The Effect of Force, Frequency and Gripping on Muscle Activity during a cyclic Push Task

Abstract

<p>Risk factors for the development of upper extremity work-related musculoskeletal disorders (UE WMSD) include high repetition, high force, and the combination of high force and high repetition (Moore et al. 1991 ; Silverstein et al., 1986; Latko et al. , 1999). This study examined the muscle activity in the upper extremity during a repetitive pushing and gripping task with differing force levels and differing frequencies. Ten males and 9 females performed a cyclic bimanual push and grip task for 120 s while 8 muscles of the upper extremity were monitored by EMG on the right side of the body. All combinations of 3 push load levels (1 kg, 2 kg, 4 kg), 3 frequencies (4/min, 8/min, 16/min) and 2 grip conditions (no grip and 30% relative to the individual's maximum) were randomized.</p> <p>The paradigm of doubling frequency and load did not lead to a doubling of muscle activity. The increase in muscle activity was somewhat linear in many muscles with several exceptions but was much less than 1: 1. The AEMG appears to reflect workload rather then the force and frequency parameters that were chosen. In the forearm, the load and frequency parameters and concurrent grip caused increased activity in all muscles. The addition of a grip superseded the effects that were expected due to load but not frequency. As well, as the frequency increased, muscle activity also increased while the amount of muscular rest (muscle activity < 1 % maximum in seconds per minute) decreased. The muscular rest of the extensor digitorum decreased during the grip the high frequency (16/min) condition regardless of load, all the trials were under 10% of rest.</p> <p>For the shoulder, anterior deltoid activity increased when gripping for all participants but significant differences were noted between genders. The anterior deltoid muscle activity was also affected by the weight of the arm, thus was not directly proportional to the 1-2-4kg load applied. Although maximum strengths for the shoulder were not recorded in this study, the finding that female subjects had higher anterior deltoid activity than male subjects was expected due to the absolute push loads likely being a greater relative effort for women. Overall, male subjects required about 70-75% of the anterior deltoid activity of females but had the same amount of muscular rest based on muscular rest (muscle activity < 1 % maximum in seconds per minute). This finding is quite interesting as often a single metric is used to analyze tasks leading to potentially misleading interpretations.</p> <p>Through examining the EMG of the upper extremity muscles, the current data suggests the need to raise the importance on the frequency of work and to also increase the importance of gripping in ergonomic assessment tools. This thesis helps further the understanding of the physiological relationship between force, frequency and concurrent gripping, with the ultimate goal of establishing acceptable values.</p>