Disturbed muscular adaptation and changed motor strategies in recurrent low back pain patients during stair walking after experimental back-muscle fatigue

Abstract

Background: The majority of low back pain (LBP) patients experience no symptoms after 4 weeks but up to 70–80% will relapse within one year and up to 40% develops recurrent LBPand reduced functional capacity.Recurrent LBP thereby is a major cause of dysfunction, decreased quality of life and economical expenditures. The evidence on causality is generally low, but motor control is a potential factor linked with LBP. The trunk-muscle activity is affected by pain but the majority of the evidence is based on investigation of non-functional motor tasks. The transfer of results to functional and complex motor tasks is challenged, since the motor output is a result of the interaction between central and peripheral dynamic mechanisms. Experimental fatigue of the backmuscles in healthy and LBP patients affects the control of the trunk and the muscle activity is earlier affected in patients during exercising towards fatigue. Disturbed control of the muscular control can influence the development of LBP but the underlying mechanisms and consequences on motor control during motor tasks requiring motor planning and higher stability and dynamic force development remains unknown. Purpose: To explore the trunk muscle activity and motor task timing in healthy and recurrent LBP patients during stair ascend and descend before and after lumbarmuscle fatiguing. Methods: 45 participants (25 LBP) performed 10 concurrent ascend and descend steps before and after fatiguing the back extensors by repetitive dynamic exercises to exhaustion. 3 ascending and 3 descending gait phases were detected. The duration of the phases and changes in i-EMG activity from surface EMG-recordings of 3 back-, 3 abdominaland 2 gluteus-muscles before and after fatigue were calculated and compared by ANOVA statistics. Results: The duration of the step tasks were shorter in LBP patients before (P< 0.01) and after (P< 0.01) fatiguing the back extensors. Fatigue furthermore resulted in increased duration during phases with preparation of single-leg weightbearing (P< 0.001) and decreased during the weight bearing phases (P< 0.001). Back extensor fatigue resulted in increased back and abdominal muscle activity in healthy (P< 0.01) and unchanged muscle activity in LBP patients in most phases, but during the descend phase with single-legged weight bearing the activity remained unchanged in healthy and decreased in LBP patients (P< 0.001). Conclusion(s): Back extensor muscle fatigue resulted in increased trunk muscle activity in healthy and unchanged trunk muscle activity in LBP patients during stair stepping and the patients could be at higher risk of inappropriate load of potential painful structures. In phases with high stability demands the patients utilized movement strategies that in short-term might reduce the stability demands, but the long-term structural consequence is unknown. Furthermore it remains unknown if these strategies will affect recurrent LBP patients during other functional motor tasks and more research is needed to explore these questions. Implications: Affected control of the spine in high stability demands is important in evaluation of recurrent LBP patients. More research is needed to explore the basic mechanisms and the potential role of motor control during functional motor tasks in LBP development.