Several sports coaches around the world have discovered that optimal performance is contingent upon "psychingup" just as much as it is on physical preparation and technical skill (Tod et al., 2003; 2015). However, sport and exercise psychologists have reported that strength athletes need to undertake some form of psyching-up prior to performance, both in training and competition (McCormick et al., 2015; Tod et al., 2015). Cognitive strategies or psyching-up strategies are reliably associated with increased strength performance (results range from 61 to 65%) (Tod et al., 2015). Typical strategies include mental imagery. This psyching-up technique has been applied to (a) reduce muscle fatigue, (b) improve strength performance in sports without sensorial input, using mental training with perceptual experiences, which includes simulations of movements and specific task perceptions and (c) enhance motor recovery in patients after injuries (Reiser et al., 2011; Rozand et al., 2014; Tod et al., 2015). Mental imagery is defined as "using all the senses to recreate or create an experience in the mind" (Cumming and Williams, 2014). This technique has become one of the most widely used simulation tools and performance enhancement strategies in sports psychological interventions (Cumming and Williams, 2014; Slimani et al., 2016). Recent research has shown that mental imagery improves motor tasks (muscular power: Slimani and Cheour, 2016; sprinting: Hammoudi-Nassib et al., 2014; and endurance: McCormick et al., 2015). The improvements associated with this technique have been related to several mechanisms, including psychological skills such as motivation (Martin and Hall, 1995; Slimani and Cheour, 2016), self-efficacy (Beauchamp et al., 2002; Slimani et al., 2016), self-confidence (Weinberg, 2008; Slimani et al., 2016), and managing competitive anxiety (Vadoaab et al., 1997). As will be discussed, a few early researches suggest that mental imagery training may improve functional recovery after short-term muscle immobilization and anterior cruciate ligament (ACL) by the reduction of strength loss (Clark et al., 2014; Frenkel et al., 2014; Newsom et al., 2003).
Mental imagery can be carried out in various forms, including the auditory, olfactory, tactile, gustatory, kinesthetic, and visual modes (Cumming and Williams, 2014). Furthermore, mental imagery can be performed using one of two basic perspectives, namely internal or external (Cumming and Williams, 2014). The internal perspective involves imaging from within the body and experiencing the motor act without overt movement, i.e., the subject imagines that he or she is really performing the motor act, that his or her muscles are contracting, and that he or she feels kinesthetic sensations (Jeannerod, 1994). The external perspective, on the other hand, involves imagining the action as if it is outside the body, i.e., the motor task is generated in the mind of subjects (Wang and Morgan, 1992). Despite the general consensus among experts that mental imagery could offer promising opportunities for the enhancement of physical strength performance (Tod et al., 2003; Feltz and Landers, 1983), there is no conclusive result regarding which modality is most effective. In fact, research on this cognitive simulation technique has evolved over the past three decades, and researchers have spent considerable efforts investigating the mental imagery perspectives and their relationship with strength performance. Despite a voluminous literature on this subject, there is no definitive understanding of the effects of mental imagery perspectives on muscle strength (Sidaway and Trzaska, 2005). In fact, the literature presents different and sometimes opposing views, and it is only recently that researchers have realized the need for a timely literature review that critically analyzes and updates current knowledge on mental imagery.
Ranganathan et al. (2002) showed stronger effects on strength for high compared with low mental effort (20.5% vs. 2%, respectively). They also showed that internal imagery induces a greater improvement in strength performance compared with that induced by external imagery techniques (10% vs. 5.3%, respectively). Furthermore, several studies have demonstrated the presence of muscular activity (electromyography: EMG) during mental imagery directed towards the production of force (Guillot and Collet, 2005; Yao et al., 2013). Accordingly, and based on the imagery perspectives and the relationship with EMG activity, internal imagery results in significantly higher muscle excitation than external imagery of the same movement (Bakker et al., 1996; Hale, 1982; Harris and Robinson, 1986). Thus, several studies have demonstrated that alternation of mental imagery and voluntary contractions could increase the volume of training and limit the development of muscle fatigue in healthy adults (Guillot and Collet, 2008; Ranganathan et al., 2004). Research in this area could provide both theoretical and practical contributions to the field. For example, it could provide athletes and coaches with principled insight on how to optimize their use of mental imagery, help understand the underlying mediators and moderators influencing the effect of mental imagery on strength performance, and stimulate future research on the multiple factors involved in the development of mental imagery theory and practice.
