Muscle Exertion and Physiological Fatigue Associated with Vigorous Shaking

John  D. Lloyd, Ph.D., M.Erg.S., CPE, CBIS
Board Certified Ergonomist & Certified Brain Injury Specialist

In this study, six adult participants, including 3 adult males (ages 30-77) and 3 adult females (ages 20-50) were asked to shake biofidelic mannequins representing a 12 month old infant (30”, 22lb) and a 3 year old toddler (37”, 35lb) to physical exhaustion.

EMG (muscle activity) electrodes were attached to the left and right biceps, triceps, anterior deltoid (shoulder) and trapezius (neck / upper back) muscles. These were calibrated by asking the participants to perform a series of exercises for a period of 3 seconds, during which maximum voluntary contraction (MVC) of the isolated major muscle groups was recorded. The MVC value was calculated as the mean value across the upper three quartiles of the MVC trial, which was used as a reference value against which muscle activity during the shaking activities is calculated.

Subjects were asked to demonstrate violent shaking of each of the CRABI12 and Hybrid-III 3yr old biofidelic mannequins to the maximum of their ability and for as long as possible until physically exhausted. The order of presentation of the mannequins was randomized to minimize order effect and participants rested for at least 5 minutes between trials to permit physiological recovery.

Each participant was asked to shake both mannequins. During the shaking tasks, muscle activity for the defined eight major muscle groups was recorded at 1000 samples per second across the duration of the task. Linear acceleration and angular velocity of the head of the biofidelic mannequin was simultaneously measured in 6 degrees of freedom (Linear x, y, z & angular yaw, pitch, roll).

EMG results across subjects, presented in the table below, indicate that the biceps and anterior deltoids are the major muscle groups associated with vigorous shaking. The triceps and trapezius muscle groups play a secondary role. Mean muscle activity at the biceps and anterior deltoids, across the duration of the task is approximately 1.34 or 134% of MVC and 0.97 or 97% respectively, with a maximum 576% and 447% MVC bilaterally for the biceps and anterior deltoid, respectively. That is, activity of the biceps and shoulder muscles was quite extreme during vigorous shaking tasks.

Table 1: Muscle activity during shaking CRABI12 and Hybrid III mannequins

The maximum duration of vigorous shaking was also recorded. The average time across subjects was 22 seconds with the CRABI and only 19 seconds with the Hybrid III 3-year old mannequin. As expected, the male participants were able to perform the task slightly longer than their female counterparts. Further, cycle time lengthened in correlation with duration of shaking.