Changes in the flexion-relaxation response induced by hip extensor and erector spinae muscle fatigue

Abstract

Background: The flexion-relaxation phenomenon (FRP) is defined by reduced lumbar erector spinae (ES) muscle myoelectric activity during full trunk flexion. The objectives of this study were to quantify the effect of hip and back extensor muscle fatigue on FRP parameters and lumbopelvic kinematics.

Methods: Twenty-seven healthy adults performed flexion-extension tasks under 4 different experimental conditions: no fatigue/no load, no fatigue/load, fatigue/no load, and fatigue/load. Total flexion angle corresponding to the onset and cessation of myoelectric silence, hip flexion angle, lumbar flexion angle and maximal trunk flexion angle were compared across different experimental conditions by 2 x 2 (Load x Fatigue) repeated-measures ANOVA.

Results: The angle corresponding to the ES onset of myoelectric silence was reduced after the fatigue task, and loading the spine decreased the lumbar contribution to motion compared to the hip during both flexion and extension. A relative increment of lumbar spine motion compared to pelvic motion was also observed in fatigue conditions.

Conclusions: Previous results suggested that ES muscles, in a state of fatigue, are unable to provide sufficient segmental stabilization. The present findings indicate that, changes in lumbar-stabilizing mechanisms in the presence of muscle fatigue seem to be caused by modulation of lumbopelvic kinematics.

Figure 1

Fatigue task. Fatigue task during which participants were instructed to push upwards with both legs with an isometric contraction set at 60% of their MVC force (peak value from the two MVC trials).

Figure 2

Angles and LEDs positioning. Illustration of LEDs positioning and the thoracic (D), lumbar (C), hip (B) and knee angles (A).

Figure 3

*L/H ratio, loading, fatigue and interaction effects during flexion (Q1-Q4). Data are presented for the (1) no load-no fatigue (2) no load-fatigue (3) load-no fatigue and (4) load-fatigue conditions.

Figure 4

*L/H ratio, loading, fatigue and interaction effects during extension (Q1-Q4). Data are presented for the (1) no load-no fatigue (2) no load-fatigue (3) load-no fatigue and (4) load-fatigue conditions.