Recently we presented a method for the assessment of in vivo forces on pectoral device implants motivated from technological and clinical advancements toward smaller implantable cardiac pacemakers and the altered structural demands arising from the reduced device size. Objective of this study was the investigation of the intra-species proportionality of in-line force and transverse reaction force of the Pectoralis major for the characterization of mechanical in vivo loadings on pectoral implants. Two Chacma baboons (23.9 ± 1.2 kg) received bilaterally one chronic and one acute pectoral sub-muscular instrumented pacemaker (IPM) implant. The Pectoralis major muscle was electrically stimulated and resulting in-line and transverse muscle force were measured. The correlation of in-line force and transverse force of the Pectoralis major was investigated using linear regression analyses. The proportionality of in-line and transverse force of the Pectoralis major was found to be subject-specific (R² = 0.17, p < 0.003). Including morphometric parameters, i.e., length along line of action, width over implant and stress, in the regression analysis provided a strong intra-species correlation between in-line and transverse force (R² = 0.71, p < 10⁻⁷). The novel intra-species correlation provides a tool toward the characterization of mechanical in vivo loading conditions of pectoral device implants.

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Background: Spaceflight has been shown to cause atrophy, reduced functional capacity, and increased fatigue in lower-limb skeletal muscles. The mechanisms of these losses are not fully understood but are thought to result, in part, from alteration in muscle usage.