Decrease of force capabilities as an index of upper limb fatigue

The aim of the study was to assess upper limb fatigue on the basis of the force change index (FCI), which expresses changes in developed force, and to demonstrate that this index differentiates muscle fatigue of the upper limb depending on external load. The study was performed on ten young men in 12 conditions of external load. Ten conditions characterized repetitive work in a two-period cycle, in which both or one of the periods were loaded, and two conditions characterized continuous work with constant load. The participants tried to maintain hand-grip force at an imposed level during a determined time in the standard upper limb posture. Changes in values of recorded force exerted during successive cycles were approximated by a regression function. In most cases there was a strong correlation between the measurement data and the logarithmic regression curve. However, several cases of lower external load showed absence of such correlation. In 75% of the cases, there were statistically significant differences between the values of FCI calculated for individual conditions of external load. That means that FCI not only expresses muscle fatigue quantitatively but also points to the differences in upper limb fatigue resulting from differences in the external load. The study results have shown that the developed index (FCI) can be applied for fatigue assessment and discrimination with a more sophisticated model of a repetitive task than just a simple two-period work and rest model.