Changes in human alpha-motoneuron excitability during sustained maximum isometric contractions.

Experiments were conducted to evaluate the change in alpha-motoneuron excitability during sustained maximum isometric contractions of human triceps surae. A test H-reflex was used to assess motoneuron excitability 10 ms after a conditioning reflex was generated. The test reflex was compared to a reference H-reflex; both test and reference reflexes were of approximately equal amplitudes at the onset of the sustained maximum efforts. Both reflexes were assumed to be influenced by similar descending and peripheral inputs. In addition, the test reflex was influenced by the conditioning reflex. For the 4 subjects tested, the test reflex decreased in amplitude within the first 30-40 s of effort, while the reference reflex remained roughly constant or increased in amplitude. The decline of the test reflex relative to the reference was indicative of an inhibitory effect due to the conditioning reflex. In that the conditioning reflex was always generated 10 ms prior to the test reflex, the two factors most likely responsible for the inhibition would be recurrent inhibition and summation of motoneuron afterhyperpolarization. A combination of these two factors could also account for the associated slowing of motoneuron firing during sustained maximum efforts.