Effect of motor imagery training of dorsiflexor muscles on motor unit behavior

PURPOSE: Motor imagery (MI) training can enhance maximal strength, making it a promising tool for both athletic performance and clinical rehabilitation. Yet, the mechanisms underlying the increase in maximal strength, usually attributed to neural adaptations, remain to be further elucidated. We therefore aimed at investigating whether four weeks of MI training was likely to affect dorsiflexor strength and tibialis anterior motor units (MU) behavior. METHODS: Twenty-four healthy adults (12 women; 24.5 ± 3.3 years) were tested at baseline, after a control period (pre-training), and after a 4-week kinesthetic MI training (post-training). Outcomes included maximal voluntary isometric dorsiflexion torque (MVC) as well as MU recruitment and derecruitment thresholds, discharge rates during recruitment, plateau, and derecruitment phases, input-output gain and net discharge-rate changes (ΔDR) recorded from high-density electromyography during submaximal contractions at 35%, 50%, and 70% MVC. RESULTS: At post-training, MVC increased by 7.1 ± 10.7% (p < 0.002) with no baseline to pre-training difference (p = 0.97). Discharge rates at recruitment (p = 0.06), plateau (p = 0.76), and derecruitment (p = 0.88) were not affected by MI training. Recruitment (p < 0.001), but not derecruitment thresholds, rose post-training. Input-output gain was stable (p = 0.36), and ΔDR increased only at 70% MVC between baseline and post-training (p = 0.002). CONCLUSION: Four weeks of MI training increased force without any change in MU discharge rate and intrinsic MU properties, yet with increased recruitment threshold of MUs.