Impact of static flexion duration on the development of a neuromuscular disorder of the lumbar spine

Many occupational groups work under adverse conditions in which their spinal columns are fully flexed under load for substantial periods of time. This study was designed to determine the response of the lumbar spine to a static load of a substantial duration under a range of load magnitudes. The impact of static loads of 20, 40 and 60N were applied over 30 min flexion, 10 min rest, and 30 min flexion (for a total of one hour exposure) followed by a 7 h rest. Lumbar viscoelastic creep (laxity) and reflex electromyographic (EMG) activity were monitored over both flexion periods and the 7 h rest period. It was found that 10 min of rest was not sufficient for complete recovery of the creep developed in the first 30 min of flexion resulting in a large cumulative creep at the end of the work-rest session. Muscle activity indicated spasms during the static flexion periods and demonstrated the development of initial and delayed hyperexcitability in each of the 3 loads. Larger magnitudes of initial and delayed hyperexcitability were observed for larger loads although the differences were not statistically significant. Thus, intense periods of static flexion will result in neuromuscular disorders regardless of load magnitude. The results of the 3:1 work-rest ratio were compared with previously obtained data of a series of short static flexion periods (1:1 ratio) of the same cumulative time to determine which can best expedite the recovery of creep. Although the valuable effects of a 1:1 work-rest ratio have been documented (Sbriccoli, 2004), a 3:1 work-to-rest duration ratio was not sufficient to attenuate or prevent the development of any of the components of a neuromuscular disorder and the associated microdamage and inflammation. In conclusion, a cumulative low back disorder was elicited from exposure to two 30 min static loads spaced by a 10 min interval