During several hours of farming, operators of single-axle tractors are subjected to large quantities of hand vibration. Vibration feedback from most power tools to the hand contains inputs from all three orthogonal motion axes and frequency-weighted RMS acceleration values for vertical, forward and lateral direction measured as ahwx, ahwy, and ahwz are recorded separately using the frequency-weighting graph, respectively[1].
Hand-tractor coupling forces are the main quantities for calculating hand-transmitted vibration; they must be continuously measured during physiological effects studies [2, 3]. According to the Griffin study[4], the vibration transmission is largely determined by the dynamic response of the hand; this, in turn, depends on the physical characteristics of the individual hand, the contact area, the grip force, the push force, the location, etc. The effect of these variables can be quantified by calculating the energy used by the hand instead of the vibration frequency on the tool handle. Friction forces tangential to the surface of the handle are linked to the grip strength originating from the grip contact pressure normal to the handle surface[5, 6].
Long-term vibration damage from vibratory equipment results in muscle fatigue affecting grip strength[7] and research performed on different occupational groups, such as cleaners, dental hygienists, dental technicians, dentists, drivers, metalworkers, and wood product assemblies, suggested that the most common symptoms were numbness and loss of grip strength [8].
Grip strength can be used as a quantitative measure of the efficiency of the sensor motor capacity of the user; it also serves as an indication of the applicability of the device to the task. Schlüssel et al. [9] described the grip strength as an integral part of the contact pressure over the entire hand and the contact surface; it was noted that the subject received increasing attention from industrial engineers and ergonomics researchers.
Research has shown that on average individual maximal handgrip strength is associated with anthropometric indicators such as gender, age, body weight, dominant hand, hand length, and hand width [10, 11] and the transfer of vibration energy to the hand is primarily influenced by the contact forces between the hands and the gripping zones.
In addition to the direct increase in hand vibration level with grip strength; the muscle tension causes phase shifts in the vibration[12–14]. Hewitt [15] has shown that vibratory tool operators such as impact hammers, grinders, and pneumatic hammers exert high grip strengths that enhance the transfer to operators of vibration energy. Research has shown that grip strength generally depends on age, with younger operators having relatively high grip strength; grip strength then decreases with age for both the dominant and non-dominant hands[16, 17]. Another study [18] found that in the dominant hand, grip strength is typically higher. According to Dong et al.,[19] grip strength measurements generally depend on the orientation of the measurement axis and that the maximum value can be significantly different from the minimum value in a given gripping action. Several studies have been conducted to find age-dependent reference (baseline) grip strength[20–23].
Workers who apply frequent forceful gripping and moving forces to vibratory tool handles may be at risk of developing circulatory, neurological, or musculoskeletal disorders. These disorders have been grouped as hand-arm vibration syndrome (HAVS). HAVS covers neurological, vascular and musculoskeletal injuries[24].
Several studies have shown that grip strength predicts upper extremity weakness and improves muscle strength, physical movement and range of motion, hand dexterity, and ability to perform day-to-day operations [25–27].
The strength decrement index (SDI) was defined by Endurance [28], which is the fractional decrease in Grip Strength due to fatigue calculated by the formula:
Where IS = initial Grip Strength
FS = final Grip Strength
Of special concern is muscle fatigue arising from repeated hand surgery. With a sufficiently vigorous and extended duration of exercise, the mechanical strength of the skeletal muscles decreases. Fatigue can be defined as a progressive decline in muscle performance during exercise, or an exercise-induced reduction in the ability to exert muscle force or power[29]. In cases where the operator uses maximum force, or experiences high-frequency fatigue, the maximum grip strength available generally shows a rapid decline during work; however, there is generally a rapid recovery from this type of fatigue[29].
Several researchers [30,31]described fatigue resistance for each hand as the time for grip strength to decrease to 50% of its maximum value. Bautmans et al [32] described "grip work" (GW) as an integral part of GS grip strength over fatigue resistance time (FR), where FR is the time for grip strength to decrease to half of its maximum value. (figure 1).
GW » 0.75 * GS * FR (4)
Where GW = grip work,
GS = maximum grip strength
FR = fatigue resistance
This paper presents data on measurements of grip strength, fatigue resistance, grip work and strength decrement index for operators of a 15 horsepower single-axle tractor operator, before and after 30 minutes of ground tilling activities.