This paper by Gordon, Huxley, and Julian investigates the variation in isometric tension with sarcomere length in vertebrate muscle fibers. The authors use isolated frog muscle fibers to measure tension and stiffness under different degrees of stretch, covering the range of lengths where the thick and thin filaments overlap. The results confirm previous findings by Ramsey and Street (1940) and show several new features that align with expectations based on filament dimensions. The study also includes experiments on the speed of shortening at different lengths. The authors describe improvements to their apparatus, including a new servo motor and more precise length measurement methods. They find a plateau in tension development at sarcomere lengths between 2.05 and 2.2 μ, with tensions decreasing above and below this range. The decrease in tension above the optimum length is attributed to the progressive development of irregularities in striation spacing during tetanus. Below the optimum length, tension declines steeply after a sarcomere length of about 1.65 μ. The validity of the results from afterloaded contractions is discussed, showing that initial length and afterload size do not significantly affect the tension. The speed of shortening under light load is also examined, with findings suggesting that the only important effect of changing length is altering the number of sites acting in parallel. The interpretation of the results in the context of the sliding filament theory is provided, suggesting that isometric tension is directly proportional to the number of bridges overlapped by thin filaments. The authors also discuss forces opposing shortening, such as internal resistance and the deformation of the sarcolemma, which may contribute to the decline of tension with decreasing length. Finally, the paper compares the results with earlier studies, noting good agreement and explaining differences in tension levels at long and short sarcomere lengths.This paper by Gordon, Huxley, and Julian investigates the variation in isometric tension with sarcomere length in vertebrate muscle fibers. The authors use isolated frog muscle fibers to measure tension and stiffness under different degrees of stretch, covering the range of lengths where the thick and thin filaments overlap. The results confirm previous findings by Ramsey and Street (1940) and show several new features that align with expectations based on filament dimensions. The study also includes experiments on the speed of shortening at different lengths. The authors describe improvements to their apparatus, including a new servo motor and more precise length measurement methods. They find a plateau in tension development at sarcomere lengths between 2.05 and 2.2 μ, with tensions decreasing above and below this range. The decrease in tension above the optimum length is attributed to the progressive development of irregularities in striation spacing during tetanus. Below the optimum length, tension declines steeply after a sarcomere length of about 1.65 μ. The validity of the results from afterloaded contractions is discussed, showing that initial length and afterload size do not significantly affect the tension. The speed of shortening under light load is also examined, with findings suggesting that the only important effect of changing length is altering the number of sites acting in parallel. The interpretation of the results in the context of the sliding filament theory is provided, suggesting that isometric tension is directly proportional to the number of bridges overlapped by thin filaments. The authors also discuss forces opposing shortening, such as internal resistance and the deformation of the sarcolemma, which may contribute to the decline of tension with decreasing length. Finally, the paper compares the results with earlier studies, noting good agreement and explaining differences in tension levels at long and short sarcomere lengths.