It is important for improvement of quality of life to enhance the regulatory ability of motor performance. This motor performance, often referred to as“steadiness”has been quantified by the standard deviation (SD) or coefficient of variation (CV) for produced force. Exercise training has been established to improve the steadiness, however, it is difficult for elderly adults to perform this type training. To make clear the mechanisms improving steadiness in synergistic muscles, the present study examined whether the steadiness of lower limbs is influenced by noise stimulation which bases on the stochastic resonance. The experiment 1 consisted of knee extension force matching tasks at 2.5% and 5% of maximal voluntary contraction (MVC) for 60-s that followed: 1) intramuscular electrical noise stimulation (ES) to the rectus femoris (ESRF), 2) ES to the vastus lateralis (ESVL), 3) ES to the vastus medialis (ESVM), and 4) ES to the all knee extensor muscles (ESKE). In experiment 2, the subjects requested to perform isometric plantar flexion at 5% of MVC with and without ES of tibialis nerve. The experiment 2 also recorded motor unit action potential of soleus using fine-wire electrodes to examine motor units firing variability during steady contractions. In experiment 1, the SD of forces (anterior-posterior and medial-lateral directions) was significantly larger in ESVL than other protocols. On the other hands, SD of plantar flexion force significantly decreased due to ES in experiment 2. Fluctuations (SD or CV) in motor unit firing rate attenuated due to ES irrespective of no difference of mean firing rate. This indicates that the enhanced steadiness by ES is caused by strategy for motor unit firing. In conclusion, 1) steadiness is influenced by noise ES, 2) the steadiness of multiple muscles changes due to noise ES applying single synergist, and 3) the improvement of steadiness by noise ES is attributed to the attenuated fluctuations in motor unit firing.

DESCENTE SPORTS SCIENCE Vol.31/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE