Effects of Mental and Physical Fatigue Inducing Tasks on Balance and Gait Characteristics
Authors: Lee Youngjae , Ulman Sophia , Kim Sunwook , Srinivasan Divya 
Published: November 2019

150-Word Abstract Fatigue may reduce human performance and increase injury risk. This study investigated the effects of mental and physical fatigue inducing tasks (MF and PF) on balance and gait characteristics of healthy young adults (n = 10; gender balanced). During quiet stance, PF led to increases in center-of-pressure mean velocity and sway area. Although MF was associated with the same trend, the differences were not statistically significant. Among gait measures, PF was associated with expected changes in mean spatiotemporal measures (shorter and faster steps). However, MF was associated with increased inter-trial variability (standard deviations) in stride and stance times, and decreased variability in step width. Furthermore, detrended fluctuation analysis (DFA) exponent of step width, indicating the presence of long-range persistent correlations across time, decreased significantly after both MF and PF. Our results suggest that balance and gait characteristics of healthy young adults are affected differentially by PF and MF. Extended Abstract Fatigue is considered a multidimensional construct including physical and mental components (Smets, Garssen, Bonke, & De Haes, 1995), which may reduce human performance and potentially lead to increased risks of falls and fall-related injuries (Helbostad, Leirfall, Moe-Nilssen, & Sletvold, 2007; Miles, Ives, & Vincent, 1997). While the effects of both physical and mental fatigue have been separately investigated in diverse populations under various experimental conditions, a few studies have compared the effects of physical vs. mental fatigue on postural control and gait in the same cohort. Thus, as a first step, we aimed to investigate the comparative effects of mental and physical fatigue on balance and gait characteristics among healthy young adults. We hypothesized that both mental and physical fatigue will have adverse effects on balance and gait characteristics of the healthy young adults. Ten healthy individuals (21-28 years old, 5 males and 5 females) from the university and local community completed the two experimental sessions. In the balance session, participants performed three pre-fatigue, quiet standing trials. Each trial lasted 45 seconds, during which participants were asked to stand as still as possible. Then, participants were instructed to complete a 30-minute mental fatigue inducing task (MF). Following the completion of MF, participants were instructed to complete five post-fatigue standing trials at 0 (immediately after the task), 1.5, 4, 7, and 10 minutes. Next, participants completed a physical fatigue inducing task (PF). They performed calf raises repeatedly at 40 bpm until voluntary exhaustion. Similar to MF, participants were instructed to complete five post-fatigue standing trials following the completion of PF. A 10-minute rest period was provided between MF and PF. In the gait session, participants performed a 6-minute baseline walking trial on the treadmill at their preferred walking speed (PWS), determined following the procedures developed by Jordan, Challis, and Newell (2007). Then, similar to the balance session, 6-minute walking trials were collected immediately following the completion of MF and PF respectively, with a 10-minute rest period provided between the tasks. Mean center-of-pressure (COP) velocity, in both anteroposterior (AP) and mediolateral (ML) directions, and 95% confidence ellipse area (95EA) of sway were computed for the balance measures. Means and variability (standard deviations) of spatiotemporal gait parameters, including spatial parameters (i.e., step length, step width, and stride length) and temporal parameters (i.e., step, stride, swing, and stance times), were computed for the gait measures. In addition, detrended fluctuation analysis (DFA) was performed on COP time series in the AP and ML directions separately, as well as on step time, length, and width from the gait data, in order to quantify long-range correlations over time. Separate repeated measures analyses of variance (ANOVA) were performed to examine the effect of fatigue (pre vs. post mental fatigue, or pre vs. post physical fatigue) on the dependent variables, with gender included as a blocking variable. Significance level, p ≤ 0.05, was used to conclude significant influences. The results showed that PF significantly increased COP mean velocity in the AP (26.4%) and ML (19.1%) directions as well as the COP 95EA (31.7%). However, no significant changes were observed for the COP-based measures after MF. The DFA exponent of the COP trajectory in both AP and ML directions showed no significant differences after MF or PF, although the decrease in DFA exponent in COPAP approached statistical significance in both cases ( p = 0.056 and 0.054, respectively after MF and PF). In addition, there was no significant gender effect in any COP-based measures. Regarding the gait measures, with the exception of stance time, all spatial and temporal parameter means changed significantly following PF. As such, the mean value of step length, stride length, step time, stride time, and swing time decreased by 1.0%, 1.0%, 1.1%, 0.8%, and 1.7%, respectively, while the mean value of step width increased by 7.4%. After MF, only the mean value of step width significantly increased by 8.6%, compared to the baseline (pre-fatigue) condition. However, the inter-trial variability in step width, stride time, and stance time changed significantly following MF: step width variability decreased by 12.5%, while stride time and stance time variability increased by 15.4% and 18.2%, respectively. Stance time variability was the only variability measure that showed a significant change (18.2% increase) after PF. The DFA exponent of step width decreased significantly following both MF (16.9%) and PF (16.6%). Lastly, no significant gender differences were found for gait measures. The findings from the study can help better understand the effects of MF and PF on balance and gait characteristics of healthy young adults. Our hypothesis that both MF and PF would affect balance and gait characteristics was partly supported. MF and PF showed significant influences on gait characteristics, but only PF showed significant influence on balance characteristics. From this exploratory study, it seems that, while PF induced by local muscle fatigue in the ankle changed average gait measures, MF may have a greater influence on control of the gait patterns (as indicated by the changed variability and DFA measures). In conclusion, these results suggest that both PF and MF may cause different effects on balance and gait characteristics of healthy young adults.