The relationships between muscle length, fractional change in length (strain) and work output during cyclic contraction were examined in scaphognathite levator muscle L2B of the green crab Carcinus maenas (L.). The muscle was subjected to sinusoidal strain at 2 Hz and to phasic stimulation in the strain cycle.
At an average length and stimulus phase which are optimum for net work output, the work from muscle L2B during shortening rises to a peak or a plateau with increasing strain. The failure of shortening work to increase continuously with strain is due, in part, to the greater shortening velocity associated with greater strain, and to the consequent reduction in muscle force during shortening at higher velocity. The work required to re-lengthen a muscle following contraction is a complex function of strain, with an initial peak followed by a work minimum and then a monotonic rise in work with further increase in strain. The early work minimum is a result of shortening inactivation which reduces muscle force and thus the work which must be done to re-lengthen the muscle. Because shortening work rises to a peak or plateau with increasing strain while lengthening work, for the most part, increases with strain, there is a sharp optimum strain (about 8%) for net work output.
Muscle relaxation becomes slower with increasing muscle length. As muscle length is increased, fusion of tension from cycle to cycle becomes more pronounced and shortening inactivation becomes a more important determinant of optimum strain.