Forearm muscle fatigue can occur during intensive work and may be a marker or precursor to chronic musculoskeletal disorders. The objective of this research was to develop a reliable and sensitive method for measuring muscle fatigue resulting from low force static and dynamic work with the hands. The ultimate goal is to develop and validate a technique that can measure muscle fatigue during precision work, such as computer keyboard and mouse use, in order to determine how changes in the task and tool design influence fatigue.

A new method and apparatus for measuring muscle fatigue with electrical stimulation was developed and evaluated. The technique involved electrically stimulating the finger flexor muscle with 2 Hz electrical stimulation and measuring the resultant twitch forces at the fingertip. In the first part of this study it was demonstrated that the muscle twitch could be used as a reliable index for measuring muscle fatigue if the muscle was properly pre-conditioned and certain precautions were taken to minimize sources of variability during twitch measurement.

In the second part of this study, die sensitivity of the measurement technique was assessed. Six subjects (3 male, 3 female) performed sub-maximal static exertions of 5%, 10% and 15% of their maximum voluntary contraction (MVC) with their ring finger against a force transducer for durations of 5,10 and 15 minutes (9 experimental conditions). Muscle twitch forces from the Flexor Digitorum Superficialis (FDS) muscle were measured before exercise, after exercise, and in subsequent periods of recovery. In general a dose-response relationship was observed between load (force level and contraction duration) and twitch forces indicating that the technique is sensitive to muscle fatigue during brief, low-level static contractions.

In the third and fourth parts of this study, muscle fatigue was measured during computer use. In one experiment, ten subjects (5 male, 5 female) used a mouse for 5 hours. The average forces applied to the mouse during this experiment was 0.47 (s.d. 0.15) Newtons or just 0.7% MVC. In the second experiment, nine subjects (3 male, 6 female) typed on a computer keyboard for 4 hours. The average forces applied to the keyboards ranged between 1.1 - 1.6 Newtons or 4.4 - 6.5% MVC. In both the mouse and keyboard experiment, significant levels of muscle fatigue were measured after three to four hours of computer use and persisted up to 80 minutes after computer activity had ceased.

This research has demonstrated that the muscle twitch can be a reliable and sensitive tool for measuring muscle fatigue following low-force, static and dynamic work with the hands.