In the present study, we chose the most common functional activity (combining the grip-strength generation and reaching movements) involving the upper extremities in daily living and analyzed the impact of aging and arm movement performance on grip-strength and grip-stability control. For both young and older adults, reaching movement performance resulted in reduced grip-force stability control (measured by CV values), decreased ability to sustain the target force (force-stability index values), and induced lower grip accuracy (absolute error) in both hands. Age-related deterioration in grip-force stability control (grip force and CV values) and the ability to sustain the target force (deviation error, absolute error, and force-stability index values) were found among older adults by comparing their scores during the three manual precision tasks with those of young adults. The results also showed that age and reaching-movement performance had an interaction effect on grip force and CV of muscle-strength performance and on the deviation error, absolute error, and force-stability index of strength-stability control. Based on these findings, this study indicated the older adults have inconsistent grip strength and instability control when performing arm-reaching movements.
Quantitative measurement of grip-strength performance and stability control during manual precision tasks
Strain-gauge or load cell force transducers were used in previous studies to measure the relationship between grip-force generation and the load force of an object when asking healthy young participants to position their arms at specific postures or performing circular or lifting tasks [5,6,7,8, 27]. The findings provided valuable and quantitative information, revealing that the participants generated higher grip strength than the target force or the load force of the object (called the safety margin) to increase friction between the skin and the object and to prevent the target object from slipping . These studies provided quantitative grip-force data in kg by calculating the deviation error values in grip performance during tasks. However, quantitative analysis of grip-strength and stability control regarding grip-strength performance of participants and of the variability of grip-stability control to the relevant target force of the object in both hands simultaneously is lacking in both young and older populations. In the present study, the CV values for grip-strength performance and deviation error and absolute error and force-stability index values for grip-stability control were calculated and applied to represent the participants’ grip-strength performance and grip-stability control in terms of the relevant target force. The CV value in the dominant hand was higher by 19.1% than that in the non-dominant hand during the backward-reach task for the young group (p = 0.017), and the absolute error value was also higher by 27.3% in the dominant hand than the non-dominant hand during the backward-reach condition (p = 0.049). Thus, the dominant hand generates higher grip strength than the non-dominant hand in relevant target force but is accompanied by unstable grip-strength generation for young, healthy adults. Furthermore, the older adult group showed a 25.6% higher force-stability index for the dominant hand than the non-dominant hand in the stationary condition (p = 0.035), representing poor grip force stability control in the dominant hand in the stationary condition. This poorer grip force stability control in the older population (by 15.3–17.7%) has been previously reported .
Additionally, compared with the grip-strength tests applied in previous studies , the grip-stability control tests in the present study were performed at 20% MVC target force; thus, the grip-force generated by both hands was much higher than the loading and lifting force of the dynamometer (dynamometer weight, 360 g). Consequently, young participants generated more grip strength, and the higher safety margins were also induced for both hands among three manual precision tasks by 0.15–0.31 kg (positive deviation error values). By contrast, the negative values for the deviation error to the target force for both hands were − 0.02 to − 0.18 kg during the three manual precision tasks. As such, the older adults instinctively decreased their grip strength to the relevant target force because they did not need to lift the dynamometer and generate higher grip strength to increase friction force to avoid slipping. If the older adults had generated a greater grip strength in terms of the load force of the object (i.e., creating a higher safety margin), it is likely that muscle fatigue would have been induced . However, the age-related changes in grip strength and stability control among manual precision tasks could be induced and are a cause for concern in older adults.
Influence of aging on grip-strength performance and stability control
The CV and force-stability index values indicated the age-induced deterioration in the quality of grip strength and stability control in both the dominant and non-dominant hands of older adults. Compared with the healthy young group, the older group showed higher CV and force-stability index values in both the dominant (5.1–10.7% and 7.2–11.1%, respectively) and non-dominant (2.6–8.3% and 8.5–10.1%, respectively) hands during the three manual precision tasks. These findings are consistent with those in previously reported studies reporting that age was the main factor affecting strength-stability control, and that older adults had higher variability in grip-strength performance and poorer stability control than did young adults during sustained grip effort, as indicated by the CV values from grip force data, representing the magnitude of force variability for each participant [22, 29]. This age-related change in grip-strength performance and strength stability control may have resulted from structural and functional degeneration in the central and peripheral nervous systems, deficits in perception feedback and receptors, and grip pattern dysfunction.
