Figure 6. Extremes in force-frequency relationship.

Force-frequency curves of the two slowest and two fastest motor units recorded in the current series (i.e. data from Fig. 5A with all intermediate units removed). Dashed lines indicate the mean frequency required to generate half-maximum tetanic force: fastest group, 19.6 Hz; and slowest group, 5.3 Hz. Note that the units in the slow group were both intrinsic motor units and those in the fast group were both extrinsic motor units, and that both slow motor units had reached maximum force by 33.3 Hz.