Safe grasping of an object using fingers is an essential task while handling the object. Maintaining the grasped object in static equilibrium is a critical aspect that governs safety by avoiding clumsy behavior such as dropping or spilling.
There is a theory known as the Mechanical Advantage Hypothesis (MAH) of grasping that says that the finger with a longer moment arm for normal force will produce greater normal force than the finger with a shorter moment arm during the moment-producing tasks. Considering the thumb as pivot point during prismatic precision grasp, the little finger possesses a greater moment arm for normal force than the adjacent ring finger. Therefore, the expectation was that the little finger would produce greater normal force than the ring finger during moment production tasks. Some past studies fully supported MAH, while a few others supported it partially. This led to the conclusion that the MAH has a potential to be task and effector-specific. Although previous studies from the literature were investigating the applicability of MAH while grasping, the root cause for supporting this theory has not been explored. In the study conducted by Ms. Banuvathy Rajakumar, Mr. Swarnab Dutta, and Dr. Varadhan SKM, from the Department of Applied Mechanics, Indian Institute of Technology Madras, Chennai, India, it was believed that the support for MAH depends on the challenge associated with the moment-producing grasping task.
In this study, a grip device was constructed such that the tangential force of the thumb would be constrained to a constant minimum magnitude. This was achieved by placing the thumb on a freely movable slider platform. So, with a systematic increase in the mass of the handle, only the tangential force of the finger would increase. The mass of the handle was varied with additional masses. External loads of mass 0.150 Kilograms, 0.250 Kilograms, 0.350 Kilograms, and 0.450 Kilograms were added to the handle one by one.
It was found that the ring finger and the little finger exerted comparable normal forces with the external loads of masses 0.150 Kilograms, 0.250 Kilograms, and 0.350 Kilograms. But for an external load of mass 0.450 Kilograms, the Mechanical Advantage Hypothesis (MAH) was found to be supported. With the addition of greater mass, under the constraint of using minimal thumb tangential force, establishing static equilibrium by the ring and little finger becomes progressively more challenging.
The authors of this paper concluded that the Mechanical Advantage Hypothesis (MAH) as a strategy utilized in human grasping is not only employed when there is any change in mass of the grasped handle or moment arm of the suspended load but also when a certain threshold difficulty is reached during the task.
Article by Akshay Anantharaman
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