# Aging Impairs Unimanual and Bimanual Hand-Grip Force Control Capabilities
## 4 Discussion
Perhaps, aging-induced loss of motor units alternatively increases the number of muscle fibers innervated by the remaining motor units, presumably resulting in increased motor unit discharge variability and higher force fluctuations. In contrast, a force control training program for older adults advanced force accuracy and increased the motor unit discharge rate [49]. These findings suggest that force control capabilities as indicated by relative force control variables are effective for representing altered neuromuscular systems in the aging population. Increased BMI in older adults was related to lower relative force accuracy and greater relative force variability at 10% MVC in the unimanual non-dominant hand. The obesity paradox indicated that being overweight in the older adult population could decreasemortality by increasing strength and bone mineral density, protecting against falls, and acting as a metabolic reserve during disease [50; 51; 52]. For older adults, a decrease in BMI was associated with low hand-grip strength [53; 54], and some studies suggested that older adults with either lower (<25 kg/m\({}^{2}\)) or higher BMI (>35 kg/m\({}^{2}\)) values experience motor deficits [55; 56]. Considering the range of BMI (i.e., 18.3-29.8 kg/m\({}^{2}\)) for older adults in our study, the current findings suggest that decreased BMI is related to fine motor impairments in the aging population. Potential mechanisms underlying age-induced hand-grip force control deficits for unimanual and bimanual conditions may include structural and functional changes in key brain regions. Previous studies reported decreased gray and white matter volumes across the prefrontal cortex, primary motor cortex, and cerebellum in older adults, related to hand motor impairments [57; 58]. Despite the greater importance of interhemispheric neural communications for successful bimanual force control in older adults [59], age-induced atrophy of the corpus callosum additionally interfered with their bilateral motor control capabilities [60; 61]. Moreover, older adults may require more neural resources to successfully perform unimanual and bimanual force control by compensating for degenerative changes in neuromuscular systems (e.g., reduced brain volume, decreased neurotransmitter interactions, and impaired sensorimotor functions) [62; 63; 64; 65; 66]. For example, while showing lower accuracy during unimanual hand-grip force control tasks, older adults revealed hyperactivation patterns across bilateral sides of premotor and sensorimotor areas compared with younger adults [67]. However, older adults who revealed fewer neural activations related to sensorimotor processing showed more impairments in unimanual and bimanual motor functions [63; 68]. Thus, future studies may use exercise protocols combined with non-invasive brain stimulation techniques (e.g., transcranial direct current stimulation) that may modulate cortical activation patterns for improving force control capabilities in older adults. Despite the finding that relative outcome variables effectively identified age-related force control deficits, several study limitations remain. First, the proportion of female participants in this study was relatively high (i.e., female ratio = 81.3%) so that the current findings could be affected by sexual bias. Therefore, future studies should determine whether gender differences influence age-related force control capabilities identified by relative outcome measures. Second, given that we focused on isometric hand-grip force control tasks, whether these findings are observed in other force control tasks (e.g., lower limb force control) should be investigated. Moreover, future studies should examine how relative force control variables are associated with structural and functional changes in key brain regions and motor neuron pools.