Unusual motor unit firing behavior in older adults.

Motor unit firing behavior was studied in the first dorsal interosseus (FDI) and tibialis anterior (TA) muscles of 10 aged subjects during slow, isometric contractions. Previous study in younger individuals had shown that motor units are recruited and derecruited in an orderly manner whereby the early-recruited units are the last to be derecruited. However, there were several examples in the old subjects in which some high-threshold motor units were derecruited at much lower levels of force. Concurrent antagonist firing in an effort to maintain the required precision is considered a likely candidate for such prolonged motor unit activation.