MECHANICS OF HUMAN QUADRICEPS MUSCLE

Mechanics of human quadriceps muscle strips (vastus lateralis; n = 10) were investigated over the whole load continuum. Mechanical experimen ts were performed at 29 degrees C and in both twitch and tetanus modes . For a given level of isotonic total load (P) and over a large part o f the contraction phase, instantaneous velocity (V) was shown to be a unique function of instantaneous length (L), regardless of time and in itial length. By considering this time- and initial length-independent mechanical property between instantaneous L and instantaneous V over the whole P continuum, a three-dimensional P-V-L relationship was cons tructed. Any variations in stimulation conditions modified the time-in dependent P-V-L diagram. Such modifications in the P-V-L relationship were characteristic of changes in contractile performance. Moreover, c haracteristics of the P-V relationship were investigated in both twitc h and tetanus modes. The curvature of the P-V hyperbola was significan tly higher in tetanus at 30 Hz than in twitch mode (P < 0.001). In con clusion, our study indicates that, in human quadriceps muscles, contra ctility can be defined as the time- and initial length-invariant part of a three-dimensional P-V-L relationship. Moreover, our data are cons istent with an increase in economy of force generation in tetanus cont ractions compared with that in twitches.