The influence of motor imagery and practice time on the accuracy and excitability of spinal anterior horn cells

Abstract

Forty-four healthy subjects were randomly and evenly allocated into four groups based on the allowed motor practice time: either 10, 30, 60, or 120 seconds. F-waves were recorded at rest (resting condition). Next, subjects were asked to practice pinch force generation at 50% maximum voluntary contraction (MVC), using rapid visual feedback for guidance (MP condition). The subjects were then asked to generate pinch force at 50% MVC without visual feedback (pinch task 1). The index of accuracy (absolute error at 50% MVC for pinch task 1) was recorded. Next, these subjects were asked to perform motor imagery, and F-waves were recorded (MI condition). Finally, these subjects repeated the non-guided pinch task again (pinch task 2). Increases in persistence and F-wave/M-wave amplitude ratios (F/M-amp) were calculated by subtraction of the resting values from those in the MI condition. Increases in absolute error at 50% MVC were calculated by the subtraction of values from pinch task 2 from pinch task 1. Increases in persistence, F/M-amp, and absolute error at 50% MVC were decreased in the 30 and 60 second motor practice groups when compared to the 10 and 120 second motor practice groups. In conclusion, improvements in accuracy that result from motor imagery are not due to excessively increased excitability of spinal anterior horn cells.