Single motor unit firing rate after stroke is higher on the less-affected side during stable low-level voluntary contractions

Abstract

Muscle weakness is the most common outcome after stroke and a leading cause of adult-acquired motor disability. Single motor unit properties provide insight into the mechanisms of post-stroke motor impairment. Motor units on the more-affected side are reported to have lower peak firing rates, reduced discharge variability and a more compressed dynamic range than healthy subjects. The activity of 169 motor units was discriminated from surface electromyography in 28 stroke patients during sustained voluntary contractions 10% of maximal and compared to 110 units recorded in 16 healthy subjects. Motor units were recorded in three series: ankle dorsiflexion, wrist flexion and elbow flexion. Mean firing rates after stroke were significantly lower on the more-affected than the less-affected side (p < 0.001) with no differences between dominant and non-dominant sides for healthy subjects. When data were combined, firing rates on the less-affected side were significantly higher than those either on the more-affected side or healthy subjects (p < 0.001). Motor unit mean firing rate was higher in the upper-limb than the lower-limb (p < 0.05). The coefficient of variation of motor unit discharge rate was lower for motor units after stroke compared to controls for wrist flexion (p < 0.05) but not ankle dorsiflexion. However the dynamic range of motor units was compressed only for motor units on the more-affected side during wrist flexion. Our results show that the pathological change in motor unit firing rate occurs on the less-affected side after stroke and not the more-affected side as previously reported, and suggest that motor unit behavior recorded in a single muscle after stroke cannot be generalized to muscles acting on other joints even within the same limb. These data emphasize that the less-affected side does not provide a valid control for physiological studies on the more-affected side after stroke and that both sides should be compared to data from age- and sex-matched healthy subjects.

Application of the ${\rm F}$-Response for Estimating Motor Unit Number and Amplitude Distribution in Hand Muscles of Stroke Survivors

“…A decrease of MUNE in the FDI muscle indicates less number of motor units being recruited which leads to reduced maximal strength. Other factors such as decrease of motor unit firing rates can also account for muscle weakness [32, 37–39]. …”

Motor Unit Activity during Fatiguing Isometric Muscle Contraction in Hemispheric Stroke Survivors

“…This muscle weakness post-stroke has been attributed to alterations in the descending voluntary command, and to anatomical and physiological changes within the muscle ( McComas et al, 1971 ; Bourbonnais and Noven, 1989 ; Dattola et al, 1993 ). Previous studies have identified impairments in voluntary muscle activation ( Riley and Bilodeau, 2002 ; Knorr et al, 2011 ; Bowden et al, 2014 ; Hoffmann et al, 2016 ), altered motor unit (MU) firing rates ( Rosenfalck and Andreassen, 1980 ; McNulty et al, 2014 ), a reduced ability to modulate MU firing ( Gemperline et al, 1995 ; Mottram et al, 2014 ; Li et al, 2015 ) and abnormal MU recruitment patterns ( Tang and Rymer, 1981 ; Hu et al, 2015 , 2016 ), all of which may contribute to muscle weakness post-stroke.…”

Mechanisms underlying firing in healthy and sick human motoneurons

“…Garland et al (2014) reviewed stroke-related changes in motor unit discharge characteristics and suggested that residual motor control strategies may remain after stroke. McNulty et al (2014) investigated motor unit firing characteristics in different muscles of stroke survivors showing that the effects on peak firing rates and their dynamic range differ not only between joints of the upper and lower limbs but also between muscles of different joints of the same limbs. Furthermore, they have shown that motor units on both paretic and non-paretic sides changed after stroke.…”