Muscle injury, especially hamstring muscle, frequently happens during sports activities and exercise which could have serious consequences if not diagnosed and treated promptly. This research investigated the quasi-static and dynamic responses of over 30 fresh semitendinosus muscle-tendonbone structures by utilizing Split Hopkinson Pressure Bars (SHPB) and a material testing system under the strain rates between 0.001 ~ 200 s − 1 . Because of the special shape of muscle-tendon-bone structures, PLA-material clampers were produced by 3D printer to properly hold and to prevent slippery during the test process. The mechanical characteristics of the whole muscle bundle, including Young's modulus and stress-strain curve, are illustrated at various strain rates. The ndings showed that the muscle properties were sensitive with strain rate at the passive deformation. Both maximum stress and Young's modulus increased with the rise of strain rate, and modulus at 200 s − 1 can be as high as 10 times compared with quasi-static conditions.

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BACKGROUND: The knee plays an essential role in movement. There are four major ligaments in the knee which all have crucial functionalities for human activities. The anterior cruciate ligament (ACL) is the most commonly injured ligament in the knee, especially in athletes. OBJECTIVE: The aim of this study was to investigate the dynamic tensile response of the porcine ACL at strain rates from 800 to 1500 s−1 for simulations of acute injury from sudden impact or collision. METHODS: Split Hopkinson Tension Bar (SHTB) was utilized to create a dynamic tensile wave on the ACL. Stress–strain curves of strain rates between 800 s−1 to 1500 s−1 were recorded. RESULTS: The results demonstrated that the elastic modulus of the porcine ACL at higher strain rates was six to eight times higher than that of porcine and human specimens at quasi-static strain rate. However, the failure stress was quite similar while the strain was much smaller than that at the lower strain rate. CONCLUSIONS: ACL is highly strain rate sensitive and easier to break with lower failure strain when the strain rates increased to more than 1000 s−1. The stress–strain curves indicated that the sketching crimps at the slack region did not happen but switched to the sliding process of collagen fibers and was accompanied by some ruptures, which can develop into tears when strain and stress were large enough. On the other hand, the viscoelastic properties of the ligament, depending on the proteoglycan matrix and the cross-link, showed a limited value in the studied strain rate range.

Muscle injuries are the most common sports injuries, and it is often observed in eccentric contraction. There are many factors that could influence the severity of muscle injuries, including strain and strain rate. This study evaluated the interaction of these two factors on the biomechanical properties of the muscle-tendon bundle and their role in injuries. A Hopkinson bar system, an MTS machine and an electrical pulse generator were utilized to collect eccentric contraction response data of over 150 frog muscle-tendon samples at strain rates ranging from 0.01 to 300 s-1. The results have shown that the maximum stress has increased and peaked at about 150 s-1. That peak value has then maintained at the following strain rates. While Young's modulus of stimulated samples reduced as the strain rate changed from 50 to 300 s-1. That trend was in contrast to unstimulated muscle bundles. In addition, strain rate has significantly influenced tendon-muscle bundle fracture. Samples tend to rupture at a minor strain of about 3.5 % with strain rates over 200 s-1. Because of the increasing stiffness of the muscle area at high strain rates, increased strain in the tendon region resulted in frequent injuries in the tendon area. On the other hand, a maximum-stress reduction was detected when the muscle bundles were stimulated at muscle strain greater than 0.2. The results showed that improper timing of stimulation could increase muscle injury.