Frequency and Resting Time Dependencies of Electrically-Induced Muscle Contraction Force

Abstract

Physical exercise is a promising countermeasure for osteoporosis and strengthens skeletal muscle as well, resulting in a reduction of the risk of fall. However, physical exercise is not always safe and practicable, in particular, for osteoporotic patients with a decline in physical ability or under bed rest situation. We focused on electrically-elicited muscle contraction to prevent bone loss without any voluntary physical activities. To provide basic knowledge to determine effective stimulation patterns in this method, this study aimed to investigate the stimulation frequency and resting time dependencies of muscle contraction induced by electrical stimulation using rats, which is caused by muscle fatigue during the stimulation. A rat quadriceps was stimulated continuously or intermittently for 30 minutes by giving a repetition of 10-s electrical stimulation at 2, 5, 10, or 20 Hz, followed by 0-, 10-, or 20-s resting time. The change in muscle contraction force was monitored over the stimulation period, and average peak-to-peak force and accumulated peak-to-peak force for 30 min were analyzed as mechanical stimulus parameters. The data showed a larger decrease in the average peak-to-peak force at higher frequencies and with shorter resting times. The largest average peak-to-peak contraction force was observed in the 2-Hz stimulation with 20s-rests, which was larger 1.8, 3.7, and 8.1 fold than those in the 5-, 10-, and 20-Hz stimulation with 20s-rests, respectively. The 10s-rest did not show any increase in average peak-to-peak force when compared with the no-rest. On the other hand, the largest accumulated peak-to-peak contraction force was represented in the 10-Hz stimulation without resting time, larger 1.8, 1.2 and 4.2 fold than those in the 2-, 5-, and 20-Hz stimulation without resting time, respectively. Resting times did not contribute to increase in accumulated peak-to-peak force. These findings would contribute to basic researches on the prevention of osteoporosis using the electrically-stimulated muscle contraction, in particular, by providing useful information for the determination of stimulation pattern.

Keywords

• electrical stimulation
• muscle contraction
• mechanical stimulation
• muscle fatigue
• bone strain
• osteoporosis
• osteogenesis