Assessment of human muscle strength for engineering purposes: a review of the basics.

Contracting skeletal muscles pull on body segments to withstand or overcome internal and external opposing forces. Various methods and techniques can be employed for measuring muscular exertions. Information on strength that can be applied to objects such as hand tools or handling loads is of great importance for ergonomic design. This paper establishes a systematic context regarding: (1) the contractile muscle actions and their associated metabolic and circulatory processes, which are often the limiting factors for exertion; (2) the internal biomechanical effects of muscle contraction in terms of the application of muscle force to body links and the internal transmission of the generated torque through the body to the point of external exertion; (3) the various kinds of muscular efforts, ranging from strictly bounded statics (isometrics) to freely executed dynamics; and (4) the ensuing possibilities to measure human muscular capabilities for the application of this information to ergonomic design of tasks and equipment.

Evaluation of supermarket bagging using a wrist motion monitor.

The supermarket industry has one of the highest numbers of repeated trauma illnesses. Checkout departments have a rate of musculoskeletal injuries 2 to 3 times higher than that of other supermarket departments. The primary objective of this study was to quantify the wrist motions required to bag groceries using a wrist motion monitor. The wrist motions included deviations, velocities, and accelerations for flexion-extension, radial-ulnar, and pronation-supination directions. The independent variables were handle type and object location. Objects with finger-thumb couplings required more extreme pronations, greater wrist velocities for pronation-supination deviations, and greater wrist accelerations for pronation-supination deviations than did other objects. Objects with 10-cm hand couplings required more extreme flexion, larger ranges of movement for radial-ulnar deviations and pronation-supination deviations, and greater wrist velocities in the radial-ulnar and pronation-supination directions than did 5-cm objects. The right and front locations required more extreme deviations than did the left and back locations. Because finger-thumb and 10-cm hand couplings require larger wrist deviations and greater velocities, these objects may pose a greater risk of developing cumulative trauma disorders to the bagger. Potential applications of this research include engineering design of grocery packaging and supermarket bagging workstations.

Usability of a one-hand ternary chord keyboard.

Five subjects learned to operate a one-hand ternary chord keyboard with 18 different characters, on average, in just over 30 min. Subjects practised inputting numeric-type text for an additional 60 h and attained an average peak keying speed of 170 characters/min. Because of the short learning time, high performance, and low space requirement, the one-hand ternary chord keyboard is an attractive input device for tasks with small character sets.

Avoiding cumulative trauma disorders in shops and offices.

Cumulative trauma disorders have been medically described for about 100 yr and have been related to physical activities for nearly 300 yr. Yet, avoiding these disorders in the shop and office is becoming of urgent concern only now, particularly because of the Occupational Safety and Health Administration's (OSHA's) investigation and enforcement program. Such disorders occur most often in soft tissues of the body, particularly at tendons and their sheaths. They may irritate or damage nerves and impede blood flow. They are frequent in the hand/wrist/forearm area; for example, in the carpal tunnel and in the shoulder and neck. Although controversy exists, occupational and leisure activities are generally believed to cause or aggravate cumulative trauma disorders. The major activity-related factors are rapid repetitive movements, forceful movements, static muscle loading, inappropriate body postures, vibrations, and cold. Yet, the quantitative thresholds above which cumulative trauma disorders are expected to occur are largely unknown and need to be researched. Furthermore, certain health conditions may make individuals predisposed to cumulative disorders. For most cumulative trauma disorders, physical activities and job procedures can be identified that are related to the occurrence of cumulative trauma disorders. This allows the establishment of generic and specific recommendations for the avoidance of conditions that may lead to cumulative trauma disorders in the workshop or the office.

Performance on a prototype keyboard with ternary chorded keys.

Ten subjects learned to operate a ternary chorded keyboard which took them, on average, just over three hours. After an additional 10 h of use, they were inputting an average 70 characters/min with an accuracy of better than 97%. The individual performances were quite different and variable, but throughout rapidly improved. The experiments indicated that people are able to memorize 59 chords, and to execute them quickly, with surprising facility.

Working strenuously in heat, cold, polluted air and at altitude.

Engineers and managers need to know how environmental conditions affect human capabilities to perform physical work. Heat reduces the ability to work intensively over extended time, but does not affect short bursts of activity. Cold, in contrast, does not affect intensive work of long duration, but reduces short bursts of energy exertions. Dehydration is a problem in both heat and cold. The effects of air pollution on work capabilities are not well researched though carbon monoxide is known to reduce them. Altitude hardly affects short burst of efforts, but the capabilities for enduring and maximal work are much reduced with increasing heights and stay diminished until one returns to sea level.

Anthropometry of the elderly: status and recommendations.

Available anthropometric information on the elderly in the United States is discussed. Demographic data as well as information on performance capabilities and limitations (such as strength, endurance, and mobility) are needed for ergonomic design of tools, utensils, equipment, and vehicles to improve the working and living environment for the elderly. Recommendations are made for collection of more information.

Cumulative trauma disorders: their recognition and ergonomics measures to avoid them.

Cumulative Trauma Disorders, CTD, are the summary results of many microtrauma. Though known since the early 1800s, they have become a matter of urgent ergonomics concern in the 1980s. They are usually caused by repeated and/or forceful exertions, often in the hand-arm-shoulder region. They predominantly occur to soft tissues, such as tendons and ligaments, and to nerves and blood vessels. They are commonly associated with certain occupational and leisure activities. Many CTD can be avoided by a set of fairly simple and straightforward ergonomics procedures.

Towards an objective assessment of the "maximal voluntary contraction" component in routine muscle strength measurements.

Routine measurements of muscle strength capabilities commonly use external dynamometers against which the subject exerts maximal voluntary contractions of muscles in static (isometric) tests. These tests require active cooperation of the subject, i.e., full motivation to "give the best". At present, no practicable techniques exist that provide objective clues indicating that indeed a maximal effort is delivered, or if only a portion of the available strength is exerted. This paper describes experiments performed with 30 subjects which indicate that the rate of strength build-up in repeated exertions may provide objective criteria to judge whether or not a subject exerts full muscular strength in a routine test.

Horizontal push and pull forces: exertable when standing in working positions on various surfaces.

Push and pull forces are reported which can be exerted by one or several men while they assume common standing positions, either braced against sturdy supports or standing on slippery floors. Coefficients of friction between shoe and floor materials are listed.