Abstract: Objective:The aim of this study was to establish models regarding the muscle functions and musculoskeletal system in lower extremity, quantify the changes in muscle length, contraction velocity, muscle stress, muscle power, and total work of the bi-articular muscles during sprinting and analyze the biomechanical characteristics of bi-articular muscle work and strain risks.Methods:the kinematic and kinetic data from elite sprinters were collected.Two models of the muscle functions and musculoskeletal system in lower extremity were built to calculate the changes in the parameters of muscle function in lower extremity.Furthermore, the muscle stress, the power, and the total amount of work done per unit area of a single muscle were also examined combined with the quantification via optimization algorithm.Resultsand Conclusion:the peak values of muscle stress, muscle length, contraction velocity, and negative muscle power per unit area in bi-articular muscles were significantly greater than other monoarticular muscles.During the early swing of the stride cycle, the gastrocnemius were eccentric contracted to counteract the loads from external forces, and thus reached a maximal negative power.These results indicated that the gastrocnemius were susceptible to injury during the early stance.For the hamstrings, muscle length and muscle stress were higher during the late swing and early stance phases.Besides, the hamstrings performed quick negative work and produced a maximal power during mid-swing.Hence, these periods were identified as the potential phases for hamstring muscle strain injury.The quadriceps performed much negative work and reached a maximal power during the early swing of the stride cycle, whereas the quadriceps were at a high risk of injury in this period of the gait.The negative work done in bi-articular muscles was significantly higher than other mono-articular muscles during a full gait circle.Additionally, during certain phases of gait, the positive and negative work were simultaneously done with bi-articular muscles on two separated joints.This study provided the biomechanical basis for the strain risk of bi-articular muscle during sprinting through a series of related indicators of the mechanical load on single muscle.