Torres et al., 2007) and to induce reduced range of motion (ROM) immediately after exercise (Lau and Nosaka, 2011; Torres et al., 2007; Whitehead et al., 2001). Besides parameters such as stretch tolerance and neural factors (Guissard and Duchateau, 2006), ROM is mainly determined by passive muscle stiffness (Proske and Morgan, 2001; Murayama et al., 2000). An increased number of residual cross-bridges between myosin heads and actin is thought to largely contribute to this exercise-induced increased stiffness (Proske and Morgan, 2001). Accordingly, increased passive muscle stiffness has been extensively reported immediately and up to 3 weeks after purely eccentric contractions (Lacourpaille et al., 2014; Whitehead et al., 2001; Chleboun et al., 1998; Green et al., 2012; Howell et al., 1993; Murayama et al., 2000; Janecki et al., 2011; Torres et al., 2007), and is associated with delayed-onset muscle soreness (DOMS) (Cheung et al. 2003).
Increased muscle stiffness may alter muscle performance (Jones et al., 1987) and is further associated with greater risk of muscle damage and strain injury (McHugh et al., 1999; Watsford et al., 2010). Hence, it may be of interest to use recovery strategies to minimize the observed symptoms (Cheung et al., 2003). In some activities such as during a judo tournament, acute recovery is of importance since athletes often participate in several matches that are generally separated by 15 to 60 min (Franchini et al., 2009; Franchini et al., 2003). Several post-exercise recovery interventions are often employed to improve the acute recovery process (e.g. low intensity exercise (Mika et al., 2007), electrostimulation (Bieuzen et al., 2012), cryotherapy (Pournot et al., 2011)). Massage is currently one of the most popular recovery techniques (Torres et al., 2012). Massage is thought to increase ROM through decreased muscle passive stiffness (Weerapong et al., 2005). However, there is little scientific evidence to corroborate an effect of massage on muscle recovery (Moraska, 2005; Weerapong et al., 2005). This may be explained by the variety of techniques and durations used, difficulty in applying constant pressure during the massage, or the therapist's level experience (Moraska, 2007).
Alternatively, another way to mechanically stimulate muscles may be the use of vibratory massages (Green and Stannard, 2010). Using vibrations may provide the advantage to precisely control the frequency, duration, and amplitude of the stimulation (Edge et al., 2009), and can be easily use as a recovery tool in the sports field, especially when using local vibration (LV) directly applied onto muscles. LV has been demonstrated to prevent DOMS symptoms when applied before eccentric exercise (Bakhtiary et al., 2007; Imtiyaz et al., 2014). LV is also a useful post-exercise recovery modality in treating inflammation and DOMS when applied during 5 days after eccentric exercise (Broadbent et al., 2010). Regarding ROM, Lau and Nosaka (2011) demonstrated that LV has an immediate positive effect on elbow ROM when applied on days 1, 2 and 3 after an eccentric arm exercise inducing muscle damage. Although the authors' main hypothesis was based on the analgesic effects of vibration (Lau and Nosaka, 2011), it may also be hypothesized that such vibration-induced increased ROM was due to reduced passive muscle stiffness through a decreased number of residual cross-bridges, some of them being broken by the mechanical vibratory stimulation. Although massages have been demonstrated to reduce passive stiffness of relaxed muscles (Eriksson Crommert et al., 2014), the effects of LV remain to be determined, especially in the context of recovery from exercise.
Elastographic methods have recently been implemented to quantify local muscle mechanical properties (Drakonaki et al., 2012). One of these methods is Supersonic shear wave imaging (SSI) (Bercoff et al., 2004). It consists of measuring the velocity of shear waves remotely induced by focused ultrasound. The squared velocity is directly related to tissue stiffness, i.e. the shear elastic modulus (in kPa). SSI technique has been recently shown to provide reliable measurements of shear elastic modulus in a variety of resting muscles (Lacourpaille et al., 2012), and with linear increase of shear elastic modulus during passive stretching (Maisetti et al., 2012).While SSI has been used to demonstrate increased passive muscle stiffness after eccentric exercise (Lacourpaille et al., 2014), this technique may allow the quantification of increased passive muscle stiffness after more natural exercise involving both concentric and eccentric actions, and may determine the potential effects of LV as a recovery method.
The first aim of the present study was to investigate the acute changes in biceps brachii muscle passive stiffness following intense barbell curl exercise involving both concentric and eccentric actions. We hypothesized that biceps brachii passive stiffness would increase (as characterized by increased shear elastic modulus), following eccentric contractions. The second aim of this study was to investigate the acute effect of LV applied immediately after exercise as a recovery technique on muscle stiffness. We hypothesized that LV massage would reduce the acute exercise-induced increased stiffness.
Eleven physically active adults (5 females and 6 males ; age 38 [+ or -] 9 years, height 1.74 [+ or -] 0.09 m, body mass 73 [+ or -] 8 kg) without prior experience of resistance training specific to biceps curl, participated in this study. Subjects with neuromuscular pathology and arm injury were excluded. Written informed consent was obtained from all subjects prior to their participation and this study conformed to the standards from latest revision of the Declaration of Helsinki and was approved by the local ethics committee.
In the present study, exercise-induced increased muscle passive stiffness was induced by 4 bouts of 10 movements of bilateral barbell curl exercise (i.e. bilateral elbow flexion/extension with a barbell) at 70% of the one-rep maximal flexion force (1RM, mean 1RM of 21.7 [+ or -] 7.2 kg), with a 1-min resting period between each bout. This was designed according to the training load and repetitions per set recommended for novice and intermediate individuals (Kraemer et al. 2002). Participants supported their back against a wall and maintained a 120[degrees] knee angle (controlled through a goniometer before each bout and visual inspection during movements). During flexion, subjects had to lift the load from full extension to full flexion over a period of 1 to 2 s. During extension, subjects were asked to lower the load slowly over a 5-s period and keeping the velocity as constant as possible. Subjects had to perform their flexion/extension movements according to the instruction of the experimenter. 1RM was determined during a...