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4 often required. Skilled hands-on work can facilitate the recovery process, of course—more about this later. Not only do hamstrings work hard, but they also often feel hard. Hamstrings are notoriously tight, and sometimes seem impervious to all attempts to lengthen them. Their resilience may be related to their function. We use them as postural muscles whenever upright, and connective tissue resilience is much more efficient than muscular contraction when continuous tension is needed. Turkeys take this a step further by developing ossifications within their leg muscles. Like us, turkeys are bipeds that spend a great deal of time standing around (Image 3). To better accommodate the demands of standing, turkeys have intermuscular septa and tendons within their leg muscles that often ossify into long, thin, intramuscular bonelike structures. Although our own human hamstrings may sometimes feel as if they've ossified, their function is both to resist stretching, and to spring back. Researchers such as Robert Schleip2 and Serge Gracovetsky3 have each described models of gait and movement based on the soft tissue's ability to store and release energy via elastic recoil. Schleip writes about how kangaroos hop much farther and faster than can be explained by the contractile force of their hamstring and leg muscles alone. Kangaroos (Image 4) use a kind of "catapult effect" to load and unload their springy leg tendons.4 Rather than relying solely on Turkeys develop bonelike structures within their hamstring tendons; they also spend much of their lives standing around. Kangaroos jump farther and faster than can be explained by muscular contraction alone—their leg tendons elastically stretch and recoil to store and release energy. 3 muscular contraction for their jumps, kangaroos use the springiness of their leg tendons to store the energy of landing, releasing it into the next hop. These springlike mechanisms have also been observed in antelope and humans. Ultrasound observation of human muscle during use (in this case, oscillatory loading motions such as hopping or jumping) shows more-than-expected tendon stretch and recoil, and less-than-expected muscle fiber shortening. