Minimal changes in excitation-contraction coupling in spastic human muscle

Area of Science:

• Physiology
• Neurology
• Biomedical Engineering

Background:

• Spasticity, a condition arising from upper motor neuron lesions, can lead to muscle deformities, pain, and contractures.
• While spasticity's origin is neural, secondary muscle alterations are evident.
• Understanding the physiological properties of spastic muscle is crucial for effective treatment.

Purpose of the Study:

• To directly quantify and compare the force-frequency relationship (FFR) of healthy and spastic human skeletal muscles.
• To investigate potential secondary changes in muscle contractile properties due to spasticity.

Main Methods:

• Intraoperative direct measurement of muscle force in healthy gracilis (n=13) and spastic biceps brachii (n=8) muscles.
• Application of nerve stimulation at frequencies ranging from 1 to 70 Hz to determine the FFR.
• Analysis of twitch contraction parameters, including time to peak tension (TPT) and half-relaxation time (HRT).

Main Results:

• Force-frequency relationship (FFR) profiles were nearly identical between healthy and spastic muscles.
• Half relaxation time (HRT) was significantly prolonged in spastic biceps muscles (p<0.05).
• Time to peak tension (TPT) did not significantly differ between the groups (p=0.12).

Conclusions:

• This study provides the first direct in-vivo report of spastic human muscle kinetic properties.
• Contractile kinetics, as measured by FFR, are largely similar in healthy and spastic muscles.
• Findings suggest that spasticity may not involve dramatic alterations in calcium handling or myosin heavy chain composition in the biceps muscle.