Interfering effects of the task demands of grip force and mental processing on isometric shoulder strength and muscle activity

Abstract

The purpose of this study was to examine the interfering effects of physical and mental tasks on shoulder isometric strength in different postures. Fifteen volunteers (seven women, eight men) performed a series of isometric shoulder exertions at 30 degrees , 60 degrees and 90 degrees of both shoulder flexion and abduction alone and with the addition of a 30% grip force, a mental task (Stroop test) and both additional tasks simultaneously. The shoulder tasks were completed either at maximal intensity, or while maintaining a shoulder posture without any additional effort. Surface electromyography (EMG) from seven muscles of the shoulder girdle and shoulder moment were collected for each 6 s shoulder exertion. When normalized to maximum exertion, no differences were found between genders and no differences existed between conditions when subjects maintained each posture without exerted force. In the maximal shoulder exertion trials, an increase in shoulder angle (in either plane) resulted in an increase in EMG in most muscles, while shoulder moment decreased in flexion and remained constant in abduction. Shoulder moments and muscle activation were greatest in the shoulder exertion alone condition followed by adding a 30% grip and the Stroop test, with the addition of both tasks further reducing the exerted shoulder moment and EMG. However, muscle activity did not always decrease with shoulder strength and remained elevated, indicating a complex coactivation pattern produced by an interfering role of the tasks. Overall, it was found that a mental task can have the same or greater effect as a concurrent grip and should be considered when assessing muscular loading in the workplace, as typical biomechanical modelling may underestimate internal loads. The results not only provide valuable shoulder strength data but also practical strength values, depending on additional tasks.

Targeted gripping reduces shoulder muscle activity and variability

“…The addition of gripping decreased deltoid activity by 4-14% of maximum voluntary excitation (MVE) as arm elevation angle increased. Although others have found similar decreases in deltoid activity with gripping, the reaction of the rotator cuff to gripping is not as clear (Antony and Keir, 2009;Smets et al, 2009;Au and Keir, 2007;MacDonell and Keir, 2005). Sporrong et al (1995Sporrong et al ( , 1996 found supraspinatus activity increased by 4-20% MVE while changes to infraspinatus activity were less consistent, in some cases decreasing by as much at 8% MVE while in other cases increasing as much as 21%.…”

“…Sporrong et al (1995Sporrong et al ( , 1996 found supraspinatus activity increased by 4-20% MVE while changes to infraspinatus activity were less consistent, in some cases decreasing by as much at 8% MVE while in other cases increasing as much as 21%. MacDonell and Keir (2005) also found infraspinatus activity to decrease by as much as 6.7% MVE with gripping during maximal shoulder exertions. In submaximal dynamic shoulder raises, Antony and Keir (2009) found infraspinatus activity to increase by 2% MVE.…”

“…During many activities of daily living, the relatively small rotator cuff muscles are exposed to large forces making them susceptible to injury (Harrast et al, 2004;Lee et al, 2000). In addition to the obvious biomechanical factors of arm posture and hand load, gripping and cognitive effort have also been shown to increase shoulder load and the risk of injury (Smets et al, 2009;Au and Keir, 2007;MacDonell and Keir, 2005;Meskers et al, 2004;Sporrong et al, 1996Sporrong et al, , 1995Kronberg et al, 1990).…”

Influence of mental workload on muscle endurance, fatigue, and recovery during intermittent static work

“…3a). While much of the research on concurrent demands have focused on low-level static exertions, a few studies have highlighted stronger interference by cognitive demands during high force production (Macdonell and Keir 2005;Zijdewind et al 2006). Although shorter endurance times were observed at 55% MVC during concurrent demand conditions compared to the control [3.09 (1.04) vs. 3.15 (.86)], these differences were not significant.…”

“…It could be argued that at low-level exertions, since force output was controlled, participants were able to maintain their exertion (and not overshoot their target) while processing the mental task. However, a few studies have demonstrated that muscular effort decreases during concurrent work at high force levels (Macdonell and Keir 2005;Mehta and Agnew 2011a, b). This interference may be attributed to the simultaneous activation of dorsolateral prefrontal cortex, which has shown increased activity during motor contractions (Dettmers et al 1996) and during cognitive processing (Rowe et al 2000).…”