Biology subjects

Enoka, R.

Publications and source records attributed to Enoka, R..

3 recordsLinked to original sources

bioRxiv2025

Voluntary Dissociation of Motor Unit Activity in the Vastii Muscles

The CNS coordinates movement through consistent activation patterns across muscles and motor units, suggesting the presence of a relatively fixed and high-dimensional number of neural constraints on voluntary actions. In the human quadriceps, the vastus medialis (VM) and vastus lateralis (VL) control the knee extensor torque and are considered a synergistic pair largely activated by shared neural inputs. However, some evidence suggests that these muscles, or even subregions within them, can be controlled independently. In this study, we investigated whether humans can dissociate neural input to VM and VL during isometric contractions. Ten participants received real-time feedback from multiple intramuscular EMG electrodes that targeted different regions of the VM and VL while attempting to activate each muscle or sub-regions selectively. We found that nine out of ten subjects were able to clearly separate VM and VL activity based on the intramuscular EMG feedback. However, motor unit decomposition from the intramuscular EMGs revealed that selective recruitment of a unique set of motor units was possible only within the proximal region of VM. In contrast, VL and distal VM showed highly correlated activation, indicating tight functional coupling. Correlation analyses confirmed that the proximal VM exhibited distinct activation profiles compared with both distal VM and VL, supporting the existence of compartmentalized control within VM. These findings demonstrate that it is possible to dissociate the activation of motor units within this synergistic muscle group during low-force isometric contractions.

neuroscience↗

bioRxiv2023

The decoding of extensive samples of motor units in human muscles reveals the rate coding of entire motoneuron pools

Movements are performed by motoneurons transforming synaptic inputs into an activation signal that controls muscle force. The control signal emerges from interactions between ionotropic and neuromodulatory inputs to motoneurons. Critically, these interactions vary across motoneuron pools and differ between muscles. To provide the most comprehensive framework to date of motor unit activity during isometric contractions, we identified the firing activity of extensive samples of motor units in the Tibialis Anterior (129{+/-}44 per participant; n=8) and the Vastus Lateralis (130{+/-}63 per participant; n=8) muscles during isometric contractions of up to 80% of maximal force. From this unique dataset, the rate coding of each motor unit was characterised as the relation between its instantaneous firing rate and the applied force, with the assumption that the linear increase in isometric force reflects a proportional increase in the net synaptic excitatory inputs received by the motoneuron. This relation was characterised with a natural logarithm function that comprised two stages. The initial stage was marked by a steep acceleration of firing rate, which was greater for low- than medium- and high-threshold motor units. The second stage comprised a linear increase in firing rate, which was greater for high- than medium- and low-threshold motor units. Changes in firing rate were largely non-linear during the ramp-up and ramp-down phases of the task, but with significant prolonged firing activity only evident for medium-threshold motor units. Contrary to what is usually assumed, our results demonstrate that the firing rate of each motor unit can follow a large variety of trends with force across the pool. From a neural control perspective, these findings indicate how motor unit pools use gain control to transform inputs with limited bandwidths into an intended muscle force.

neuroscience↗

bioRxiv2022

Metabolic costs of walking and arm reaching in persons with mild multiple sclerosis

Movement slowness is a common and disruptive symptom of multiple sclerosis (MS). A potential cause is that individuals with MS slow down to conserve energy as a behavioral adjustment to heightened metabolic costs of movement. To investigate this prospect, we measured the metabolic costs of both walking and seated arm reaching at five speeds in persons with mild MS (pwMS; n = 13; 46.0 {+/-} 7.7yrs) and sex- and age-matched controls (HCs; n = 13; 45.8 {+/-} 7.8yrs). Notably, the cohort of pwMS was highly mobile and no individuals required a cane or aid when walking. We found that the net metabolic power of walking was approximately 20% higher for pwMS across all speeds (p = 0.0185). In contrast, we found no differences in the gross costs of reaching between pwMS and HCs (p = 0.492). Collectively, our results suggest that abnormal slowness of movement in MS - particularly reaching - is not the consequence of heightened effort costs alone. Our findings are consistent with the possibility that demyelination of reward regions of the central nervous system in MS disrupt the dopamine-mediated impetus to move more quickly and thereby prompt slower movements. NEW & NOTEWORTHYIndividuals with multiple sclerosis (MS) often move more slowly than those without the disease. A possible cause is that movements in MS are more energetically expensive and slowing is an adaptation to conserve metabolic resources. Here, we find that while walking is more costly for persons with MS, arm reaching movements are not. These results bring into question the driving force of movement slowness in MS and implicate other motor-related networks contributing to slowing.

physiology↗