Spinal stiffness increases with axial load: another stabilizing consequence of muscle action

Area of Science:

• Biomechanics
• Spinal Mechanics
• Orthopedics

Background:

• Spinal stability is essential for function and injury prevention.
• Lumbar motion segment stiffness is a key factor influencing spinal stability.
• Previous models often simplify or neglect the effect of axial loading on segment stiffness.

Purpose of the Study:

• To investigate the role of lumbar spinal motion segment stiffness in spinal stability.
• To analyze how axial preloading affects lumbar spinal stiffness and stability.
• To determine the minimum muscle stiffness required for lumbar spinal stability under various loading conditions.

Main Methods:

• Modeling lumbar spinal stability under various external loads (30 Nm, 60 Nm) and body weight.
• Employing two assumptions for motion segment stiffness: constant equivalent beam and preload-dependent stiffness.
• Utilizing experimentally determined relationships for preload-dependent stiffness.
• Calculating muscle forces for equilibrium using two physiologically plausible muscle activation strategies.

Main Results:

• Stability analyses indicated that critical muscle stiffness values decreased when preload effects were included.
• This decrease in required muscle stiffness suggests an increase in overall spinal stability.
• The findings were consistent across different loading conditions and muscle activation strategies.

Conclusions:

• Lumbar spinal stiffness, including muscular stiffness, plays a vital role in maintaining spinal stability.
• The stiffness-increasing effect of axial preloading significantly enhances spinal stability.
• Stability analyses should incorporate the influence of preloading on spinal stiffness for more accurate assessments.