Minimal Changes in Excitation-Contraction Coupling in Spastic Human Muscle.

Spasticity results from upper motor neuron lesions and can create a deforming force, pain, and is often accompanied by contracture. While 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 years; surgery due to brachial plexus injury) and spastic biceps brachii muscle (n=8; aged 53.3 ± 10.3 years; 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 were modelled with sigmoid functions, and differences between muscles were assessed with an extra sum-of-squares F-test. Time to peak tension (TPT) did not significantly differ between groups (p=0.12), whereas half relaxation time (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 not dramatic calcium handling or myosin heavy chain changes in the biceps muscle secondary to spasticity.