The electrically evoked long latency reflex of the biceps brachii muscle: The impact of train stimuli, preceding stimuli, and voluntary muscle contraction

Long latency reflex (LLR) responses were examined over the biceps brachii (BB) at different contraction levels after electrical single or train stimuli over the ipsilateral superficial radial nerve with an inter-stimulus interval of 3 ms. Two constant motor waves were present, LLR2 with a peak latency value of 53 ± 4 ms and LLR3 with 85 ± 10 ms. LLR responses showed a significant increase (twofold) in amplitudes after train stimuli compared to up to a fourfold increase after train stimuli were combined with a weight load of 1.5 kg. When LLR were investigated after subsequent (1/s) stimuli by selective averaging, a significant increase in LLR2 amplitude values was seen after the third compared with the first stimulus for trains of 3 stimuli. In the present study, 3 factors exerted an influence on LLR, namely temporal summation of synaptic potentials (by train stimuli), facilitation (with higher stimulus repetition rates), and volition (resulting in muscle contraction). The augmentation behaviour of LLR may be useful for the investigation of central nervous system diseases such as e.g. movement disorders.

► Long latency reflexes (LLR) were studied over the biceps brachii muscle.
► LLR amplitudes displayed an increase after train vs. single electrical stimuli.
► LLR gained in amplitudes from one stimulus to the next (frequency-dependent).
► LLR increased in amplitude depending on the muscle contraction level.
► LLR were influenced by temporal summation, synaptic facilitation, and volition.