The Ehlers-Danlos syndrome is a group of inherited connective tissue disorders caused by defects in collagens or tenascin-X (TNX). Muscle involvement can be expected based on interactions between muscle and extracellular matrix molecules; however, muscle function has not yet been investigated quantitatively. This study aims to investigate effects of TNX deficiency on muscular characteristics in TNX knockout (KO) mice, a mouse model of Ehlers-Danlos syndrome. At lower muscle lengths, maximally dissected medial gastrocnemius muscle-tendon complex of TNX KO mice showed lower active force, lower maximal rate of relaxation, and longer time delay between first stimulation pulse and initial force rise, supporting the hypothesis that relatively more slack needs to be taken up, as well as more elastic length changes occurring. In addition, study of the minimally dissected lower leg muscles shows that TNX deficiency strongly affects the mechanical interaction between antagonistic, as well as synergistic, muscles, which is consistent with the concept of altered myofascial force transmission due to increased compliance of myofascial components. Altered properties of the force transmission pathways of muscle (being either part of the myotendinous or myofascial pathways) due to TNX deficiency directly affect muscle function in TNX KO mice. Such effects are likely to contribute to muscle weakness experienced by patients with Ehlers-Danlos syndrome.