## 3 Absorption Energy or Absorption Power  
Another metric notion, specific to vibrations, is the energy absorbed per unit of time (power). Some results regarding this are presented below. The term energy absorbed per time unit for 8 h and 4 h of vibration exposure represents the total amount of energy accumulated during the vibration exposure period. This is the energy related to the time unit, calculated with Equation (2), as per reference [1], where \(T(8)\) = 28,800 s (time of exposure to vibrations, corresponding to 8 h \"weighted energy\"), \(a_{\rm hw}\) = weighted acceleration [m/s\({}^{2}\)], and \(T\) = the period of exposure to vibrations at the analyzed frequency [s]. \[a_{\rm hw}(\rm ech.8h)=\left\{\frac{1}{T(8)}\!\int_{0}^{T}\left[a_{\rm hw}(t )\right]^{2}dt\right\}^{1/2} \tag{1}\]  
The \"limit of exposure to vibrations\" is rendered by the degree of sensitivity of the individual exposed to said vibrations, analyzed according to the responses of people exposed to vibrations (%), at a certain time interval. It should be mentioned that there are situations in which the previously mentioned undesirable conditions appear before reaching the vibration sensitivity threshold, in conformity with [1]; therefore, it is recommended to avoid these values. Research conducted over long periods of time and studies such as those by [PERSON] and [PERSON], in conformity with the studies presented in [1], showed that hands with a bent elbow transmit fewer vibrations to the shoulder than fully outstretched hands. Another notion often encountered in biomechanics is VPA--vibration power absorption. In conformity with [1], it represents the vibration absorption power, and, sometimes, it is used as the average vibration power absorption density (VPAD) because the VPAD may be a better measure of energy than the total power absorption of the hand-arm system, particularly for measurements of the soft tissues of the fingers [37]. These values can be determined for the hand-arm system (in particular, the palm) for frequencies between 40 and 100 Hz. For a more detailed bilinear analysis of mechanical vibrations in time-frequency coordinates, WVD can be used [1], but there are disadvantages in terms of exposure time. While using the Wigner-Ville time-frequency distribution (WVD), the energy concentrations (acceleration R.M.S. depends on the frequency) can be measured easily. The continuation of studies regarding the energy absorbed by the hand and its subs assembly was carried out by other researchers [17]. Among these, some examined the transmissibility of hand-arm vibrations in terms of absorption with the arm extended and bent, the excitation coming from electric tools. The absorption power of the biomechanical models of the hand system, determined by both the mechanical impedance of the input drive point in the system and the transmissibility of vibrations to the arms, was higher at a frequency of 25 Hz than at other frequencies transmitted via the hands (fingers and palm) for both positions during VPA. It can be noticed that the values of vibration transmissibility to the hands were larger above 100 Hz (suggested by the standard over 250 Hz), but, in comparison, the arm's transmitted values are bigger under 25 Hz; these data were all reported by SR EN ISO 5349/2003. The conclusions of these researchers showed that the power absorbed by the hand while using the device with the arm extended was about 2.5 times higher than that absorbed, for the same type of force, with the arm bent, according to [1]. It was also observed that the higher the rotation speed of the tool during operation, the higher the power absorbed by the hand. This was regardless of the position of the arm and only in the range of 5-16 Hz. Hence, it was necessary to evaluate the transmission of vibrations to the subassemblies of the hand-arm system and not the system as a unit. The results thus depended on the source of excitation, the method of attachment, and their natural frequencies.