Can cognitive activities during breaks in repetitive manual work accelerate recovery from fatigue? A controlled experiment

Abstract

Neurophysiologic theory and some empirical evidence suggest that fatigue caused by physical work may be more effectively recovered during "diverting" periods of cognitive activity than during passive rest; a phenomenon of great interest in working life. We investigated the extent to which development and recovery of fatigue during repeated bouts of an occupationally relevant reaching task was influenced by the difficulty of a cognitive activity between these bouts. Eighteen male volunteers performed three experimental sessions, consisting of six 7-min bouts of reaching alternating with 3 minutes of a memory test differing in difficulty between sessions. Throughout each session, recordings were made of upper trapezius muscle activity using electromyography (EMG), heart rate and heart rate variability (HRV) using electrocardiography, arterial blood pressure, and perceived fatigue (Borg CR10 scale and SOFI). A test battery before, immediately after and 1 hour after the work period included measurements of maximal shoulder elevation strength (MVC), pressure pain threshold (PPT) over the trapezius muscles, and a submaximal isometric contraction. As expected, perceived fatigue and EMG amplitude increased during the physical work bouts. Recovery did occur between the bouts, but fatigue accumulated throughout the work period. Neither EMG changes nor recovery of perceived fatigue during breaks were influenced by cognitive task difficulty, while heart rate and HRV recovered the most during breaks with the most difficult task. Recovery of perceived fatigue after the 1 hour work period was also most pronounced for the most difficult cognitive condition, while MVC and PPT showed ambiguous patterns, and EMG recovered similarly after all three cognitive protocols. Thus, we could confirm that cognitive tasks between bouts of fatiguing physical work can, indeed, accelerate recovery of some factors associated with fatigue, even if benefits may be moderate and some responses may be equivocal. Our results encourage further research into combinations of physical and mental tasks in an occupational context.

Figure 1. Experimental protocol and test battery.

The protocol (Figure 1a) included six 7-minute bouts of a repetitive reaching task, interspersed by 5-minute breaks comprising two ratings of perceived fatigue and a 3-minute mental task. A test battery (TB, Figure 1b) was performed before, immediately after, and one hour after the work period. Both x-axes show time in minutes. EMG, electromyography; ECG, electrocardiography; BP, blood pressure; MVC, maximal voluntary contraction; PPT, pressure pain threshold; TC, test contraction.

Figure 2. Trapezius EMG amplitude during the work period.

EMG amplitude during the first and last minute of each of the six work bouts; mean across subjects and mental task conditions. X-axis shows time after commencing work; black zones mark physical work bouts. Filled and empty symbols with full-drawn and dashed line: ipsi- and contralateral trapezius EMG, respectively.

Figure 3. Heart rate during the work period.

Heart rate (measured by inter-beat ECG intervals) during work and breaks in the easy (MT1, green squares), medium (MT2, blue circles), and difficult (MT3, red triangles) mental task protocols. X-axis shows time after commencing work; black zones mark physical work bouts.

Figure 4. Ratings of perceived fatigue before, immediately after, and one hour after the work period.

Perceived fatigue rated on, (a) the Borg CR10 scale, (b) aching in SOFI, and (c) spent in SOFI; before and after work in the easy (MT1, green), medium (MT2, blue) and difficult (MT3, red) mental task protocols. Mean values; bars illustrating SD between subjects. Text and numbers above columns mark differences in recovery between mental task conditions reaching a p-value less than 0.10. As an example, for spent the change from post-work0h to post-work1h differed between M1 and M3 at a p = 0.05 level of significance.