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

The availability of animal models with disrupted genes has increased the ne ed for small-scale measurement devices. Recently, we developed an experimen tal device to assess in situ mechanical properties of isometric contraction s of intact muscle complexes of the mouse. Although this apparatus provides valuable infer mation on muscle mechanical performance, it is not appropri ate for determining contractile properties during shortening and lengthenin g contractions. In the present study we therefore developed and evaluated a n experimental apparatus for assessment of shortening and lengthening contr actile properties of intact plantar and dorsal flexors of the mouse. The cu rrent through a custom-built, low-inertia servomotor was measured to assess contractile muscular torque ranging from -50 to +50 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 effec ts on the measured torque. Furthermore, misalignment of the axis of rotatio n 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 expe rimental conditions. Peak power per unit muscle mass (mean +/- SD) of intac t dorsal and plantar flexors was 0.27 +/-0.02 and 0.19 +/-0.03 W(.)g(-1), r espectively. The angular velocity at maximal peak power generated by the do rsal flexor complex and the plantar flexor complex was 1100 +/- 190 and 700 +/- 90 degrees (.)s(-1), respectively.