Effects of speed and distance of muscle shortening on force depression during voluntary contractions

The purpose of this study was to determine the influence of speed and dista nce of muscle shortening on the amount of force depression for voluntary co ntractions. Two experimental tests were performed. In the first test, subje cts performed isometric knee extensor contractions following muscle shorten ing produced by isokinetic knee extensions over the range 25-50 degrees. Tn the second test, subjects performed isometric knee extensor contractions f ollowing muscle shortening produced by isokinetic knee extensions at two sp eeds: 20 and 240 degrees/s. Knee extensor moments, surface electromyographi cal (EMG) signals of quadriceps femoris, and interpolated twitch moments we re measured during all contractions and were compared with the correspondin g values obtained during purely isometric contractions. Force depression fo llowing muscle shortening for the voluntary contractions tested in this stu dy did not depend on the distance or the speed of muscle shortening. These results are in contrast to the corresponding results in the literature obta ined using artificial electrical stimulation in which force depression was always found to be directly related to the distance of shortening and inver sely related to the speed of shortening. The difference in force depression as a function of the distance and speed of muscle shortening between volun tary and artificial electrical stimulation may be associated with changes i n activation following the voluntary shortening contractions, whereas activ ation is controlled and constant in all artificial stimulation protocols. ( C) 2000 Elsevier Science Ltd. All rights reserved.