In terms of the physiology of brain function, several motor and sensory areas, including the contralateral primary motor cortex, primary sensory cortex, premotor cortex areas, bilateral prefrontal cortex, supplementary motor area, and cerebellum areas, are involved in the grip-strength generation of the hands . In addition, as grip strength increases, the magnitude of neural excitation and the activation of regions in the ipsilateral supplementary motor area, globus pallidus internus, and the subthalamic nucleus also increase [30, 31]. However, with aging, atrophy of the motor cortical regions and corpus callosum  and reduction of grey matter and dendritic density  occur in the brain, which could result in motor and functional impairments in older adults . Functional degeneration in the central nervous system may also occur with aging. For example, a previous study reported that neuron activation of the contralateral primary motor cortex, primary sensory cortex, posterior cingulate motor areas, and premotor cortex areas reduces with age, and that increasing neuron excitation in these areas improves grip-force levels in older adults . In the present study, all participants were asked to execute grip-force stability control at 20% MVC during the three manual precision tasks; thus, when resistance was applied, the older adults could induce higher neuron activation and recruit more neural networks (ipsilateral primary motor cortex, putamen, subthalamic nuclei, substantia nigra, lateral globus pallidus, and contralateral cerebellum) than young adults . Although degeneration of the central nervous system can cause poor strength performance and strength-stability control, other age-related changes of the peripheral nervous system may also affect grip strength and induce unstable grip-stability, reduce hand dexterity, and result in abnormal compensatory strategies and discoordination of the hands [12, 36]. Examples include the decline in nerve conduction and functions of the sensory system and the reduction in the number and sensitivity of somatosensory receptors in the skin, muscles, and joints . In a previous study, a group of young adults was asked to hold an object and generate grip force at a target force level; these participants distributed their grip forces across all fingers to maintain stable grip-force strength and stability control . However, with aging, the frequency, hand strength, and movement time of grip patterns significantly change , and antagonist muscle activation is also induced ; this phenomenon may result in discoordination of the grip-force generated by all fingers and reduce grip-stability control in older adults.
Grip-force generation is frequently associated with coordination movements involving both the upper extremities and simultaneous cognitive tasks . This concurrent activity combining cognition and movement control was reported as postural (arm posture or movements) and suprapostural (stable grip strength generation and hold) tasks, which is one of the dual tasks described in previous studies [1,2,3]. Early studies have also revealed that the grip strength and moments are affected, and grip-force control in young people varies when assuming different postures or performing arm movements [2,3,4,5].
Arm-reaching performance impacts grip-strength performance and stability control in young and older adults
The concurrent activity combining the cognitive attention involved in grip strength generation and arm reaching performance was reported as part of dual tasks in previous studies [1,2,3]. Many activities of daily living, such as walking and maintaining balance, are dual tasks  that involve the concurrent use of cognition, posture, and motor processes  and require the simultaneous application of several physiologic systems and cognition. In the present study, compared with that in the stationary condition, arm reaching performance resulted in inconsistent grip strength (higher CV values) in both the dominant (0.8–5.6%) and non-dominant hands (1.3–7.1%) of participants under forward- and backward-reach conditions. Poor grip-force stability control relative to the target force (higher force stability index values) was observed in both the dominant (5.9–10.8%) and non-dominant hands (9.2–12.5%) of both the young and older adult groups under forward- and backward-reach conditions, compared with that under the stationary condition. Several studies have also reported similar findings and revealed that grip force generation and control are unstable when positioning at specific postures or performing arm movement [5,6,7,8]. This phenomenon may be induced by shifting the attention resource from grip strength control to arm movements when performing grip-force stability control tests for the three manual precision tasks, and each manual precision task condition required different levels of effort. Additionally, attentional shifting can cause a delay in the onset time of movement, which increases movement time, induces compensative movements, and exacerbates the risk of accidents [41,42,43,44].
