In situ assessment of shortening and lengthening contractile properties of hind limb ankle flexors in intact mice.

The availability of animal models with disrupted genes has increased the need for small-scale measurement devices. Recently, we developed an experimental device to assess in situ mechanical properties of isometric contractions of intact muscle complexes of the mouse. Although this apparatus provides valuable information on muscle mechanical performance, it is not appropriate for determining contractile properties during shortening and lengthening contractions. In the present study we therefore developed and evaluated an experimental apparatus for assessment of shortening and lengthening contractile properties of intact plantar and dorsal flexors of the mouse. The current through a custom-built, low-inertia servomotor was measured to assess contractile muscular torque ranging from -50 to mN.m. Evaluation of the fixation procedure of the animal to the apparatus via 3-D monitoring of the muscle-tendon complex length showed that the additional shortening in length due to a contraction with maximal torque output has only minor effects on the measured torque. Furthermore, misalignment of the axis of rotation of the apparatus relative to the axis of rotation in the ankle joint, i.e. eccentricity, during a routine experiment was estimated to be less than 1.0 mm and hence did not influence the measured torque output under our experimental conditions. Peak power per unit muscle mass (mean +/- SD) of intact dorsal and plantar flexors was 0.27 +/- 0.02 and 0.19 +/- 0.03 W.g-1, respectively. The angular velocity at maximal peak power generated by the dorsal flexor complex and the plantar flexor complex was 1100 +/- 190 and 700 +/- 90 degrees.s-1, respectively.

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 mouse hindlimb to the measurement device. Halothane-anaesthetized Swiss wild-type mice were positioned on the thermostatic measurement platform, and fixated with a hip and foot fixation system. The novel fixation unit was evaluated by measuring knee and ankle displacements during a contraction. A mathematical muscle model was used to quantify the effects of these displacements 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. Measured 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 plantar 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 plantar (14.9+/-0.6 ms) than in dorsal (11.8+/-2.0 ms) flexors.