Short latency heteronymous excitatory and inhibitory reflexes between antagonist and heteronymous muscles of the human shoulder and upper limb

The aims were (i) to investigate heteronymous excitatory and inhibitory Gro up Ia reflexes linking agonist/antagonist muscle pairs acting at the should er and elbow: clavicular pectoralis major (Pmajor) and posterior deltoid (P deltoid): biceps brachii (Bi) and Tri brachii (Tri), and linking muscles ac ting at the elbow (Bi and Tri) with muscles acting at the shoulder (Pmajor and Pdeltoid). (ii) To test the hypothesis that the excitability of the: re flexes would vary between different tasks in a functionally relevant manner . The study was performed on 45 adults. Reflexes were recorded in the surfa ce EMG when the target muscle was contracting at 10% maximum voluntary cont raction. Reflexes were recorded in Bi and Tri with the elbow joint in one o f three positions: 105 degrees, 80 degrees, or 55 degrees from full extensi on. Group Ia reflexes were evoked using a small, brief tap to the tendon of the muscle being stimulated. Reflexes were recorded by cross-correlation o f the surface EMG and pseudo-random series of taps. All subjects demonstrat ed short latency inhibition and excitation between agonist/antagonist muscl e pairs. inhibition was significantly more frequent than excitation. Excita tion and inhibition occurred with equal frequency between muscle pairs acti ng between elbow and shoulder. Minimum central delays for excitatory reflex es were 1 ms, consistent with monosynaptic projections and for inhibitory r esponses were 2 ms consistent with disynaptic linkage. Later excitatory and inhibitory reflexes with central delays of up to 15 ms also occurred. The probability of evoking excitation or inhibition in Tri or Bi changed with t he different elbow positions. (C) 2001 Elsevier Science B.V. All rights res erved.