The projection pattern from large muscle spindle afferents (Ia) onto c
at alpha-motoneurones innervating the elbow, wrist and digit muscles i
s correlated with new electromyographical and x-ray kinematic data on
the motor behaviour of the forelimb. This correlation suggests that th
e Ia-synergistic groups may be one substrate to organize movement comp
onents mobilized in motor behaviour. This hypothesis is based on the d
ata on the active stabilization of the radio-ulnar joints during the s
tance phase, on the data on the Ia-organization of a motor nucleus tak
ing part in different motor behaviours and on the results showing how
the Ia convergence onto the long digit extensor muscles fits the execu
tion of finely tuned movements at ball joints.