Background: The epidemiology classify cyclic lumbar flexion as a risk factor for the development of cumulative low back disorder. Experimental biomechanical data confirming the epidemiology in humans are lacking. The purpose of this study, therefore, is to investigate the flexion relaxation response to sustained cyclic lumbar flexion in humans.
Methods: Twelve normal college aged males performed deep cyclic lumbar flexion at 0.1 Hz for 9 min while recording lumbar paraspinal electromyogram and kinematic data.
Findings: The most important observation of the study was the significant increase in the angular excursion of myoelectric silence during the deep part of the flexion phase. The observed increase in myoelectric silence consisted of earlier cessation of EMG during flexion and delayed activation of trunk extensors during extension. EMG magnitude, during flexion, increased approximately 30% over trials (P < 0.0001), and a smaller (10%), but, significant (P < 0.02) EMG magnitude increase was also observed during the extension phase of the cycle. Spasms, an indication of micro damage to viscoelastic tissues, were sporadic and appeared more frequently later in the session and mostly during the silent period.
Interpretations: It was concluded that increased myoelectric silence during prolonged cyclic flexion-extension demonstrates an enhanced flexion-relaxation phenomenon which reduces lumbar stability and may be detrimental to low back health. The presence of spasms confirm that sustained cyclic lumbar flexion results in micro damage in the viscoelastic tissues. Overall, a neuromuscular disorder was evoked due to a relatively short period of unloaded cyclic lumbar flexion.