Abstract
Motor imagery involves imagination of an executed action without physically executing these actions. This provides the possibility to practice a certain skill or movement without needing to physically perform it. In sports, motor imagery has become a common addition to the physical training sessions. Previous studies have shown that mental practice is beneficial for learning and performance of several motor skills. However, the effect of motor imagery on muscle strength has received relatively little attention. Therefore, the objective of this thesis was to investigate the effect of motor imagery of a strength training task on muscle strength. In total, 11 studies were included. The general finding was that motor imagery enhanced the MVC of the muscles involved in the practiced task. Motor imagery without physical practice had a positive effect on muscle strength, while physical practice itself was still more effective. As a result, a combination of motor imagery and physical practice might be optimal to improve strength performance. These strength increases after motor imagery are likely due to neural plasticity in the cerebral cortex and enhanced spinal excitability. However, further investigation of these mechanisms is necessary to increase the understanding of the beneficial effect of motor imagery on muscle strength.
Introduction
The use of mental practice to enhance performance has become more common over the years. “Mental practice refers to the cognitive rehearsal of a task in the absence of overt physical movement”, (Driskell, Copper, & Moran, 1994). Mentally rehearsing movements can be beneficial for, for example, musicians when they ‘think through’ a passage before a concert, or for athletes when they imagine all the steps required to perform an exercise. Especially in sports, motor imagery has become a common addition to the physical training sessions. According to Mizuguchi et al. (2012), 70-90% of the elite athletes use motor imagery to improve their sport performance. Furthermore, motor imagery has been used by athletes as part of the preparation for big events including the Olympic games:
“I’m standing on top of the hill. I can feel the wind on the back of my neck. I can hear the crowd,” Cook said. “Kind of going through all those different senses and then actually going through what I wanted to do for the perfect jump. I turn down the in-run. I stand up. I engage my core. I look at the top of the jump. I was going through every little step of how I wanted that jump to turn out.”
[Emily Cook, a U.S. freestyle aerials Olympian before the 2014 Olympic games, quoted in The New York Times, 2014]
Motor imagery involves imagination of an executed action without actually executing these actions (Xu et al. 2014). The use of motor imagery provides the possibility to practice a certain skill or movement without needing to physically perform it. This could be particularly useful to practice certain exercises which may not suitable for repeated attempts. This could be because the exercises are dangerous in nature, require most of the muscle stored energy, or the risk of injury is too high. For example, a one-repetition maximum in weightlifting requires the right technique, all the effort of the athlete and has a risk of injuries. Therefore, such an exercise can’t be performed multiple times successively. Motor imagery thus provides an opportunity to extend training but allowing the athlete to practice the exercise without the potential downsides of physical practice (Ridderinkhof & Brass, 2015).
In practice, motor imagery is used in many different forms and diverse names are given in literature. The literature distinguishes two main types of mental imagery: the first-person perspective (1PP) and the third-person perspective (3PP). During 3PP (also known as external imagery or visual imagery), the individual imagines the action as a spectator watching oneself perform. This is similar to watching oneself perform an exercise on a monitor. Using 1PP (also known as internal imagery or kinaesthetic imagery), the individual imagines the action from within the body, trying to experience all the sensations of the movement. For example, during a single maximum repetition (1RM) of a squat, 1PP imagery emphasizes the recalling of the feeling of the barbell on the back, the aching of the muscles and the overall feeling of the body during the exercise. Which perspective is used depends on the task and the preference of the athlete. 3PP imagery can be beneficial in motor skills where it is important to obtain an overview of the exercise and the surroundings (e.g. a soccer match). In motor skills where the precise kinematic components of the movement are essential for performance, 1PP will be more beneficial. In practice, 1PP and 3PP imagery are often combined in training (Ridderinkhof & Brass, 2015).