Although many practical imagery interventions have been shown to improve strength performance, little is known about the mechanisms underlying these improvements. According to the literature, such mechanisms are marked largely by references to the role of neurophysiological variables. There is also little question that neural factors play an important role in muscle strength gains and motor recovery after injuries. One of the historical reasons for the lack of evidence is that mental imagery has not been subject to extensive empirical examination. The situation has evolved somewhat over the past two decades, and researchers have expended considerable effort investigating the mental imagery and the mechanisms underlying strength increases.
As it is now well known, common neural substrates underlie motor performance and mental imagery (Guillot et al., 2008; Guillot and Collet, 2008; Zijdewind et al., 2003), and understanding the neural correlates of goal-directed action, whether executed or imagined, has been an important aim of cognitive brain research since the advent of functional imaging studies (Gabriel et al., 2006). In addition, despite the consensus between sports psychologists regarding the increase in strength conditions with internal mental imagery and the correlations between neural adaptations and strength performance improvement, there is no conclusive result concerning which modality (perspective) is most effective in neurophysiological adaptations. To date, each type of mental imagery has been considered to have different properties with respect to both psychophysical (Jeannerod, 1995) and physiological (Stinear et al., 2006) perspectives and to the nature of the neural networks that they activate (Guillot et al., 2009; Solodkin et al., 2004). Accordingly, many studies have shown that external imagery perspective produces a little physiological response (Lang et al., 1980; Ranganathan et al., 2004; Wang, 1992) and is not as effective in enhancing muscle force as internal imagery training did (Ranganathan et al., 2002).
Previous reviews examined the effects of mental imagery on various outcomes (i.e., motor learning and performance, motivation, self-confidence and anxiety, strategies and problem-solving, and injury rehabilitation) (Bowering et al., 2013; Khaled, 2004; Kossert and Munroe-Chandler, 2007; Zimmermann-Schlatter et al., 2008) and neurophysiological adaptations (Guillot and Collet, 2005). Thus, six imagery models and frameworks were reviewed by Guillot and Collet (2008). Although some psychophysiological models related to endurance performance are currently available in the literature (Smirmaul et al., 2013), similar models related to strength performance are still lacking. The purpose of the present systematic review is to examine the influence of mental imagery on the outcome of muscular strength. There are three reasons why such a systematic review will advance current understanding. First, previous reviews have not examined the effects of mental imagery on strength performance in healthy participants as well as strength loss for persons with immobilization and ACL (Braun et al., 2013; Tod et al., 2003, 2015). Second, much research is currently interested in the relationship between mental imagery and muscular strength to provide guidelines for coaches, sports psychologists, and therapists to create effective imagery intervention for use with their athletes or patients. Third, unlike narrative review, systematic review involves a detailed and comprehensive plan and search strategy derived a priori, with the goal of reducing the risk of bias by identifying, appraising, and synthesizing all relevant studies on the present topic. Thus, a systemic review about effects of mental imagery on muscular strength in healthy and patients subjects is a well planned way to answer this specific research question using a systematic and explicit methodology to identify, select, and critically evaluate results of the studies included in the literature review (Khan et al., 2000). While narrative review works have an important role in continuing education because they provide readers with up-to-date knowledge about a specific topic or theme (Khan et al., 2000). Furthermore, this review aims to (a) identify the effects of mental imagery on strength performance and EMG activity in healthy participants and patients with immobilization and ACL, (b) evaluate the moderator and mediator variables related to the mental imagery-strength performance relationship...