Strength gains by motor imagery with different ratios of physical to mental practice

The study aimed to determine the extent to which strength gains from high-intensity resistance training could be achieved using motor imagery (MI) of maximal isometric contractions (IMC). Prior research demonstrated that MI is effective for various motor tasks, including strength training. Studies by Yue and Cole (1992) and others showed significant strength gains from IMC training, particularly in smaller muscle groups. However, the effectiveness of MI in larger muscle groups and its optimal ratio to physical training remained unclear. The study hypothesized that comparable strength gains could be achieved using different combinations of physical (MVC) and mental (IMC) strength training compared to exclusive physical training, potentially reducing stress on the musculoskeletal system while maintaining neural activation. The research aimed to test this hypothesis by comparing groups with varying ratios of MVC and IMC training.

The literature review examined previous studies on the effectiveness of imagined muscle contraction (IMC) training on strength gains. Early studies like Yue and Cole (1992) demonstrated comparable strength gains with IMC and MVC training in the little finger. Subsequent research explored IMC effects on larger muscle groups with varying success. Smith et al. (2003) found significant gains using IMC, while Ranganathan et al. (2004) observed smaller increases in elbow flexion strength. Studies also investigated IMC's effects on preventing strength loss during immobilization (Newsom et al., 2003) and the underlying neurophysiological mechanisms. Neuroimaging studies revealed similar neural activation patterns during physical and mental motor actions, implicating central neural processes in IMC strength gains. The review highlighted the need to determine the optimal balance between MVC and IMC training for maximum effectiveness in practical settings.

Forty-three healthy sport students (20 female) participated. All subjects first completed a 4-week standardized strength training program with submaximal loads. This was followed by a 4-week intervention with four groups: a control group (CO) with no further training; a MVC-only group (MO); and three groups (M75, M50, M25) combining MVC and IMC training at different ratios. Each subject performed two of four exercises: bench press, leg press, triceps extension, and calf raise. Isometric MVC force was measured before (pretest), immediately after (Posttest 1), and one week after (Posttest 2) the 4-week intervention. The IMC groups were matched to the MO group for the total number of trials. IMC training involved the same temporal pattern as MVC training but involved mentally imagining maximal contractions, with visual observation to ensure no actual contractions occurred. Subjects rated the vividness of their imagery after each session. Statistical analyses included ANOVAs and correlations to evaluate strength gains and the influence of various factors.

The standardized strength training program prior to the intervention led to significant strength gains (p<0.001). At Posttest 1, the MO group showed the largest strength gain (4.3%), followed by the IMC groups (3.0–4.2%). All intervention groups showed significant improvements compared to the CO group (p<0.05). The interaction between group and practice (trained vs. untrained exercises) was not significant at Posttest 1 or Posttest 2. At Posttest 2, the MO group showed additional gains (8.3%), while IMC groups maintained their gains. A moderate negative correlation (r=-0.61, p=0.005) between pretest MVC and percentage strength gain was found for the calf raise. Strength gains were not significantly influenced by gender. However, a moderate positive correlation was observed between reported imagery vividness and strength gains (Posttest 1: r=0.30, p=0.041; Posttest 2: r=0.40, p=0.019). Subjects with higher imagery vividness showed significantly better strength improvement.

The findings support the hypothesis that high-intensity strength training sessions can be partly replaced by IMC training. The comparable strength gains in the IMC and MO groups indicate a functional equivalence between motor imagery and motor performance. The smaller effect sizes compared to some previous studies are likely due to the initial 4-week standardized training program. The maintained strength gains in the IMC groups after one week of rest suggest that the training effects are primarily neural rather than solely due to muscular hypertrophy. The significant correlation between imagery vividness and strength gains suggests that the richness of mental representation influences training effectiveness. The variability in individual responses highlights the need to consider task-specific demands and individual differences in future research.

This study demonstrates that high-intensity isometric strength training can be partially replaced by IMC training sessions without significantly reducing strength gains, at least at the recreational level. The effectiveness of IMC training was enhanced by the prior 4-week physical training phase, highlighting its potential as a supplementary training method. Future studies should investigate the effects of IMC training on dynamic strength exercises and the role of individual factors in determining response to IMC training.

The study focused on isometric strength training, which may not fully generalize to dynamic exercises commonly used in athletic training. The sample consisted of recreationally trained individuals, limiting the generalizability to highly trained athletes. While measures were taken to control for muscle activity during IMC training, it cannot be entirely ruled out that some very minimal muscle activation may have occurred. Individual differences in responsiveness to training were observed, highlighting the need for further investigation into factors like imagery ability and task-specific effects.