Force depression in human quadriceps femoris following voluntary shortening contractions

The purpose of this study was to investigate whether the isometric muscle f orce, redeveloped following maximal-effort voluntary shortening contraction s in human skeletal muscle, is smaller than the purely isometric muscle for ce at the corresponding length. Isometric knee extensor moments, surface el ectromyographic (EMG) signals of quadriceps femoris, and interpolated twitc h moments (ITMs) were measured while 10 subjects performed purely isometric knee extensor contractions at a 60 degrees knee angle and isometric knee e xtensor contractions at a 60 degrees knee angle preceded by maximal-effort voluntary shortening of the quadriceps muscles. It was found that the knee extensor moments were significantly decreased for the isometric-shortening- isometric contractions compared with the isometric contractions for the gro up as a whole, whereas the corresponding EMG and ITM values were the same. This study is the first to demonstrate force depression following muscle sh ortening for voluntary contractions. We concluded that force depression fol lowing muscle shortening is an actual property of skeletal muscle rather th an a stimulation artifact and that force depression during voluntary contra ction is not accompanied by systematic changes in muscle activation as eval uated by EMG and ITM.