Prolonged Intermittent Trunk Flexion Increases Trunk Muscles Reflex Gains and Trunk Stiffness

Area of Science:

• Biomechanics
• Neuromuscular Physiology
• Occupational Health

Background:

• Prolonged trunk flexion is common in various occupations and can lead to low back pain.
• Understanding the effects of flexion on trunk neuromuscular control is crucial for injury prevention.
• The role of passive support in mitigating these effects remains under investigation.

Purpose of the Study:

• To investigate the impact of prolonged, intermittent trunk flexion on neuromuscular control.
• To evaluate the benefits of passive upper body support during flexion tasks.
• To analyze changes in trunk stability, range of motion, and reflex responses.

Main Methods:

• Twenty-one healthy volunteers underwent 1 hour of intermittent trunk flexion (60s flexion, 30s rest) at 80% lumbar flexion.
• Flexion was performed with and without passive upper body support.
• Lumbar range of motion, trunk stability during forward perturbations, and trunk translational admittance and reflex gains were assessed using inertial measurement units and system identification.

Main Results:

• Unsupported flexion led to decreased trunk admittance and increased lumbar range of motion and reflex gains.
• Supported flexion also increased lumbar range of motion and reflex gains, but to a lesser extent than unsupported flexion.
• The findings suggest unsupported flexion increases cumulative low back load, potentially by enhancing intrinsic and reflexive stiffness.

Conclusions:

• Prolonged intermittent trunk flexion alters neuromuscular control, decreasing trunk admittance and increasing lumbar range of motion and reflex gains.
• Passive upper body support during prolonged trunk flexion is advisable to reduce the associated increase in low back load.
• These findings have implications for ergonomic recommendations in occupational settings involving repetitive trunk flexion.