Simultaneous assessment of isometric forces in fast- and slow-twitch muscles of single rat hindlimbs in situ.

An apparatus, consisting of a pair of small strain gauge transducers, was designed for the simultaneous assessment of isometric contractile function in two muscles, composed of a predominance of either fast- or slow-twitch fibers, and within a single rat hindlimb in situ. This facilitates assessment of mechanical performance of two separate muscles under identical conditions. In anesthetized rats (N = 10), the voltages and frequencies required to produce isometric twitch and tetanic forces from the soleus (SOL) and extensor digitorum longus (EDL) muscles were determined. The apparatus was then used to demonstrate the simultaneous assessment of forces produced by the SOL and EDL from the same hindlimbs (n = 5) during 30 min of fatigue and 30 min of recovery. With this apparatus, data collected were comparable to published data. The apparatus can be used for the simultaneous assessment of isometric contractile function and fatigue in both a fast- and a slow-twitch muscle of a single rat hindlimb in situ.