Studies have shown that mental practice can be beneficial for learning and performance of several motor skills (see meta-analysis Feltz & Landers, 1983; Driskell et al., 1994). Furthermore, imagery can be used to influence other aspects of performance like enhancing motivation and self-confidence, coping with pain and reducing stress and anxiety (Martin, Moritz, & Hall, 1999). Motor imagery has been shown to enhance performance in different sports varying from golf, dart throwing and basketball shooting to weightlifting, down-hill skiing and soccer (Ridderinkhof & Brass, 2015). However, motor imagery was found less effective when the executed task contained fewer cognitive elements (Feltz & Landers, 1983; Driskell et al., 1994). This is often the case with ‘simpler’ strength tasks, such as a bicep curl or a leg press. In the broad field of motor imagery, the effect of motor imagery on strength tasks has received relatively little attention. Yue and Cole (1992) were one of the first to study the effect of motor imagery on a strength task. After a 4-week motor imagery training program, they recorded a maximal force increase of 22% of the abductor digiti minimi. Yue and Cole thus showed that an increase in strength can be achieved without physical practice. In recent years more studies examining the effect of motor imagery on muscle strength have been published. Nevertheless, a clear overview and insight in the underlying mechanisms is lacking. Therefore the objective of this thesis is to answer the following question: what is the effect of motor imagery of a strength training task on muscle strength in healthy subjects? This thesis will review the literature on this topic published in the last 15 years.
Method
Relevant articles published from 2003 till February 2018 were identified through PubMed, Google Scholar and the VU university library. The following terms were used in various combinations: “motor imagery, mental imagery, practice, muscle, strength”. In total, 11 studies were included.
Results
The results of the included studies will be discussed following their methodologies. First, the effect of motor imagery alone on muscle strength will be discussed. Second, the effect of motor imagery will be compared to the effect of physical practice. Lastly, the effect of motor imagery in combination with physical practice will be compared with physical practice alone. An overview of the included studies is shown in Table 1.
Effect motor imagery
Three of the included studies (Bahari, Damirchi, Rahmaninia, & Salehian, 2011; Yao, Ranganathan, Allexandre, Siemionow, & Yue, 2013; Grosprêtre, Jacquet, Lebon, Papaxanthis, & Martin, 2018) have studied the effect of motor imagery of a strength training task on muscle strength using an imagery group and a control group. In the imagery groups, the subjects practiced motor imagery of an isometric elbow flexion (Bahari et al., 2011; Yao et al., 2013) or an isometric plantar flexion of the ankle (Grosprêtre et al., 2018), multiple times a week. They repeated these training sessions for the length of the study, varying from one week up to six weeks. During these training sessions the subjects were asked to imagine a maximum voluntary contraction (MVC) of the given strength task 30-100 times, in different series with rest in between. To check if the muscles were activated during the imagery sessions, EMG of the involved muscles was recorded (Grosprêtre et al., 2018; Yao et al., 2013) or activity was visually checked by the researchers (Bahari et al., 2011). The MVC was measured before and after the training sessions. The general finding was a significant increase in MVC in the groups that have practiced motor imagery and no significant change in the control group (Bahari et al., 2011; Yao et al., 2013; Grosprêtre et al., 2018). However, the amount of strength increase in the imagery groups varied. Yao et al. (2013) and Grosprêtre et al. (2018) measured an increase in MVC of respectively 10,8% and 9,6%, while Bahari et al. (2011) showed a higher increase of 30,4%.
Motor imagery compared to physical practice
Motor imagery of a strength training task thus appears beneficial for enhancing muscle strength, compared to no practice at all. The following studies have compared the difference in MVC after a period of physical practice and after mental practice of the same exercise (Shackell & standing, 2007; Smith, Collins, & Homes, 2003; Leung, Spittle, & Dawson, 2013; Ruiter et al., 2012; Ranganathan, Siemionow, Liu, Sahgal, & Yue, 2014). Smith et al. (2003) and Ranganathan et al. (2004) measured the maximal abduction force of the little finger, while Schakell and Standing (2007) recorded the MVC of the hip flexors and Leung et al. (2013) of the elbow flexor. For the imagery groups during imagined abduction of the little finger, two sets of either 10 (Smith et al., 2003) or 25 (Ranganathan et al., 2004) repetitions were carried out. For the hip and elbow flexion, four sets of 6-8 repetitions were executed, with 1-3 minutes rest in between for both the imagery and physical practice groups (Schakkell & Standing, 2007; Smith et al., 2003). The mental practice executed was not a simple visualization of oneself performing the task but it was emphasized to include the kinaesthetic approach, where the subjects urged the muscles to contract maximally, imagining an increase in the number of weight lifted. The subjects of the physical practice groups were placed in the same position, but physically executed the exercise instead of imagining it. Again the MVC was recorded before and after the period of practice. The MVC increased between 28-53% after a period of physical practice, while the MVC after motor imagery only increased with 16-35%. These findings indicate that motor imagery does have a positive effect on muscle strength, although physical practice of the actual exercise is still more effective.
