Skeletal muscle transverse strain during isometric contraction at different lengths

An important assumption in 2D numerical models of skeletal muscle contracti on involves deformation in the third dimension of the included muscle secti on. The present paper studies the often used plane strain description. Ther efore, 3D muscle surface deformation is measured from marker displacements during isometric contractions at various muscle lengths. Longitudinal strai ns at superficial muscle fibers( - 14 +/- 2.6% at L-0, n = 57) and aponeuro sis (0.8 +/- 0.9% at L-0) decrease with increasing muscle length. The same holds for transverse muscle surface strains in superficial muscle fibers an d aponeurosis, which are comparable at intermediate muscle length, but diff er at long and short muscle length. Because transverse strains during isome tric contraction change with initial muscle length, it is concluded that th e effect of muscle length on muscle deformation cannot be studied in plane strain models. These results do not counteract the use of these models to s tudy deformation in contractions with approximately - 9% longitudinal muscl e fiber strain, as transverse strain in superficial muscle fibers and in ap oneurosis tissue is minimal in that case. Aponeurosis surface area change d ecreases with increasing initial muscle length, but muscle fiber surface ar ea change is - 11%, independent of muscle length. Assuming incompressible m uscle material, this means that strain perpendicular to the muscle surface equals 11%. Taking the relationship between transverse and longitudinal mus cle fiber strain into account, ii is hypothesized that superficial muscle f ibers flatten during isometric contractions. (C) 1999 Elsevier Science Ltd. All rights reserved.