Effects of Motor Imagery Versus Motor Imagery Combined with Real Muscle Contraction on Strength Development and Vertical Jump Performance in Junior Soccer Players: A Randomized Controlled Trial

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Abstract
This study examined the effects of motor imagery (MI) training, performed alone or in combination with muscle contractions, on quadriceps performance in junior soccer players. Forty-five players (14.1 ± 0.8 years) were randomly assigned to MI alone (EXP-1; n = 15), based on the Physical, Environment, Task, Timing, Learning, Emotion, Perspective (PETTLEP) model (3 sessions/week, 20–25 min/session), MI combined with muscle contractions (EXP-2; n = 15; 3 session/week, 35–45 min/session), and a control group (CG; n = 15). Physical performance was assessed via vertical jumps (countermovement jump [CMJ], squat jump [SJ], drop jump [DJ]) and maximal isometric knee extension strength. A two-way repeated-measures general linear model (time × group) was used to assess mean differences. If necessary, we used a Mixed ANCOVA (Analysis of Covariance) to control for the influence of important parameters after significant mean differences were observed at the first examination. After 12 weeks, relevant-significant (p < 0.05 and ηp2 > 0.15) interaction effects were observed for all variables. The interaction effects ranged from ηp2 = 0.40 (maximum strength left) to ηp2 = 0.77 (DJ). Regarding DJ, we found only a partial group effect between CG and EXP-2 (p = 0.044). Significant partial effects for strength parameters were calculated for the average strength (CG vs. EXP-2: left: p = 0.008; right: p = 0.018) and for the maximal strength (CG vs. EXP-2: left: p = 0.046). These findings suggest that motor imagery training may improve neuromuscular performance (maximal strength and vertical jump performance) in junior soccer players, whether performed alone or combined with muscle contractions.

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