Previous studies have also indicated the impact of different tasks on grip-strength performance and reported varying responses when participants performed different dual tasks [43,44,45,46,47]. For example, previous studies have investigated motor or grip-force control among young and older people under the following conditions: performing a force-tracking task combined with an n-back test , executing a forward-reach task combined with posture change , reaching and grasping an object while performing a counting task , recovering balance to determine the reach-to-grip response , and gripping and lifting ability during a single-leg stance with the eyes closed . The findings of these studies indicated that such different tasks resulted in poor grip strength, inaccurate performance, and high variability in repeated force-tracking [11, 21, 44,45,46,47]. Additionally, the compensatory mechanism for neuron network reorganization in older adults may be induced. For example, several additional areas of the bilateral hemispheres are recruited, and higher levels of neuron activation are generated [35, 48,49,50]. Previous studies also reported that the ability for neuron modulation in appropriate motor networks in the brain is reduced [34, 51] when older adults perform motor and cognition tasks. However, the potential impact of compensatory mechanisms on grip strength and stability control remains unknown and has rarely been discussed with regard to young and older populations. In the present study, results indicated changes in stability (CV value) and accuracy (force stability index value) in grip strength performance and stability control in both hands during forward- and backward-reaching movements among young adults and revealed insufficient grip strength and poor stability control relative to the target force in both hands among older adults. These findings may explain the real impact of compensatory mechanisms of both central neuron network and arm movements on grip strength and stability in young and older populations. Finally, age and arm-reaching conditions have an interaction effect in terms of CV in grip strength performance and force-stability index in stability control, which means that the older adults exhibited unstable grip strength during arm-reaching movements and this finding was statistically significant. Factors such as age, cognitive impairment, and frailty can also affect grip strength performance . Therefore, we advise clinical therapists to develop appropriate health promotion exercises or rehabilitation strategies to achieve stable grip strength control during arm movements and prevent loss of grip on objects, for the avoidance of future accidents in this older age group.
Study limitations and suggestions for future research
The findings of this study may help occupational and physical therapists develop appropriate rehabilitation programs or health-promotion exercises for improving grip strength performance and stability control in older people. Previous studies have indicated the correlation between age-related deterioration in grip strength and frailty and illness [14,15,16,17]. However, the present study did not recruit older people with disabilities or frailty and did not analyze the relationship between grip strength and stability control deficits and functional impairments of the hands and upper limbs. Future studies should recruit older people with disabilities or frailty, collect grip strength and stability control parameters, and conduct an upper extremity evaluation using clinical motor and functional assessment scales. It is important to analyze the relationship between these parameters and the clinical scale scores. Such investigations would generate further evidence-based data regarding the quality of grip strength among older adults with functional limitations and associated disorders of the upper extremities. Additionally, this study demonstrated unstable grip strength and stability control among arm movements in older adults compared with those in young adults by analyzing several grip-stability control indexes (CV, force stability index). Future studies should conduct direct measurements, collect continual grip-force data, and calculate the grip-stability control index to determine the quality of grip-strength generation during physical performance in frail and pre-frail elderly people, rather than only evaluating maximal grip force using traditional tests. These data should also be incorporated into comprehensive geriatric assessments during frailty screening processes. Furthermore, the purpose of this study was to evaluate the impact of aging and reaching movements on grip strength and stability control in both hands, and, to our knowledge, no study has reported that hand dominance is one of the factors that affect the grip stability control of young and older adults. Therefore, we did not perform full factorial analysis of Condition x Age x Hand in this study, and only separately analyzed the two-way Condition x Age interaction for both the dominant and non-dominant hand. However, aging, manual precision tasks, and hand dominance could have interaction effects on grip stability control, although future studies will be needed to assess these aspects. This may also help clinical therapists in developing appropriate rehabilitation programs or health-promotion exercises to improve grip-stability control during daily living activities in frail and pre-frail older people.