The electrically evoked long latency reflex (LLR) of the anterior tibial muscle: the impact of train stimuli and voluntary muscle contraction

Introduction: Previously, we have shown that the long latency reflex (LLR) recorded over the biceps brachii muscle can be augmented by (1) an increase in the level of isometric muscle contraction and (2) by electrical train stimuli as compared to single stimuli delivered over the superficial radial nerve (Alaid et al., Neurosci Lett, 2012;526:91 – 95).

Objective: To investigate, whether the electrically evoked reflex responses over a leg muscle would behave in a similar way to changes in isometric muscle contraction and to train stimuli like over arm muscles.

Materials/methods: LLR were recorded over the anterior tibial muscle of the right leg 50 ms before and 450 ms after supramaximum electrical stimuli (0.2 ms, 9 – 12mA, right N. suralis; 6 volunteers, averages of 60 traces). Single stimuli and trains-of-3 with interstimulus-intervals (ISI) of 3 ms were repeated at 1 Hz. Statistics: Friedman-ANOVA, Wilcoxon-Test.

Results: Short latency response (SLR), medium latency response (MLR), and LLR were present only over the voluntarily activated muscle. SLR, MLR, and LLR gained in amplitude with loading weight from 0 up to 10 kg (p < 0.05; Fig. 1). The amplitudes of SLR, MLR, and LLR were significantly larger after train stimuli than after single stimuli (p < 0.05; Fig. 1).

Discussion: Like in arm muscles, motor units of leg muscles can be recruited to take part in reflex responses due to higher levels of depolarization (i) by volition and (ii) due to temporal summation of excitatory synaptic potentials evoked by train stimuli.

Fig. 1: Box plots of the amplitudes of SLR, MLR, and LLR (n = 6 subjects) obtained after single and train stimuli with different levels of isometric muscle contraction that was varied by weight load.