Minimal changes in excitation-contraction coupling in spastic human muscle

Spasticity results from upper motor neuron lesions and can create a deforming force and pain, and is often accompanied by contracture. Although the origin of spasticity is neural, there is ample evidence of secondary muscle changes. Here, we use direct measurement of the force-frequency relationship (FFR) to characterize human muscle's physiological properties. This study directly quantified the FFR of both healthy and spastic human skeletal muscles. Muscle force was measured intraoperatively in healthy gracilis (n = 13; aged 39.4 ± 10.6 yr; surgery due to brachial plexus injury) and spastic biceps brachii muscle (n = 8; aged 53.3 ± 10.3 yr; surgery due to stroke or traumatic brain injury). Nerve stimulation was applied at frequencies ranging from 1 to 70 Hz. Twitch contraction parameters, including time to peak tension (TPT) and half-relaxation time (HRT), were also compared. The FFR of the two muscles was modeled with sigmoid functions, and differences between muscles were assessed with an extra sum-of-squares F test. TPT did not significantly differ between groups (P = 0.12), whereas HRT was prolonged in the spastic biceps (P < 0.05). Despite small differences in twitch kinetics, both muscles exhibited nearly identical FFR profiles. This study represents the first direct in vivo report of spastic human muscle kinetic properties and shows that these contractile kinetics are similar in healthy and spastic muscles. This may suggest that there are no dramatic calcium handling or myosin heavy chain changes in the biceps muscle secondary to spasticity.NEW & NOTEWORTHY This study presents the first in vivo intraoperative measurement of the kinetic properties of spastic human muscle. Despite slower relaxation in spastic biceps, the force-frequency relationship was similar to that of the healthy gracilis muscle. This suggests that spasticity does not substantially alter frequency-dependent force summation, possibly due to similar fiber-type compositions and limited changes in calcium handling or myosin isoforms in human spastic muscle.