Analysis of shared synaptic inputs to different motor unit subgroups in triceps surae during a postural standing task

Previously, we have observed differential firing behaviors and recruitment locations of distinct motor unit (MU) subgroups within the same muscles. This study examined the amount of shared synaptic inputs to these MU subgroups. Twenty-four participants (10 healthy young adults: 5 females, 5 males; means ± SD: age 27 ± 2.5 yr and 14 healthy older adults: 6 males and 8 females; means ± SD: age 74.8 ± 5.3 yr) stood on a force platform and leaned in five directions maintaining their center of pressure for 35 s per direction. High-density surface electromyography recordings from the medial gastrocnemius, lateral gastrocnemius, and soleus were decomposed into single MU action potentials. MU tracking classified MUs as "common" or "unique" across leaning directions. Synaptic input was estimated using a coherence analysis (proportion of common input; PCI). Three PCI analyses (common, unique, and between the MU subgroups) quantified neural connectivity. MU subgroup significantly affected PCI (F = 25.92, P < 0.0001), with common MUs exhibiting higher PCI than unique (MD = 0.08, CI [0.07, 0.09], P < 0.0001) and between-subgroup MUs (MD = 0.04, CI [0.03, 0.05], P = 0.0058). Unique MUs had significantly lower PCI than between-subgroup MUs (MD = 0.04, CI [0.03, 0.06], P = 0.048). Taken together, both subgroups receive shared neural inputs for task-specific force production, more so in the common motor units. Importantly, a main effect of age (F = 4.56, P = 0.04) was observed, with older adults exhibiting higher PCI, though post hoc analyses did not reveal significant differences (P = 0.45-0.67).NEW & NOTEWORTHY Our study reveals differing amounts of shared neural inputs to MU subgroups within the same muscle during standing balance. The proportion of common input (PCI) varies with leaning direction, indicating task dependency and different descending and proprioceptive inputs. Surprisingly, age did not significantly affect PCI values in any MU subgroup or the entire muscle, suggesting that age-related differences in recruitment and firing patterns are more related to factors such as proprioceptive feedback than a common input.