Accurate assessment of in situ isometric contractile properties of hindlimb plantar and dorsal flexor muscle complex of intact mice

An isometric torque sensor for measuring in situ contractions of plantar or dorsal flexors of intact mouse hindlimb has been developed and evaluated. With this device, muscle torque can be accurately measured within the range of -14 mN.m to +14 mN.m. Special attention was paid to fixation of the mou se hindlimb to the measurement device. Halothane-anaesthetized Swiss wild-t ype mice were positioned on the thermostatic measurement platform, and fixa ted with a hip and foot fixation system. The novel fixation unit was evalua ted by measuring knee and ankle displacements during a contraction. A mathe matical muscle model was used to quantify the effects of these displacement s on the contractile parameters. Measured ankle and knee displacement, due to non-absolute fixation, resulted in a calculated muscle fibre shortening of 2.5%. Simulations of a contraction with this degree of fibre shortening, using the mathematical muscle model, showed only minor effects on maximal torque generation and the temporal parameters (half-relaxation time and 10- 50% rise time). Furthermore, we showed that muscle torque in our set-up is hardly affected by eccentricity between ankle and measurement axis. Measure d tetanic muscle torques of intact dorsal and plantar flexors were 3.2+/-0. 4 mN.m and 11.8+/-1.6 mN.m, respectively. The half-relaxation time of plant ar flexors was significantly higher than that of dorsal flexors (12.9+/-2.7 ms versus 8.8+/-1.2 ms), whereas the 10-50% rise time was longer in planta r (14.9+/-0.6 ms) than in dorsal (11.8+/-2.0 ms) flexors.