In contrast, Ruiter et al. (2012) concluded that the increase in MVC was higher after practicing motor imagery than after physically practicing the knee extension. Ruiter et al. (2012) compared the effect of motor imagery with a group that performed physical practice of the same exercise, a placebo group who performed relaxation exercises and a control group. The imagery group received guided imagery sessions of 15 minutes, where the subjects mentally practiced fast knee extensor contractions. Before and after the training sessions the MVC- isometric torque during a knee extension was recorded. Both the imagery group and the physical practice showed an increase in strength, but the increase was slightly higher in the imagery group (9,3% compared to 6,6%).
Combination of motor imagery and physical practice
Since both imagery and physical practice enhanced MVC of the muscles involved in the exercise, multiple studies have tried a combination of physical practice and motor imagery in the hope of optimally enhancing the performance. Lebon, Collet and Guilot (2010) studied the combination of motor imagery and physical practice on the leg and chest press. The combination group performed the exact same exercises and repetitions as the physical practice group. The only difference was that the combination group performed motor imagery of the same exercises in the rest period immediately after physical practice. The performance on the leg press, both the maximum number of repetitions and the MVC, increased more with the combination than with physical practice alone. These results are shown in figure 1. Although both the physical practice and the combination group enhanced the MVC performance on the leg press, the MVC of the combination group was significantly higher than the MVC of the physical practice group. Nevertheless, for the performance of the chest press no significant differences were recorded between the combination and physical practice group.
Figure 1. MVC of the leg press.
*p < 0.05, ***p < 0.001. CTRL: control (only physical practice); MI: motor imagery (combination). (Lebon, Collet, & Guillot, 2010)
Wright and Smith (2009) used the PETTLEP approach for the motor imagery of the imagery and combination groups. The PETTLEP model provides practical guidelines to practice motor imagery with the following components: physical, environment, task, timing, learning, emotion and perspective. The intervention consisted of two sets of either physical practice, two sets of motor imagery, or one set of each for the combination group. The findings of Wright and Smith (2009) were slightly in favour of the combination of imagery and physical practice rather than physical practice alone. The significant increases in MVC after the practice period were 28% and 26,5%, respectively. Lastly, different ratios of motor imagery to physical practice were studied in relation to strength gains. Reiser, Büch and Munzert (2011) studied combinations of motor imagery and physical practice with 75%, 50% and 25% of the time spent on motor imagery and the remaining time was spend on physical practice. For example, in the 75% group the subjects performed motor imagery in three of the four sets and physically executed the fourth set. No significant changes were found in either of the groups before and after the practice period (Reiser, Büsch, & Munzert, 2011).
Study
Subjects (age/gender) Sample size (groups + #) Training duration Exercise Outcome measures Results
Bahari et al. (2011) 22,5 years
(SD = 1,36)
F (n = 0)
MI
(n = 8)
CTRL
(n = 8) 5 sessions per week (T = 4) Elbow flexion (90°) MVC-isometric
MI:
↑30,4%*
CTRL:
↑5,5% n.s.
Grosprêtre et al. (2018) 22,5 years
(SD = 2,6)
F (n = 9)
MI
(n = 9)
CTRL
(n = 9) 7 sessions per week (T = 1) Plantar flexion (90°) MVC-isometric torque
MI:
↑9,64%*
CTRL:
↑3,6% n.s.
Lebon et al. (2010) 19,75 years (SD = 1,72)
MI + PP
(n = 9)
PP
(n= 10) 3 sessions per week
(T = 4) Bench Press + leg press MVC-concentric
MR (80% MVC)
MI +PP:
BP↑9%*,
LP ↑26%*
PP:
BP↑12%*, LP↑21%*
MI +PP
(13 BP, 37 LP)
PP
(13,4 BP, 29 LP)
Leung et al. (2013) 18-35
F (n = 10) MI
(n = 6)
PP
(n = 6)
CTRL
(n = 6) 3 sessions per week (T = 3) Bicep curl standing (90°) MVC-concentric and isometric
MI:
↑16%*
PP:
↑39%*
Ranganathan et al. (2004) 28,83 years
(SD = 5,35)
F (n = 12) MI ABD
(n = 8)
MI ELB
(n = 8)
CRTL
(n = 8)
PP ABD
(n = 5) 5 sessions per week (T = 12) Abduction little finger, elbow flexion (100°) MVC-isometric MI ABD :
↑35%*
MI ELB:
↑13,5% n.s.
