Influence of Lumbar Muscle Fatigue on Trunk Adaptations during Sudden External Perturbations

Abstract

Introduction: When the spine is subjected to perturbations, neuromuscular responses such as reflex muscle contractions contribute to the overall balance control and spinal stabilization mechanisms. These responses are influenced by muscle fatigue, which has been shown to trigger changes in muscle recruitment patterns. Neuromuscular adaptations, e.g., attenuation of reflex activation and/or postural oscillations following repeated unexpected external perturbations, have also been described. However, the characterization of these adaptations still remains unclear. Using high-density electromyography (EMG) may help understand how the nervous system chooses to deal with an unknown perturbation in different physiological and/or mechanical perturbation environments. Aim: To characterize trunk neuromuscular adaptations following repeated sudden external perturbations after a back muscle fatigue task using high-density EMG. Methods: Twenty-five healthy participants experienced a series of 15 sudden external perturbations before and after back muscle fatigue. Erector spinae muscle activity was recorded using high-density EMG. Trunk kinematics during perturbation trials were collected using a 3-D motion analysis system. A two-way repeated measure ANOVA was conducted to assess: (1) the adaptation effect across trials; (2) the fatigue effect; and (3) the interaction effect (fatigue × adaptation) for the baseline activity, the reflex latency, the reflex peak and trunk kinematic variables (flexion angle, velocity and time to peak velocity). Muscle activity spatial distribution before and following the fatigue task was also compared using t-tests for dependent samples. Results: An attenuation of muscle reflex peak was observed across perturbation trials before the fatigue task, but not after. The spatial distribution of muscle activity was significantly higher before the fatigue task compared to post-fatigue trials. Baseline activity showed a trend to higher values after muscle fatigue, as well as reduction through perturbation trials. Main effects of fatigue and adaptation were found for time to peak velocity. No adaptation nor fatigue effect were identified for reflex latency, flexion angle or trunk velocity. Conclusion: The results show that muscle fatigue leads to reduced spatial distribution of back muscle activity and suggest a limited ability to use across-trial redundancy to adapt EMG reflex peak and optimize spinal stabilization using retroactive control.

Keywords: habituation; high-density electromyography; muscle fatigue; reflex; spinal stability.

Figure 1

Illustration of the perturbation protocol. Participants were positioned in a semi-seated position with their trunk attached to a manual trigger by a cable using a pulley system. A visual feedback was provided using a screen indicating the target of 20% of their trunk flexion maximal voluntary contraction.

Figure 2

Representation of muscle reflex variables extracted from one high-density electromyography (EMG) electrode during one perturbation trial.

Figure 3

Stages of high-density EMG data analyses. (A) Representation of one 64-electrode matrix used in the recording of erector spinae muscle activity. (B) Myoelectric signals from 64 electrodes of one matrix in a random healthy participant during one perturbation trial. (C) Centroid migration from topographical representation of root mean square (RMS) reflex values computed within 100 ms windows at each trial. Note the difference between muscle activity recruitment pattern (color variation) between pre- and post-fatigue. (D) Dispersion representation from the 15 centroid position before and after the fatigue protocol.

Figure 4

Mean RMS peak results on the right side with (A) and without (B) the first perturbation trial on the right side. Error bars indicate standard errors. Significant post hoc results are illustrated by *p ≤ 0.001.

Figure 5

Representation of the mean EMG activity traces for the right erector spinae before (perturbation trials 1–15) and after (perturbation trials 16–30) the fatigue task. The red dotted line represents the perturbation onset. (A.U. Arbitrary Unit).

Figure 6

Representation of six random participants’ centroid displacement between perturbation trials on the right erector spinae muscles. Blue line represents centroid displacement before the fatigue task. Red line represents centroid displacement after the fatigue task. Stars represent the first trials and squares represent the last trials of each condition (pre- and post-fatigue).