This study investigated the effects of numerical imagery alone on spinal motor nerve function and motor skills. The motor imagery of a force regulation task with pinch movements of the thumb and index finger can be divided into three types of imagery: numeric imagery, which is the image of digital numbers displayed in the pinch force value; muscle contraction imagery, which is the image of muscle contraction; and sensory imagery, which is the image of the sensation of pressing on the sensor. A questionnaire was administered as a pretest to nine healthy participants (mean age: 20.8 years) who had not previously used numeric imagery. During a resting trial, F waves were elicited from the abductor pollicis brevis muscle by stimulation of the median nerve on the non-dominant hand side. The F-wave was subsequently elicited during an imagery trial in which a numerical image was provided. The task of reaching 50% contraction strength was performed before and after the imagery task. Spinal motor nerve function did not differ between the resting and imagery tasks. Moreover, a difference in absolute error between the pinch task before and after the imagery task was absent. However, the participants with decreased motor accuracy were characterized by increased excitability of the spinal motor nerve function compared to those with improved motor accuracy.