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Overhead Working Height Level Effects Performance and Onset of Shoulder Discomfort

Volunteer subjective rating of perceived discomfort (RPG) yielded excellent reliability as a predictor of shoulder fatigue while surface electromyography (EMG) provided low reliability in a laboratory simulation of an industrial overhead task according to a study by Sood et al.

In a second related experiment involving three different heights of overhead work, perceived discomfort increased at a faster rate with the highest working height.  Also, there was a significant increase in task error with increasing work height.  However, the relationship between work height and both RPG and error rates was non-linear.  Surface EMG fatigue measures did not reveal a preferred working height or trend. 

The authors felt study findings were influenced by the design of the experiment task that required low levels of muscle exertion combined with precision motor control.

Experiment 1 Study Design: Reliability of subjective and objective fatigue measurements

Ten subjects (5 males, 5 females) were recruited for the study.  They had a mean age of 24.2, manual work experience or regular performers of upper extremity exercise, and void of upper extremity injury/symptoms.

Laboratory simulation of an industrial overhead task was created (considerations included movement to precise targets and obstacle avoidance as observed in tasks performed at an auto assembly plant).  A keyboard was attached to the bottom of a height adjustable overhead platform with the keys pointing downward.  Two thin, rigid wires were strung over the keyboard to simulate obstacles.  A non-functioning electric drill was equipped with a wooden dowel as a bit and rubber hose to simulate a power supply line. 

The platform was adjusted in vertical height such that the subject assumed an upper extremity postural position of approximately 110 degrees of shoulder flexion and 70 degrees of elbow flexion when holding the drill to reach the keyboard keys.

The subject tapped four keys in sequence (S, 5, L, U) nine times (total of 36 key contacts) with the drill bit over a 27 second period.  This was followed by 27 seconds of light manual work with the hands at waist height without use of the drill.

Surface EMG was applied to the anterior deltoid, medial deltoid and upper trapezius.  EMG’s were recorded during the last 6 seconds of a tapping period.  Subjects also provided a RPD for the shoulder on a 10 point scale.  RPDs were given every fifth cycle (4.5 minutes).  The task was continued until the subject reported a RPD of 9 on two consecutive evaluations, or the subject reported a RPD of 10, or 1 hour had elapsed.

Experiment 2 Study Design: Effect of working height on shoulder fatigue

Twelve subjects (6 males, 6 females) were recruited for the study.  They had a mean age of 27.3, manual work experience or regular performers of upper extremity exercise, and void of upper extremity injury/symptoms.

The same apparatus described in experiment 1 was used.  The vertical platform height was adjusted to three different working heights to allow the drill bit to reach the keyboard keys when the subject was in an approximate postural position of:

90 degrees of shoulder flexion/90 degrees of elbow flexion

110 degrees of shoulder flexion/70 degrees of elbow flexion

130 degrees of shoulder flexion/45 degrees of elbow flexion

The same tapping tasks and stop criteria described in experiment 1 was followed.  Each subject participated in four sessions separated by 2 to 7 days.  EMG and RPD levels were recorded.  Key tap errors were recorded.

Other Findings

In the first experiment, all participants completed the one-hour sessions.  The mean final RPD rating was 3.9.

In the second experiment, 10 of the 12 subjects completed the one-hour sessions at all three heights.  For 9 of the 12 participants, the highest RFP was less than 5 for all three heights.

Study Limitations

Reservations effecting of this research include:

the supraspinatus was not evaluated by EMG

only one type of overhead task was performed at the three heights

Article Title: Fatigue during prolonged intermittent overhead work: reliability of measures and effects of working height

Publication: Ergonomics 50(4): 497-513, 2007

Authors: D Sood, M A Nussbaum, and K Hager

This article originally appeared in The Ergonomics Report™ on 2007-05-11.