Effect of Social Stress on Motor Function in Older Adults: an fNIRS Investigation
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Stress has adverse impacts on mental and physical health and quality of life, especially in older adults. Stress can impair cognitive function including short and long-term memory, and this functional declines can further be associated with decreased neuromuscular performance (Mehta & Parasuraman, 2014) and increased fatigability (Keller-Ross et al., 2014). Since older adults are more susceptible to the effect of stress because their limited mobility caused by aging can worsen under stress (Noven at al., 2014), it is important to examine the effect of acute stress on neuromuscular function in older adults. In the present study, we tested the effect of social stress on neuromuscular function of both upper and lower extremity in older adults before and after a short bout of social stress. Thirty participants (15 males, 15 females, mean age: 73.3 (5.6) yrs) performed ten trials of submaximal voluntary contraction at 30% of their maximum voluntary contraction force level before and after the Trier Social Stress Test (TSST) session. Handgrip and knee extension motor performance was measured on separate days. TSST consisted of five minutes speech and five minutes serial arithmetic subtraction tasks. We measured force steadiness and electromyography (EMG) of working muscles to evaluate motor function. Additionally, electrocardiogram (ECG), the Visual Analogue Scale of Stress (VAS), and salivary cortisol were collected to evaluate the effect of the TSST. Neural activation pattern changes of prefrontal and sensorimotor area during exercise and TSST sessions was recorded using functional Near Infrared Spectroscopy (fNIRS). To confirm whether the TSST session increased stress level in our participants, we first analysed the stress metrics. Heart rate increased during the TSST and returned to prestress level instantly after the TSST session. Perceived stress level using the VAS increased after TSST. While not significant, salivary cortisol level increased after the TSST session. Findings indicate that handgrip force steadiness improved after the TSST session, whereas knee extension force steadiness remained unchanged. On the other hand, handgrip EMG root mean square (RMS) did not change after stress while knee extension EMG RMS was found to increase. Neural activation during handgrip exercise increased at the left motor area, and neural activation during knee extension increased at the left sensory area after the TSST session. Change of heart rate and VAS indicates that participants stress level was increased after stress and the improved motor performance during handgrip exercise after the stress is consistent with a previous study that reported increased memory function after stress in older adults (Pulopulos et al., 2015). However, the differential effects of stress based on upper or lower extremity indicates increased sensitivity of certain motor tasks to social stress than other. While social stress is known to affect response time but not memory function (Guez et al., 2016), different spatial activation pattern between handgrip and knee extension exercises observed in the present study suggest that different neural strategies were adapted to compensate for the effects of acute social stress to maintain motor performance.
