EMG

1. EMG Voluntary reaction Perturbation M2 M3 M1 Time Lecture 12: Pre-Programmed Reactions An unexpected perturbation of a joint gives rise to a sequence of EMG events in a stretched muscle. The first one (M1) comes at a short latency (under 40 ms). Then two peaks (M2 and M3) come at a latency of between 50 and 100 ms. M2 and M3 are addressed as pre-programmed reactions. Later, a voluntary correction comes.

2. EMG Voluntary reaction Perturbation M2 M3 M1 React Let go Time Effect of Instruction on Muscle Reactions to a Perturbation Pre-programmed reactions demonstrate a strong dependence on the instruction. If the subject is instructed to resist perturbations, the pre-programmed reactions are large (solid lines). If the subject is asked to let the limb move, the pre-programmed reactions are much smaller (dashed lines). Note that the M1 reaction is the same.

3. Pre-Programmed Reactions • Also called triggered reactions, M2–M3, functional stretch reflex, transcortical reflex, long-loop reflex • Come at an intermediate latency (40–100 ms) • Are involuntary in nature but can be modified by instruction • Can be triggered by stimuli of virtually any modality • Generate a quick, crude compensation for the perturbation • Are followed by voluntary corrections

4. Central command ∆C Tonic stretch reflex mechanism Afferents Muscle A Possible Scheme A subject is holding a position in a joint against a load with a central command to a muscle. If the subject knows that a perturbation can occur, he/she can prepare an addition to the central command that would compensate for the predicted perturbation. The pre-programmed command (∆C) is triggered by peripheral signals generated by the perturbation and attenuates the mechanical effects of the perturbation.

5. Hearing Central command Vision ∆C Tonic stretch reflex mechanism Proprioceptors Muscle Pre-Programmed Responses Can Be Triggered by Any Sensory Signal The actual source of the triggering signal for a pre-programmed reaction is not important as long as the signal carries sufficient information. It can be provided by proprioceptors, a flash of light, a loud tone, etc.

6. Effects of a Perturbation Applied During a Fast Movement If a perturbation occurs during a fast voluntary movement, EMG changes are seen at a characteristic for the pre-programmed reactions latency. Generally, they involve an increase in the activity of a muscle that acts against the perturbation, and a decrease in the activity of a muscle that is assisted by the perturbation (bold lines).

7. M2-M3 M1 EMG Time Perturbation Vibration Effects of Vibration on M1-2-3 Responses Muscle vibration has different effects on different components of the responses to an external perturbation. The early response (M1) is suppressed just like the H-reflex, whereas the pre-programmed response (M2-M3) is unchanged.

8. Examples of Pre-Programmed Reactions • The “waiter’s response” • Grasp adjustments • Two-hand object holding • Reactions to postural perturbations (ankle and hip strategies) • Corrective stumbling reaction

9. Effects of Intention on Quick Muscle Reactions Pre-programmed postural corrections to a perturbation created by platform movement are context-dependent. If the cup is loaded with play dough, the corrections will be different than if the cup is full of hot tea.

10. A B Stim Stim Surface Corrective Stumbling Reaction A mechanical or electrical stimulation of the paw during locomotion induces different reactions in the swing and the stance phases. In the swing phase (A), there is a flexor reaction, so that the leg steps over a fictitious obstacle. In the stance phase (B), there is an extensor reaction, leading to the shortening of the stance phase for this limb.