# A Review on Ergonomics in Agriculture. Part II: Mechanized Operations
## 2 Spinal Anatomy, Biomechanics and Health Effects of Whole-Body and Hand-Arm Transmitted Vibration
### Biomechanics and Physiological Effects of Vibration Exposure  
All objects have a characteristic vibration frequency, which is called resonant frequency. When this value is reached, the system oscillates with a larger amplitude compared to the cases at which an equal oscillating force is applied, but at other frequencies. This phenomenon is called resonance in physics. For human beings, due to the existence of different body parts, there are various resonant frequencies [26]. As regards the vertical vibration, the critical range is 4-8 Hz. Strong resonance is observed in the neck region between 3 and 5 Hz. As far as the low back is concerned, the range of the resonant frequencies is around 4.5 Hz [27]. As the levels of vibration gradually increase, the muscles tend to be unnaturally tensed for the purpose of dampen the vibration.  
In a sitting position, values between 4 and 7 Hz tend to be the most harmful for the spine [28]. In addition, the transfer of the vertical vibration towards the spine appears to be higher at the standing posture than the seated one [29]. Vibration is transmitted from the source to the operator through the contact surfaces. In operating agricultural machineries with a seat, buttocks are the main contact surfaces. The effects of the exposure to vibration depend on different external factors, like the composition and the type of the seat, the kind of shoes as well as the entire body posture. The thresholds of resonant frequency also diversify among individuals. Overall, the consequences of vibration on operators rely on its magnitude, direction, frequency and duration.  
Exposure to vibration for relatively short periods turns out not to be very harmful and usually results in increase of heart rate, loss of balance and headaches. On the other hand, prolonged and repeated exposure to WBV can lead to permanent physiological alterations. In particular, since the intervertebral disks and vertebral structures serve as springs and dampers so as to dissipate the energy from WBV, the human spine appears to be affected to a great extent. The two mechanisms that have been recognized are stiffening, as a response to shocks, and softening, as WBV enhances [30]. MSDs, especially in the region of low back, are the most frequently reported effects of WBV [31]. Investigations have demonstrated that at 4.5 Hz, around which the resonant frequency of low back exists, the fatigue of the muscles takes place. This, in turn, modifies the response of the muscles to sudden loads, thus, becoming more vulnerable to injury [27]. Focusing on the microlevel of the spine structure, prolonged movement of intervertebral discs owing to vibration can stress the annular fibers. Subsequently, the levels of pressures increase in the disks and the consequent forces may induce the material to fail. The material fail of the disk can make it to protrude from the vertebral system. This movement can press the spinal nerves and develop low back pain [32].  
As far as the HAVS is concerned, it can be defined as a syndrome which includes circulatory disorders, such as \"white finger\", as well as sensory and motor disorders like numbness and clumsiness  
Figure 1: Basic spinal anatomy.  
to carry out complex tasks. MSDs can be developed among workers who operate handheld agricultural machineries, as for example handheld olive beaters and power tillers [14; 33]. All these components may not be presented in the worker at the same time. Diseases, such as carpal tunnel syndrome, osteoarthritis of the wrist, hand and elbows and bone cysts, have been attributed to HATV [34]. The main risk factor for HAVS is considered to be the duration of the exposure to intense vibration. If the exposure to vibration is discontinued, a partial recovery can occur [34]. Contributing risk factors involve also smoking and previous medical conditions, which can cause neurovascular pathology [35]. Pain in the region of upper limbs is an ordinary complaint of agricultural workers, especially when adverse ergonomic factors exist.