Jahaan Ali, Ian Smith, Geoffrey Power, Walter Herzog
INTRODUCTION When a muscle undergoes an unfused tetanus, it may display a decline in force following the initial rise. This effect, known as sag, occurs independent of fatigue and its presence or absence is regularly used to distinguish fast from slow motor units [1]. Its use has been further expanded to distinguish fast fatigable from fast fatigue-resistant motor units [2]. Despite its great utility, the precise mechanism responsible for sag is unknown. It is understood that in a series of
more » ... in a series of unfused tetani, sag is reduced or absent in all contractions beyond the first contraction [2], and that changes in relaxation play a significant role in the development of sag [1]. A potentially related effect is the progressive increase in twitch relaxation rates during stimulation at frequencies too low to cause summation [3]. Acceleration of relaxation may allow more frequent cycling between contraction and relaxation and be important for locomotion. The purpose of this study was to induce fast twitch relaxation and attenuate sag using a 16-pulse contraction, and then investigate a possible relationship between sag and fast twitch relaxation by comparing the recovery times of these two effects. It was hypothesized that twitch relaxation times and sag would return to control levels over a similar and rapid time frame.