Elastic modulus and stress relationships in stretched and shortened frog sartorii

The longitudinal dynamic elastic modulus (delta stress/delta strain) of 11 sartorius muscles (Rana pipiens; 3 degrees C) was measured at static strains, 0.57 less than L/Lo less than 1.53. Pseudorandom white-noise displacements less than +/-0.031% Lo (mean, peak to peak) were imposed on tetanically stimulated and resting muscles to obtain the isometric moduli. The active elastic modulus at each length was determined as the difference between the low-frequency (15-80 Hz) asymptotes of the tetanic and resting modulus functions. Resting moduli were found to increase with stretch at a declining rate, suggesting that some resting elasticity is attributable to active crossbridges. For isometric, tetanically stimulated muscles above L/Lo = 1.0, the ratio dynamic elastic modulus/active stress was nearly constant (65.4); the data predict zero modulus at a stretched length corresponding to zero active stress. Hence, the modulus per crossbridge exhibits sarcomere length invariance. On the other hand, active muscles below L/Lo = 0.77 manifest a significant (P less than or equal to 0.05) additional modulus beyond that found at the same active stress in the stretched muscle. Sarcomeric structural rearrangements are suggested as a possible source of this additional modulus.