Delayed grip relaxation and altered modulation of intracortical inhibition with aging

Abstract

Grip relaxation is a voluntary action that requires an increase in short-interval intracortical inhibition (SICI) in healthy young adults, rather than a simple termination of excitatory drive. The way aging affects this voluntary inhibitory action and timing of grip relaxation is currently unknown. The objective of this study was to examine aging-related delays in grip relaxation and SICI modulation for the flexor digitorum superficialis (FDS) muscle during grip relaxation. The main finding was that young adults increased SICI to relax their grips, whereas older adults did not increase SICI with a prolonged grip relaxation time (p<0.05 for both SICI modulation and grip relaxation time). A secondary experiment showed that both young and older adults did not change H reflex excitability during grip relaxation. Our data suggest that grip relaxation is mediated by increased cortical inhibitory output in young adults, and aging-related impairment in increasing cortical inhibitory output may hamper timely cessation of muscle activity. Our data also suggest a lesser role of the spinal circuits in grip muscle relaxation. This knowledge may contribute to understanding of aging-related movement deterioration and development of interventions for improving modulation of SICI to improve muscle relaxation and movement coordination.

Relationship between intracortical inhibition and motor behavior; implications for incomplete spinal cord injury

“…This may be indicative of plastic or physiological changes regarding the availability and transmission of the neurotransmitter GABA, or the action of baclofen that facilitates GABAB activity [3,6] . However, these inhibitory networks are speculated to play a role in movement regulation and behavior in both healthy and diseased states [18][19][20][21][22] . These inferences, together with novel information obtained in our recent study [7] , allow for interpretation of how intracortical inhibition is linked to some of the motor symptoms that are prominent following SCI.…”

“…Thus, SICI is likely to influence cortical mechanisms governing force gradation and deficits to this circuit in SCI [7] may contribute to impaired precision performance and submaximal force control. Further, impaired precision control seen in SCI may be due to the influence that reduced SICI has on voluntary muscle relaxation [16,22] . A recent study in aging populations revealed that, in contrast to young healthy individuals, older adults did not show increases in SICI with voluntary grip relaxation and had longer grip relaxation times.…”

“…A recent study in aging populations revealed that, in contrast to young healthy individuals, older adults did not show increases in SICI with voluntary grip relaxation and had longer grip relaxation times. [22] Thus, it may be possible that the reduction in the ability of SCI populations to recruit SICI circuits impairs relaxation time following voluntary contraction. Finally, Beck and colleagues [23] proposed that SICI plays a role in movement selection prior to motor execution.…”

Older Compared With Younger Adults Performed 467 Fewer Sit-to-Stand Trials, Accompanied by Small Changes in Muscle Activation and Voluntary Force

“…Because the phase duration of ascent was similar between groups, this early activation in hip stabilizers is indicative of a trunk strategy to older adults keep up with the metronome. We interpret the age-specific 37% longer activation of the five muscle groups combined (Figure 4) as an inability to relax muscles after force generation (Motawar et al, 2016). We conjecture that the lower MVIF, over-activation in dorsiflexor's RMS-amplitude and longer muscle activation duration in older vs. younger are related to the dramatically fewer number of STS trials (Table 1), and indeed an additional correlation confirmed this (r range = −0.51 to −0.66, all p < 0.01, details at Supplementary Material 2).…”

“…Indeed, in a number of activities of daily living, not only do the prime agonists but the antagonists also become over-activated resulting in the age-typical agonist-antagonist co-activation (Hortobágyi et al, 2003 ; Bautmans et al, 2011 ; Bryanton and Bilodeau, 2017 ; Chandran et al, 2019 ). Such an activation pattern is also associated with impaired excitatory and inhibitory control of muscle contraction and relaxation (Motawar et al, 2016 ) that can reduce power generation in old age (Clark et al, 2010 ). Imaging and magnetic brain stimulation studies suggest that age-specific imbalances in excitatory-inhibitory corticospinal circuits and cortical over-activation can accompany higher muscle activation and delay muscle relaxation (Hortobágyi and Devita, 2006 ; Motawar et al, 2016 ).…”