Architectural, histochemical, and contractile characteristics of a unique biarticular muscle: the cat semitendinosus.

I. The semitendinosus (ST) muscle of the cat consists of two anatomically distinct sets of fibers connected in series by a dense connective tissue band that divides the “muscle” into a proximal and distal compartment, each having separate innervation. 2. Isometric and isotonic contractile properties of the ST were studied for three stimulation conditions: 1) proximal activated (STp), 2) distal-activated (STd), 3) whole muscle activated (ST). In addition, the architectural and histochemical profiles of each compartment were determined. 3. The fiber type composition was similar in the STp and STd, with the majority of the fibers staining darkly for alkaline myofibrillar adenosine triphosphatase ( ATPase). The muscle fibers of the STd were twice as long as the fibers of the STp. The cross-sectional areas of the two compartments were similar. 4. The in situ maximum isometric tension (P,) was identical for all three stimulation conditions. However, the maximum twitch tension (P,) was greatest when ST was activated and least when only the STp was activated. The time to peak tension (TPT) and half relaxation times (95 RT) were the same for all three conditions. The absolute maximal shortening velocity (V,,,) revealed an orderly progression of speeds in relation to the fiber length. When the V,,, of the STd (424 t 26 mm/s) and STp (224 t 23 mm/s) were added, the sum was approximately the same as the V,,, obtained for the ST (624 t 30 mm/s). The intrinsic speed of shortening (millimeters per second per 1,000 sarcomeres) recorded at the common tendon of insertion was the same for all three conditions. 5. The identical nature of the P, values demonstrated the significance of the architectural design, specifically, the physiological cross-sectional area, in determining the potential force production of a muscle. In contrast, the P, values were significantly different for each stimulation condition. These differences were probably influenced in part by the “stiffness-compliance” properties of the muscle-connective tissue unit, which varied with different activation states of the ST muscle. 6. The similarity in TPT and 1/2 RT values for all three conditions illustrated the independence of the isometric speed-related parameters from the architecture of the muscle. The differences in the absolute V,,, data, however, revealed a positive relationship between the maximum speed of shortening and the fiber length. 7. These data demonstrate the necessity of the neuromotor command being matched with the architectural as well as the biochemical properties of each sarcomere (e.g., myosin ATPase and sarcoplasmic reticulum) of the muscles in the control of forces and velocities in vivo.