High-repetition cyclic loading is a risk factor for a lumbar disorder

Area of Science:

• Biomechanics
• Neuromuscular Physiology
• Occupational Health

Background:

• Epidemiological studies link prolonged cyclic lumbar flexion to chronic neuromuscular disorders and worker disability.
• Cyclic loading of the lumbar spine is a common occupational exposure.

Purpose of the Study:

• To physiologically and biomechanically assess repetitions of cyclic lumbar flexion as a risk factor for acute neuromuscular disorders.
• To investigate the development and recovery of neuromuscular responses to cyclic lumbar loading.

Main Methods:

• An in vivo feline model was subjected to cyclic lumbar loading (0.25 Hz) for 10 minutes, followed by 10-minute rest periods.
• Three experimental groups underwent three, six, or nine repetitions of the load/rest cycle, followed by a 7-hour recovery period.
• Lumbar viscoelastic tissue displacement and reflex electromyographic (EMG) activity of the multifidus muscle were monitored.

Main Results:

• Creep in lumbar viscoelastic tissue accumulated during loading and partially recovered during rest.
• Loading periods showed decreased reflex EMG activity with superimposed spasms.
• Delayed neuromuscular hyperexcitability, indicative of acute inflammation, was observed in the six- and nine-repetition groups after 7 hours of recovery.
• A mathematical model accurately predicted the observed physiological responses (R² ≈ 0.90).

Conclusions:

• Delayed muscular hyperexcitability signifies an acute neuromuscular disorder resulting from excessive repetitions of cyclic lumbar flexion.
• Insufficient rest can lead to the chronification of this acute disorder.
• Workers performing repetitive lumbar flexion tasks should limit long-term exposure to prevent cumulative trauma disorder.