A study published in December 2019 in the Journal of Experimental Biology shows that the closed fist produces no more force than an open palm slap.
"To find out, the researchers measured the force produced when 12 male boxers and skilled martial artists hit a punching bag as hard as they could, with both open palm and a closed fist. Surprisingly, both methods produced the same level of maximum strength.
The clenched fist delivered the same force to a smaller Surface, meaning it could inflict more tissue damage and be more likely to break bones.
This suggested that people use a closed fist for punching in order to maximize physical damage to their opponents, not to maximize the force they can produce."
The derived proportions of the human hand may provide supportive buttressing that protects the hand from injury when striking with a fist. Flexion of digits 2–5 results in buttressing of the pads of the distal phalanges against the central palm and the palmar pads of the proximal phalanges.
Additionally, adduction of the thenar eminence to abut the dorsal surface of the distal phalanges of digits 2 and 3 locks these digits into a solid configuration that may allow a transfer of energy through the thenar eminence to the wrist.
To test the hypothesis of a performance advantage, we measured: (1) the forces and rate of change of acceleration (jerk) from maximum effort strikes of subjects striking with a fist and an open hand; (2) the static stiffness of the second metacarpo-phalangeal (MCP) joint in buttressed and unbuttressed fist postures; and (3) static force transfer from digits 2 and 3 to digit 1 also in buttressed and unbuttressed fist postures.
We found that peak forces, force impulses and peak jerk did not differ between the closed fist and open palm strikes. However, the structure of the human fist provides buttressing that increases the stiffness of the second MCP joint by fourfold and, as a result of force transfer through the thenar eminence, more than doubles the ability of the proximal phalanges to transmit ‘punching’ force. Thus, the proportions of the human hand provide a performance advantage when striking with a fist.
We propose that the derived proportions of hominin hands reflect, in part, sexual selection to improve fighting performance.
Michael H. Morgan, David R. Carrier
Journal of Experimental Biology 2013 216: 236-244; doi: 10.1242/jeb.075713