Changes in reciprocal and transjoint inhibition induced by muscle fatigue in man

The effects of localised muscle fatigue on group I reflex pathways were studied in the human upper limb. Activation of group I afferents originating from biceps and extensor carpi radialis (ECR) resulted in an inhibition of flexor carpi radialis (FCR) motoneurones, probably through a disynaptic pathway. Reciprocal inhibition (from ECR to FCR) and transjoint inhibition (from biceps to FCR) were compared before and during localised fatigue induced in the muscle from which group I afferents originated. Fatigue of wrist extensors did not modify the reciprocal inhibition, while during fatigue of elbow flexors the transjoint inhibition was less pronounced. This striking difference between reciprocal and transjoint inhibition is discussed in relation to the pattern of diffusion of voluntary contractions during fatigue in the human upper limb.