Muscle fatigue in some neurological disorders

Area of Science:

• Neurology
• Muscle Physiology
• Rehabilitation Science

Background:

• Upper motor neuron dysfunction, common in multiple sclerosis (MS) and spinal cord injuries (SCI), leads to muscle weakness and inactivity.
• Long-term inactivity can alter muscle properties, potentially increasing fatigability and affecting muscle contraction dynamics.

Purpose of the Study:

• To investigate the effects of inactivity on tibialis anterior (TA) muscle fatigability and relaxation properties in patients with upper motor neuron dysfunction.
• To compare the fatigability and relaxation responses of TA muscles in patients with MS and SCI to those in healthy individuals.

Main Methods:

• Induced fatigue in the tibialis anterior (TA) muscle using repetitive electrical stimulation.
• Measured muscle tension and half-relaxation time during tetanic contractions at 40 Hz.
• Compared responses between healthy subjects and patients with MS and SCI.

Main Results:

• Patients with MS and SCI exhibited greater TA muscle fatigability compared to healthy subjects.
• Both groups showed increased half-relaxation time with repetitive stimulation, but this increase was more pronounced in patients with MS and SCI.
• These findings suggest a shift towards fatigable motor units and impaired Ca2+ uptake mechanisms in the muscles of inactive patients.

Conclusions:

• Long-term inactivity in patients with upper motor neuron dysfunction leads to increased muscle fatigability.
• Inactivity exacerbates the slowing of muscle relaxation after prolonged activity, likely due to compromised Ca2+ handling in muscle fibers.
• These findings highlight the detrimental effects of inactivity on muscle function and may inform rehabilitation strategies.