Muscle strains are one of the most common complaints treated by physicians. A muscle injury is typically diagnosed from the patient history and physical exam alone, however the clinical presentation can vary greatly depending on the extent of injury, the patient's pain tolerance, etc. In patients with muscle injury or muscle disease, assessment of muscle damage is typically limited to clinical signs, such as tenderness, strength, range of motion, and more recently, imaging studies. Biological markers, such as serum creatine kinase levels, are typically elevated with muscle injury, but their levels do not always correlate with the loss of force production. This is even true of histological findings from animals, which provide a "direct measure" of damage, but do not account for all the loss of function. Some have argued that the most comprehensive measure of the overall health of the muscle in contractile force. Because muscle injury is a random event that occurs under a variety of biomechanical conditions, it is difficult to study. Here, we describe an in vivo animal model to measure torque and to produce a reliable muscle injury. We also describe our model for measurement of force from an isolated muscle in situ. Furthermore, we describe our small animal MRI procedure. 1. These procedures can be used for rats or mice 7,17,18 . To begin, place the animal supine under inhalation anesthesia (~4-5% isoflurane for induction in an induction chamber,then~2% isoflurane via a nosecone for maintenance) using a precision vaporizer (cat # 91103, Vet Equip, Inc, Pleasanton, CA). Apply sterile ophthalmic cream (Paralube Vet Ointment, PharmaDerm, Floham Park, NJ) to each eye to protect the corneas from drying. During the procedure, the animal is kept warm by use of a heat lamp placed outside the cage and kept at least 6 inches from the animal at all times. 2. Prep the skin by removing hair and by cleaning with alternating scrubs of betadine and 70% alcohol to prevent seeding skin bacteria into the soft tissue or bone. Confirm proper anesthesia by lack of a deep tendon reflex (no foot withdrawal in response to pinching the foot). A needle is manually placed through the proximal tibia in order to stabilize the limb onto the rig (25G or 27G for mouse). The needle should not enter the anterior compartment of the leg. 3. Lock the needle into a fixed position, such that the animal is supine and the toes are facing straight up. A custom-made device is used to secure the needle and thereby stabilize the leg. 4. Place the foot of the limb onto a custom-machined footplate (Figure 1). The axis of the footplate is attached to a stepper motor (model T8904, NMB Technologies, Chatsworth, CA) and a torque sensor (model QWFK-8M, Sensotec, Columbus, OH). The foot should initially be aligned so that it is orthogonal to the tibia, as in Figure 1. 5. Use transcutaneous electrodes (723742, Harvard Apparatus, Cambridge, MA) or subcutaneous electrodes (J05 Needle Electrode Needles, 36BTP, Jari Electrode Supply, Gilroy, CA) to stimulate the f...

Background Muscle strains are one of the most common complaints treated by physicians. High-force lengthening contractions can produce very high forces resulting in pain and tissue damage; such strains are the most common cause of muscle injuries. The hamstring muscles are particularly susceptible as they cross two joints and regularly perform lengthening contractions during running. We describe a patient with return to full function after a large hamstring tear. Case Description We report the case of a 26-year-old man who presented 1 year after a noncontact, left-sided proximal hamstring tear incurred while sprinting. He received no medical treatment or formal rehabilitation. He was able to return to all sports and activities 1 to 2 months after injury, but noted a persistent deformity of the proximal thigh, which led him to seek evaluation. Physical examination, MRI functional tests, and specific muscle tests 1 year after his injury documented a major hamstring tear at the musculotendinous junction with muscle retraction, but no avulsion of the proximal tendon attachment. Literature Review Surgery often is recommended for major proximal hamstring tendon tears, especially when more than one tendon of origin is ruptured from the ischial tuberosity. Myotendinous tears are treated nonoperatively, but may be associated with decreased strength, prolonged recovery, and recurrence. Purpose and Clinical Relevance We describe the case of a young man who sustained a hamstring tear, with retraction, at the proximal myotendinous junction, where the biceps femoris and semitendinosus arise from the conjoint tendon. He achieved full functional recovery without medical attention, but had a persistent cosmetic deformity and slight hamstring tightness. This case suggests a benign natural history for this injury and the appropriateness of noninvasive treatment.