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Walter Herzog and Rachid Ait-Haddou

The target article by Dr. Prilutsky is based on three incorrectly derived mathematical rules concerning force-sharing among synergistic muscles associated with a cost function that minimizes the sum of the cubed muscle stresses. Since these derived rules govern all aspects of Dr. Prilutsky's discussion and conclusion and form the basis for his proposed theory of coordination between one-and two-joint muscles, most of what is said in the target article is confusing or misleading at best or factually wrong at worst. The aim of our commentary is to sort right from wrong in Dr. Prilutsky's article within space limitations that do not allow for detailed descriptions of mathematical proofs and explicit discussions of the relevant experimental literature.

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Marlene Schoeman, Ceri E. Diss, and Siobhan C. Strike

A unilateral transtibial amputation causes a disruption to the musculoskeletal system, which results in asymmetrical biomechanics. The current study aimed to assess the movement asymmetry and compensations that occur as a consequence of an amputation when performing a countermovement vertical jump. Six unilateral transtibial amputees and 10 able-bodied (AB) participants completed 10 maximal vertical jumps, and the highest jump was analyzed further. Three-dimensional lower limb kinematics and normalized (body mass) kinetic variables were quantified for the intact and prosthetic sides. Symmetry was assessed through the symmetry index (SI) for each individual and statistically using the Mann-Whitney U test between the intact and prosthetic sides for the amputee group. A descriptive analysis between the amputee and AB participants was conducted to explore the mechanisms of amputee jumping. The amputee jump height ranged from 0.09 to 0.24 m. In the countermovement, all ankle variables were asymmetrical (SI > 10%) and statistically different (p < .05) for the amputees. At the knee and hip, there was no statistical difference between the intact and prosthetic sides range of motion, although there was evidence of individual asymmetry. The knees remained more extended compared with the AB participants to prevent collapse. In propulsion, the prosthesis did not contribute to the work done and the ankle variables were asymmetrical (p < .05). The knee and hip variables were not statistically different between the intact and prosthetic sides, although there was evidence of functional asymmetry and the contribution tended to be greater on the intact compared with the prosthetic side. The lack of kinetic involvement of the prosthetic ankle and both knees due to the limitation of the prosthesis and the altered musculoskeletal mechanics of the joints were the reason for the reduced height jumped.

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Ty B. Palmer, Jose G. Pineda, and Rachel M. Durham

improved musculoskeletal mechanics gained when the knees are in a position closer to full extension. 34 It is important to note that although the quadriceps is an important muscle group for the isometric squat, 17 the hip extensors have also been reported to be a major contributor to the forces produced

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Mark L. Latash

.1159/000147637 10.1159/000147637 Nichols , T.R. ( 2002 ). Musculoskeletal mechanics: A foundation of motor physiology . Advances in Experimental and Medical Biology, 508, 473 – 479 . 10.1007/978-1-4615-0713-0_53 Nieder , A. ( 2005 ). Counting on neurons: The neurobiology of numerical competence . Nature

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Mark L. Latash, Shirin Madarshahian, and Joseph M. Ricotta

( 10 ), 1602 – 1615 . 10.1016/j.clinph.2009.11.092 Nichols , T.R. ( 2002 ). Musculoskeletal mechanics: A foundation of motor physiology . Advances in Experimental and Medical Biology, 508, 473 – 479 .