PHYSICAL TASKS:STATIC EFFORTS AND FATIGUE

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STATIC EFFORTS AND FATIGUE

Design Limits for Static Work

Static efforts at work are often fatiguing and cannot be sustained over a long period of time (Rohmert 1960; Monod and Scherrer 1965; Pottier et al. 1969 and Monod 1972). Figure 7 illustrates the relationship between a percentage of the maximum voluntary contraction used and the time duration. This relationship has been determined for arm, leg, and trunk muscles by Rohmert (1960), for an upperlimb pulling action by Caldwell and Smith (1963), and for biceps brachii, triceps brachii, the middle finger flexor, and quadriceps femoris by Monod and Scherrer (1965). The results of these

Physical Tasks Analysis, Design, and-0016

three studies indicate that the limit time approaches infinity at a force of 8–10% maximum voluntary contraction and converges to zero at 100% of the maximum strength.

As discussed by Kahn and Monod (1989), the maximum duration of static effort or the maximum maintenance time (limit time) varies inversely with the applied force and may be sustained for a long time if the force does not exceed 15–20% of the maximum voluntary contraction (MVC) of the muscle considered. The relation between force and limit time has been defined by Monod (1965) as:

Physical Tasks Analysis, Design, and-0017

where t is the limit time (min), F the relative force used (%), ƒ the force (%) for which t tends to infinity (called the critical force), and k and n are constants. Rohmert (1960) subsequently proposed a more elaborate equation:

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In both cases the maximum maintenance time is linked to the force developed by a hyperbolic relation, which applies to all muscles.

Intermittent Static Work

In the case of intermittent static efforts during which the contraction phases are separated by rest periods of variable absolute and relative duration, the maximum work time, given the relative force used and the relative duration of contraction, can be predicted as follows (Rohmert 1973):

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where p is the static contraction time as a percentage of the total time. Rohmert (1962) had devised another method for estimating the minimum duration of rest periods required to avoid fatigue during intermittent static work:

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where tr is the rest time as a percentage of t, which is the duration of contraction (min). Kahn and Monod (1989) concluded that the main causal factor in the onset of fatigue due to static efforts (isometrically contracting muscles) is local muscle ischemia. Furthermore, the onset of local muscle fatigue can be delayed if changes in recovery time are sufficient to allow restoration of normal blood flow through the muscle.

Static Efforts of the Arm

As of this writing, only limited guidelines regarding the placement of objects that must be manipu- lated by the arm have been proposed (Chaffin et al. 1999). Figure 8 depicts the effect of horizontal reach on shoulder muscle fatigue endurance times. This figure illustrates that the workplace must be designed to allow for the upper arm to be held close to the torso. In general, any load-holding tasks should be minimized by the use of fixtures and tool supports. Strasser et al. (1989) showed experi- mentally that the local muscular strain of the hand–arm–shoulder system is dependent upon the direction of horizontal arm movements. Such strain, dependent on the direction of repetitive manual movements, is of great importance for workplace layout. The authors based their study on the premise

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that in order to avoid unnecessary strain imposed by unfavorable postures and working directions in repetitive material-handling tasks, patterns of static and dynamic musculoskeletal loads need to be determined. Figure 9 shows the results of this normalization procedure applied to the experimental data. The static components of the EA values within an angle range between 110° and 200° are also

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significantly higher than those of 20°, 30°, 40°, 60°, and 230°. With regard to the musculature of the shoulder region represented by two recordings of the trapezius (right part of Figure 9), an essentially lower dependence of the muscular strain on the direction of the repetitive horizontal arm movements was observed.

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