CTRL:
ABD ↓3,7% n.s.
ELB ↓3,6% n.s.
PP ABD:
↑53,2%*
Reiser et al. (2011) 22,7 years
(SD = 2,3)
F (n = 20)
MI 75**
(n = 12)
MI 50**
(n = 12)
MI 25**
(n = 12)
PP
(n = 12) 3 sessions per week (T = 4) Bench press (90°), leg press (45°), triceps extension (90°) and calf raise MVC-isometric MI 75/50/25:
↑3,0-4,2% (1) n.s.
↑2,6-4,0% (2) n.s.
PP:
↑4,3% (1) n.s.
↑8,3% (2) n.s.
Table 1: Overview of the included studies.
Ruiter et al. (2012) 18-24 years
F (n = 21)
MI
(n = 10)
PP
(n = 9)
PC
(n = 9)
CTRL
(n = 10) 3 sessions per week (T = 4) Knee extension (90°) MVC-isometric torque (Nm)
MI:
↑9,3%*
PP:
↑6,6%*
PC:
↑7,2% n.s.
CTRL:
↓5,4% n.s.
Shackell & Standing (2007) 20,10
(SD = 1,69)
F (n = 0) MI
(n = 10)
PP
(n = 10)
CTRL
(n = 10) 5 sessions per week (T = 2) Hip flexor machine MVC-concentric
MI:
↑23,7%*
PP:
↑28,2%*
CTRL:
↑3,5% n.s.
Smith et al. (2003) 29,33 years
(SD = 8,72)
MI
(n = 6)
PP
(n = 6)
CTRL
(n = 6) 2 sessions per week (T = 4) Abduction little finger MVC-isometric
MI:
↑23,27%*
PP:
↑53,36%*
CTRL:
↓5,36% n.s.
Yao et al.
(2013) 18-35 years IMI
(n = 6)
EMI
(n = 6)
CTRL
(n = 6) 5 sessions per week (T = 6) Elbow flexion (~100°) MVC-isometric
IMI:
↑10,8 %*
EMI:
↑4,8% n.s.
CTRL:
↓3,3% n.s.
Wrigth & Smith (2009) 20,74
(SD = 3,71)
PI
(n = 10)
MI
(n = 10)
PP
(n = 10)
PI + PP
(n = 10)
CTRL
(n = 10) 2 sessions per week (T = 6) Bicep curl machine MVC-concentric PI:
↑23,29%*
MI:
↑13,75% n.s.
PP:
↑26,56%*
PI + PP:
↑28,03%*
CTRL:
↑5,12% n.s.
ABD: abduction of the little finger, CTRL: control group, ELB: elbow flexion, F: females, MI: motor imagery group, MR: maximal number of repetitions, MVC: maximal voluntary contraction, PC: placebo group, PI: PETTLEP imagery group, PP: physical practice group. T: is the total duration of the training period in weeks. (1): posttest 1 immediately after treatment , (2): posttest 2, one week after treatment.
↑ = increase of
↓ = decrease of
* = significant (p<0,05)
n.s. = not significant
** = number indicates % of practice time spend on MI. Remaining % is spend on MVC practice
Discussion
This thesis aimed to investigate the effect of motor imagery of a strength training task on muscle strength in healthy subjects. The overall picture that emerges is that the practice of motor imagery is in general beneficial for increasing MVC. Motor imagery without physical practice has a positive effect on muscle strength, while physical practice itself has shown to be more effective. As a result, a combination of motor imagery and physical practice seems the most promising for athletes to improve their strength performance.
Strength gains after resistance training result both from neurological and morphological adaptations. During the first period of resistance training (4-6 weeks), strength will increase but morphological adaptations (muscle thickness, fascicle angle or fascicle length) are usually absent (Mcardle & katch, 2014). These early strength increases are therefore likely the result of adaptations in the nervous system (Nuzzo et al., 2017). Since motor execution is blocked during motor imagery, the strength increase after motor imagery practice has to be explained by neurological adaptations (Leung et al., 2012). These neurological adaptations will be clarified in more detail later in this thesis.
