Fish are a major inspirational source of underwater propulsion techniques and have been widely investigated by scientists and engineers in the past several decades. In order to satisfy the maneuverability requirements in the complicated environments, fish have evolved unique segmented muscle to produce undulatory locomotion. In this paper, we focused on the realistic muscle structure of fish propelled by body and/or caudal fin (BCF) swimming type, developed a three-dimensional W-shaped model of the musculo-tendinous system, and mathematically analyzed the relationship between muscle contraction and body flexion. By regulating the key parameters, the local muscle strain producing the prescribed kinematics is calculated. It is found that for a desired body flexion, local contraction can be reduced by lengthening the muscle bundle of the musculo-tendinous system longitudinally. In other words, for a fixed muscle contraction, a larger bending amplitude can be achieved by elongating the W-shaped model. This also explains the morphological differences of segmented muscle within anguilliform, carangiform, and thunniform swimming fish. Fish with better accelerating ability are likely equipped with relatively longer W-shaped muscle. The calculating results indicated that the musculo-tendinous system enlarges the contraction effect of muscle bundles and amplifies the body curvature, it indeed has functional benefits on fish swimming performance. Based on this, it is meaningful to mimic the special structure of fish muscle to improve the swimming performance of fish-inspired soft robots. These findings provide a better understanding on how fish swim and can be used for designing the soft actuators of robotic fish.