  Cumulative local muscle fatigue may lead to potential musculoskeletal
disorder (MSD) risks {\color{red}, and subject-specific muscle fatigability
needs to be considered to reduce potential MSD risks.} This study was conducted
to determine local muscle fatigue rate at shoulder joint level based on an
exponential function derived from a muscle fatigue model. Forty male subjects
participated in a fatiguing operation under a static posture with a range of
relative force levels (14% - 33%). Remaining maximum muscle strengths were
measured after different fatiguing sessions. The time course of strength
decline was fitted to the exponential function. Subject-specific fatigue rates
of shoulder joint moment strength were determined. Good correspondence
($R^2>0.8$) was found in the regression of the majority (35 out of 40
subjects). Substantial inter-individual variability in fatigue rate was found
and discussed.
