Changes in the flexion relaxation response induced by lumbar muscle fatigue

Abstract

Background: The flexion relaxation phenomenon (FRP) is an interesting model to study the modulation of lumbar stability. Previous investigations have explored the effect of load, angular velocity and posture on this particular response. However, the influence of muscular fatigue on FRP parameters has not been thoroughly examined. The objective of the study is to identify the effect of erector spinae (ES) muscle fatigue and spine loading on myoelectric silence onset and cessation in healthy individuals during a flexion-extension task.

Methods: Twenty healthy subjects participated in this study and performed blocks of 3 complete trunk flexions under 4 different experimental conditions: no fatigue/no load (1), no fatigue/load (2), fatigue/no load(3), and fatigue/load (4). Fatigue was induced according to the Sorenson protocol, and electromyographic (EMG) power spectral analysis confirmed that muscular fatigue was adequate in each subject. Trunk and pelvis angles and surface EMG of the ES L2 and L5 were recorded during a flexion-extension task. Trunk flexion angle corresponding to the onset and cessation of myoelectric silence was then compared across the different experimental conditions using 2 x 2 repeated-measures ANOVA.

Results: Onset of myoelectric silence during the flexion motion appeared earlier after the fatigue task. Additionally, the cessation of myoelectric silence was observed later during the extension after the fatigue task. Statistical analysis also yielded a main effect of load, indicating a persistence of ES myoelectric activity in flexion during the load condition.

Conclusion: The results of this study suggest that the presence of fatigue of the ES muscles modifies the FRP. Superficial back muscle fatigue seems to induce a shift in load-sharing towards passive stabilizing structures. The loss of muscle contribution together with or without laxity in the viscoelastic tissues may have a substantial impact on post fatigue stability.

Figure 1

Scaled EMG raw signal and total trunk angle from a subject performing a FRP task in a no load, no fatigue condition.

Figure 2

MedF/time and RMS/time slopes of the ES-L2 (A, C) and the ES-L5 (B, D) during the Sorensen fatigue protocol. The results are the average of all subjects.

Figure 3

Effect of fatigue induced at each muscle level (no fatigue = black; fatigue = grey) on total trunk angle at FRP onset and at FRP cessation. *P < 0.05.

Figure 4

Effect of loading at each muscle level on total trunk angle at FRP onset and at FRP cessation. *P < 0.05. (no fatigue = black; fatigue = grey).

Figure 5

Effect of loading on the FRR during flexion movement and extension movement for each ES level (no load = black; load = grey).

Figure 6

Effect of fatigue on the FRR during flexion movement and extension movement for each ES level (no fatigue = black; fatigue = grey).