Fig. 3. Peak tetanic force of the ventral interior medial muscle versus length. Different symbols represent four different muscle preparations. Fitted curve is a four-parameter Weibull nonlinear regression (r²=0.48, P<0.0001). Peak force increased linearly by more than threefold as strain increased from about 60 to 108% of in vivo resting muscle length. Optimal length for force production (L_o) was about 108% of resting length. Muscles produced 90% or more of maximum force at lengths from about 80 to 135% of L_o, and 60% or more at lengths from about 60 to 160% of L_o.