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This paper attempts to simulate fatigue in human arm muscles by modeling their basic building blocks - the motor units. Human muscles can be considered to consist of 3 types of motor units: slow-twitch, fatigue-resistant (S); fast-twitch, fatigue-resistant (FR); and fast-twitch, fatigable (FF) motor units. The S units typically generate the smallest forces that can be maintained almost indefinitely. FR units are capable of generating higher forces more rapidly, but can still maintain force for some time and hence, are generally considered as fatigue-resistant. The FF units are able to generate the highest force, but can only sustain it for a short period of time, making them fatigue-susceptible. Whole muscle force and force fatigue has been modeled by a system of coupled differential equations first proposed by Wexler, et al in 1998, and have been refined by Ding and colleagues since then.

A framework to study the response of seated operators to whole-body vibration (WBV) is presented in this work. The framework consists of (i) a six-degree-of-freedom man-rated motion platform to play back ride files of typical heavy off-road machines; (ii) an optical motion capture system to collect 3D motion data of the operators and the surrounding environment (seat and platform); (iii) a computer skeletal model to embody the tested subjects in terms of their body dimensions, joint centers, and inertia properties; (iv) a marker placement protocol for seated positions that facilitates the process of collecting data of the lower thoracic and the lumbar regions of the spine regardless of the existence of the seatback; and (v) a computer human model to solve the inverse kinematics/dynamic problem for the joint profiles and joint torques. The proposed framework uses experimental data to answer critical questions regarding human response to WBV.