Some of the included studies have recorded surface EMG signals (Bahari et al., 2011; Grosprêtre et al., 2018; Ruiter et al., 2012; Smith et al., 2003; Yao et al., 2013) before and after motor imagery and/or physical practice. In resistance training studies, an increase in EMG in the first 3-4 weeks of a strength training program was often noticed and has been related to an increase in the neural drive to a muscle (Folland & Williams, 2007). This increase in the neural drive can indicate that more motor units are recruited or that there is a higher firing rate of the spinal neurons, or a combination of both (Bahari et al., 2011). Bahari et al. (2011), Grosprêtre et al. (2018) and Smith et al. (2003) reported an increase in EMG activity of the muscles involved in the exercise during or after motor imagery, although Yao et al. (2013) and Ruiter et al. (2012) found no change in EMG. An increase in EMG after motor imagery practice might indicate that neural adaptations have occurred, which could be beneficial for strength increase. However, there were contradicting findings in the EMG signals after physical practice. Besides an increase in EMG, no change or a decrease in EMG activity have been recorded as well (Folland & Williams, 2007). As a result, it is difficult to draw conclusions about the neural adaptations from the EMG signals alone.
The success of motor imagery in learning and performance of motor skills is likely due to the partly overlapping neural circuits during motor imagery and motor execution (Ridderinkhof & Brass, 2015; Zhang et al., 2011). Motor imagery and motor practice both show activity in some cortical and sub-cortical areas including the supplementary motor area (SMA), premotor cortex (PMC), the inferior and superior parietal lobe, motor cortex (M1), basal ganglia and the cerebellum (Munzert, Lorey, & Zentgraf, 2009; Zhang et al., 2011). According to Grosprêtre et al. (2018), the enhancement of motor performance after motor imagery practice was primarily assigned to the plasticity of the cerebral cortex. These cortical representations (homunculi) are highly dynamic and can change over time. With disuse or impairment, less afferent information will be sent to the brain, which will induce a reduction of the muscle’s topographical representation in the somatosensory and motor cortex (Ruffino, Papaxanthis, & Lebon, 2017). The sensorimotor adaptions during motor imagery indicate that cortical changes can also occur without physical practice (Ruffino et al., 2017). There are small differences found in the intensity and location of the activated areas between motor imagery and physical practice. The cortical areas concerned with movement ideation and planning are activated more during motor imagery, while the regions which are closest to the motor output contribute more during actual physical practice (Ridderinkhof & Brass, 2015). These differences in activation might be elicited from the absence of sensory feedback during motor imagery (Ruffino et al., 2017).
Moreover, the sub-cortical adaptations should also be taken into consideration, since changes in spinal excitability are regarded as an important part of muscle strength increase after training (Grosprêtre et al., 2018). Transcranial magnetic stimulation (TMS) has been used to study neural transmission from the cerebral cortex to the muscles and to provide an index of the responsiveness of this tract, better known as corticospinal excitability (Caroll, Selvanayagam, Riek, & Semmler, 2011). Grosprêtre et al. (2018) used V-wave and H-reflex to mark descending neural drive and spinal excitability. After one week of motor imagery practice of a strength task, the individuals with the greatest increase in MVC showed the greatest increase in the spinal excitability (H-reflex) at rest. This is suggested to be due to the direct increase in transmission between Ia afferents and alpha-motoneurons or to the decrease of pre-synaptic inhibition at the spinal level (Ruffino et al., 2017). However, more research is necessary to clarify the exact neural adaptations. Furthermore, Grosprêtre et al. (2018) found an increase in the V-waves of 40-90% after motor imagery practice, which indicates an increase in the cortical descending neural drive to the spinal motoneuron pool. As a result, the strength increases after motor imagery are likely caused by cortical reorganization which may also result in an increase of the cortical neural drive to the spinal motoneuron pool.
In conclusion, the practice of motor imagery of a strength training task can lead to an increase in MVC of the muscles involved in the exercise. This may be helpful in rehabilitation when physical practice itself is not possible and in sports when it is better to avoid multiple attempts. Furthermore, motor imagery may be a beneficial addition to the usual trainings program in various sports. The strength improvements after motor imagery are likely due to neurological adaptations caused by neural plasticity in the cerebral cortex and enhanced spinal excitability. Further investigations of these neural mechanisms may improve our understanding of the benefits of motor imagery practice on muscle strength.