The main objective of the current study was to check and confirm whether the support for the mechanical advantage principle depends on the challenge associated with the task. The little finger produced a greater normal force than the ring finger when the thumb was restricted to produce a normal force of 7N closer to the mass of the handle. We believed that the reason could be due to the challenge of maintaining the handle equilibrium by producing lesser normal force by the thumb. In addition to the restriction on the normal force of the thumb, there was restriction imposed on its position and tangential force. The cause and effect behind the results will be discussed in the following paragraphs.
Some of the studies in the past supported the mechanical advantage principle in certain conditions. In a five-finger prehension study, when a load of greater mass (2kg) was suspended closer (1.9cm) to COM of the handle, ulnar fingers exerted apparently comparable normal force7. However, when the same external load was suspended at a farther distance (7.6cm) from COM, causing a greater moment, the little finger produced greater normal force than the ring finger. Similarly, in another multi-finger prehension study, MAH was supported even when a load of lesser mass (less than 2kg) was suspended at a greater distance (8.9cm) from COM of the handle14. Thus, this does not mean that the support for MAH is always dependent on the moment arm or mass of the suspended load or magnitude of moment requirement.
Our previous study on the systematic increase in the mass of the handle with the load suspended exactly below COM of the handle could help to understand this situation better12. Although external loads of mass ranging from 0.150kg to 0.450kg were suspended exactly below COM of the handle, MA principle was supportive only when an external load of mass 0.450kg was added. From the results, it may be posited that the support for mechanical advantage principle could be due to an abstract sense of challenge associated either with the mass of the suspended load, moment arm or magnitude of moment requirement.
Apart from this, the hypothesis was also supported when the thumb platform was made to operate in the region beyond the range of motion of carpometacarpal (CMC) joint of thumb during the pattern tracing study15. The study was comprised of two conditions: tracing trapezoid pattern and inverted trapezoid pattern. Depending on the condition, either trapezoid or inverted trapezoid pattern was displayed on the computer monitor. The task was to hold the handle with the unsteady thumb platform at the HOME position for a few seconds and translate the platform vertically towards the index finger side (during trapezoid condition) or little finger side (during inverted trapezoid condition). CMC joint of the thumb possesses a limited range of motion in the downward direction. Therefore, tracing the BOTTOM static portion of the inverted trapezoid pattern was quite challenging than tracing the TOP static portion of the trapezoid pattern. Although a greater compensatory moment was required due to the shift in the position of the thumb platform from HOME, the challenge associated with operating the thumb beyond the range of motion of its CMC joint could also be the reason for supporting MAH.
In the current study, maintaining the handle in static equilibrium was challenging by imposing restrictions on the thumb's normal force. The magnitude of target normal force to be produced by the thumb during comfortable grasp condition, was chosen from the results of our previous study on the systematic increase in the mass of the handle. As per the previous study, when there was no restriction on the normal forces, the average normal force produced by the thumb was approximately 14N when the total mass of the handle was 0.700kg. The results showed a statistically comparable normal forces by the ulnar fingers. Therefore, for the current study, we expected that the ulnar fingers would continue to produce a statistically comparable normal force during comfortable grasping. Whereas, in the case of uncomfortable grasp condition, the target normal force was set to 7N. Since the total mass of the handle with the external load was 0.700kg, the total tangential force shared by the fingers and thumb for holding the handle, including the cable mass, was approximately 6.86N. Therefore, for uncomfortable grasp condition, the instruction was to exert a thumb normal force of 7N.
In addition to the restriction on the thumb normal force, a constraint was already imposed on the handle design. That is, there are two different interfaces on the thumb side of the handle: the thumb-platform interface and platform-railing interface. Since the slider platform was mounted on the vertical railing fitted over the handle frame, the friction at the platform-railing interface was very low (µ ~ 0.001 to 0.002). Therefore, the tangential force produced by the thumb to hold the platform was maintained at a constant low magnitude. Additionally, throughout the entire trial, the slider platform had to be held at the HOME position by aligning the horizontal lines on the platform and the handle frame. In the presence of all these three constraints, maintaining the static equilibrium of the handle was quite challenging to perform.
During comfortable grasp condition, the task of maintaining the static equilibrium of the handle was not challenging enough as the target thumb normal force was almost double that of the mass of the handle. Therefore, as seen in our preliminary study10, the ring and little fingers shared statistically comparable normal forces to balance the horizontal equilibrium (see Introduction section). However, during uncomfortable grasp condition, the little finger produced greater normal force than the ring finger, supporting the mechanical advantage hypothesis. Since the target thumb normal force during uncomfortable grasp condition was 7N lesser than the target normal force set for comfortable grasp condition, the ulnar finger normal forces decreased. The decrease in the ulnar finger normal forces would be accompanied by a drop in the supination torque, as ulnar finger normal forces are contributors to supination torque. In response to this, there would be a pronation torque in the anti-clockwise direction due to the virtual finger tangential force. However, to maintain the rotational equilibrium of the handle, a sufficient compensatory supination torque was required without a substantial increase in the ulnar finger normal forces. Perhaps, by increasing both ring and little finger normal forces together, virtual finger normal force might increase, which might indirectly disturb the normal force produced by the thumb.
Therefore, during uncomfortable grasp condition, the aim was to produce a sufficient supination torque without showing a greater increase in the total normal force of the ulnar fingers. Employing the mechanical advantage principle would be the best solution from the mechanics perspective. It involved increasing the normal force of the little finger than the ring finger. Thus, sufficient supination torque was produced while simultaneously producing minimal total normal force. It is inferred that when multiple constraints are imposed simultaneously, MAH is supported.
It is possible to untangle the intricate details behind the results of both the conditions from an anatomical or biomechanical standpoint. The tendons of the extrinsic muscle, flexor digitorum profundus (FDP), extend to the distal interphalangeal (DIP) joints of the index, middle, ring, and little fingers. FDP muscle is responsible for the flexion of DIP joints of the four fingers and thus accountable for the normal force production in those fingers. Whereas the intrinsic muscles of the hand such as lumbricals, hypothenar, thenar, dorsal and palmar interossei muscles are involved in the precise (or dexterous) manipulation of the object16–18.
In the case of comfortable grasp condition, since the thumb exerted a relatively high normal force of 14 N, extrinsic muscles responsible for forceful grip production would attempt to increase the virtual finger normal force. In particular, the forces of ulnar fingers increase more than the radial fingers (index and middle) due to the task requirement of compensatory supination torque. In the case of uncomfortable grasp condition, since maintaining the handle equilibrium was quite challenging, dexterous control of ulnar finger normal forces was required for the minimal total normal force production and sufficient compensatory torque production. Among the ulnar fingers, the little finger has an additional group of intrinsic muscles referred to as hypothenar muscles (flexor digiti minimi, abductor digit minimi, opponens digiti minimi) in addition to the lumbrical muscle.
Since the little finger has the added advantage of a separate group of intrinsic muscles for the dexterous manipulation compared to the ring finger, CNS might have attempted to use the little finger compared to the ring finger as it has both anatomical and mechanical advantages. Hence, the little finger might have produced a greater normal force than the ring finger, supporting the mechanical advantage hypothesis. The unique muscle architecture of the little finger may be why the system chooses to employ the mechanical advantage principle, particularly when the task becomes challenging, as in the current study.