The dynamic response of the hand/arm system due to exposure of two types of vibrations, one from an impact hammer and one from a grinder, were studied. The aim of the investigation was to study whether the dynamic response was dependent of the type of exposure and/or the frequency of the vibration. The dynamic responses studied were driving point impedance, transfer function from handle to finger, wrist and elbow. The energy per time (dissipated power) absorbed in the hand-arm system as well as the influences of grip and push forces on the impedance were also studied.

When grip and push forces were kept constant, no differences were found, in either transfer function or driving-point impedance, between impact and harmonic exposure. Increasing grip as well as push forces increased the impedance, both in resonance frequency and in magnitude. This means that the dissipated power depends on both (1) frequency contents of the vibration and (2) forces (grip and push).

Exposures with predominantly lower frequencies < 50 Hz) caused a greater load on the elbow and shoulder joints than exposures with higher frequencies (≥ 100 Hz). Up to about 250 Hz, the finger acts like a rigid body. When people exert force they tighten their muscles and this means that their hand-arm acts like a “stiffer spring” and thus increases the resonance frequency. The hand-arm system is also a non-linear system.

Relevance to industry:​ The paper discusses the dynamic response of the hand-arm system to exposure of vibration from an impact tool (impact hammer) and a non-impact tool (grinder). Factors like frequency content of the vibration, and grip and push forces influence the energy absorbed in the hand-arm system. Whenever possible, a tool that requires low grip and push forces should be used. This can be helpful in choosing the proper tool for the job.