In the medial gastrocnemius muscle of intact rats, division of motor units (MUs) into slow (S) or fast (F) types is typically based on presence of a sag phenomenon in 40 Hz unfused tetanic contraction. MUs with sag are classified as F, while those without sag as S. However, in rats one month after spinal cord injury this phenomenon almost completely disappears and cannot be used as a basis for MUs differentiation, whereas the twitch contraction time increased significantly. Analysis of myosin heavy chain (MHC) isoform composition confirmed transformational changes of muscle fibres after spinal cord transection and indicated unchanged proportion of type I MHC isoforms, disappearance of type IIa MHC isoforms, and increase of type IIb MHC isoforms. We proposed an additional method for division of MUs into types when standard criteria are not applicable. It was observed that relative effectiveness of force summation during 20 Hz tetanus, described as a ratio of the force of the last contraction of this tetanus to the force of the first contraction, did not change after spinal cord injury. This ratio for S MUs both in intact and spinal rats exceeded 2.0, whereas for F units was lower than 2.0. Calculations of this ratio made for better fused tetani, evoked by 30 Hz or 40 Hz stimulation, showed overlapping values. We conclude that this 20 Hz tetanus index appears to be an alternative method useful for division of motor units into S and F types.