Changes in motor unit discharge rate are not associated with the amount of twitch potentiation in old men

1 Schools of Kinesiology and 3 Physical Therapy, and 2 Departments of Anatomy and Cell Biology, and 4 Physiology, The University of Western Ontario, London, Ontario, Canada N6G 1H1 This study examined, in nine old men (82 ± 4 yr), whether there is an association between the magnitude of change in motor unit discharge rate and the amount of twitch potentiation after a conditioning contraction (CC). The evoked twitch force and motor unit discharge rate during isometric ramp-and-hold contractions (10-18 s) of the triceps brachii muscle at 10, 20, and 30% of the maximal voluntary contraction were determined before and 10 s, 2 min, 6 min, and 11 min after a 5-s CC at 75% maximal voluntary contraction. After the CC, there was a potentiation of twitch force (approximately twofold), and the discharge rate of the 47 sampled motor units declined ( P