Stretch reflexes and joint dynamics in rheumatoid arthritis

Area of Science:

• Neuroscience
• Rheumatology
• Biomechanics

Background:

• Rheumatoid arthritis (RA) is characterized by joint inflammation and can affect muscle function.
• Understanding the contribution of reflex and passive tissue properties to joint stiffness in RA is crucial for developing targeted therapies.

Purpose of the Study:

• To investigate the reflex and passive tissue contributions to joint stiffness in the upper limbs of RA patients.
• To differentiate between neural and mechanical factors influencing joint stiffness in RA.

Main Methods:

• Servo-controlled sinusoidal stretch perturbations were applied to the metacarpophalangeal joint in RA patients and controls.
• Surface electromyography (EMG) of the first dorsal interosseus muscle was recorded.
• Measurements included reflex gain, phase difference, coherence square, joint mechanical gain, and mechanical phase difference.

Main Results:

• RA patients demonstrated decreased reflex gain and increased reflex delay, particularly at higher stretch frequencies.
• Muscle weakness in RA patients contributed to the observed decrease in reflex gain.
• Increased joint stiffness was observed in RA patients during muscle contraction, but not at rest, suggesting altered mechanical properties of the active muscle-joint system.

Conclusions:

• The increased joint stiffness in rheumatoid arthritis is primarily attributed to changes in the mechanical properties of the active muscle-joint system.
• There is evidence of significant loss of neural drive to the motor unit population in RA patients.
• Reflex properties do not appear to be the primary driver of increased joint stiffness at rest in RA.