Effect of cooling on H- and T-reflexes in normal subjects.

The local application of cold has been used to decrease spasticity and facilitate neuromuscular function, but previous attempts to identify its effect on the stretch reflex have not been entirely successful. We examined the effects of cold on the Hoffmann (H) reflex and on the tendon tap (T) reflex in 16 subjects. A series of H/M recruitment curves and T-reflexes were recorded via surface EMG electrodes before and during cooling of the triceps surae. Skin and intramuscular temperatures were recorded with average decreases of 18.4C and 12.1C, respectively. Peak-to-peak amplitude of the M, H, and T compound action potentials (CAPs) was measured. In all cases, the amplitude of the maximal M-wave decreased (p less than 0.001) in response to cooling. These changes in the recording of CAPs should be considered when cooling experiments result in alterations in H or T waveforms. When using the M-wave as a covariant in our analysis, there were no significant changes in the H-reflex amplitude; the height of CAPs elicited by T decreased (p = 0.025). Our findings do not support earlier claims that simple cooling facilitates the excitatory alpha motoneuron pool as measured by the H-reflex; we do confirm that muscle spindle activity, as measured by the T-reflex, is decreased by muscle cooling.