Two mechanically distinct types of fast twitch muscle fibres of the frog and their temperature sensitivity, as detected by sinusoidal analysis.

The mechanical responses to sinusoidal oscillations were recorded from tetanically contracting fast twitch fibres from the anterior tibialis muscle of the frog, Rana japonica. Two distinct fibre types were recognized. One type of fibres (tentatively referred to as 'worker') were similar to skinned rabbit skeletal muscle fibres in that three exponential processes (each represents a single exponential viscoelastic decay) were resolved. The second fastest process (process b) had a negative polarity and caused the fibres to produce net work during oscillations in a range of frequencies. In the other type of fibres (tentatively referred to as 'idler'), this negative process was much smaller, and no net work was produced at any frequency. No intermediate fibre has been found so far. In the 'worker' type of fibres, the tension response to oscillation had a modest amount of harmonic components at frequencies just above the range for work production. In the 'idler' type, the corresponding harmonic components were much greater. In spite of these large apparent differences, the two types of fibres showed similar temperature sensitivity, raising a possibility that the basic contractile mechanisms and their rate constants are common to both types of fibres. In the 'idler' type, the rate constant for the fastest process (process c) is possibly distributed over a wide range, thus masking the work-producing process (process b).