Assumptions about the cross-sectional shape of skinned muscle fibres can distort the relationship between muscle force and cross-sectional area.

Comparisons of muscle force output are often performed after normalization to muscle physiological cross-sectional area (CSA). Differences in force per CSA (i.e., specific force) suggest the presence of physiological differences in contractile function. Permeabilized mammalian skeletal muscle fibres frequently exhibit substantial declines in specific force with increasing CSA, suggesting that smaller fibres are intrinsically stronger than larger fibres of the same group. However, the potential for CSA assessment error to account for CSA-dependent differences in specific force has not received adequate attention. Assessment of fibre CSA typically involves measurement of fibre width and perhaps also height, and CSA is calculated by assuming the cross-sections are either circular or elliptical with major and minor axes aligned with the optical measurement system. Differences between the assumed and real cross-sectional shapes would cause variability in the ratio of assessed CSA (aCSA) to real CSA (rCSA). This variability can insidiously bias aCSA such that large aCSAs typically overstate rCSAs of the fibres they represent, and small aCSAs typically understate the rCSAs of the fibres they represent. As aCSA is the denominator for the specific force calculation, scatterplots of specific force vs aCSA would be expected to show declines in specific force as aCSA increases without a corresponding effect in a scatterplot of specific force vs rCSA. When comparing active and passive muscle forces between data subsets defined by aCSA, the impact of CSA assessment error should be considered before exploring other physiological mechanisms.