Discharge properties of human motor units during sustained contraction at low level force

The characteristic of discharge behaviors of motor units (MUs) during low level contraction was investigated. The discharge of MUs in the m. vastus medialis was observed during the sustained contraction at 4 different levels below 10% MVC (2, 4, 8 and 10% MVC) for 15 min. The spike interval of all observed MUs gradually elongated during an initial several minutes of the contraction and the characteristic discharge patterns following the elongation were observed. i.e. continuous discharge throughout the contraction (CONT), decruitment (D-N), and re-recruitment following decruitment (D-REC). The relationship between recruitment threshold force (F(th)) and discharge pattern was not significant at 2% MVC but, at 10% MVC, there were significant differences in F(th) between D-N and CONT, and between D-REC and CONT MU populations. In pooled data, the MUs with the shorter mean spike interval at the beginning of the contraction (MSI(0), below 90 ms) tend to discharge continuously, but the MUs with longer MSI(0) showed various discharge patterns. In conclusion, during low level contraction MUs discharge characteristically, and the MU with high excitation levels tend to discharge continuously, but individual MU represents an intrinsic discharge pattern at not a high excitation level.