Repetitive doublets in human flexor carpi radialis muscle.

Single-motor-unit activity was recorded from flexor carpi radialis of two human subjects. 2. A large number of units showed repetitive doublets at the onset of slow recruitment. A unit starting with doublets would transfer to a normal firing pattern as the force increased. 3. At different speeds of ramp contractions, the number of doublets discharging at the onset of contraction decreased as the speed of contraction increased. 4. Both low- and high-threshold units discharged repetitive doublets. Motor units which could discharge doublets showed higher maximal firing rates than those units which did not fire doublets. 5. Short interspike intervals were also observed at the onset of ballistic movements. From the comparison of these short interspike intervals and the short intradoublet intervals we suggest that the two arise from two distinct phenomena in the spinal cord. 6. Linked potentials were observed both with single spikes and doublets. Their origin may lie in the spinal cord or the muscle unit itself or both.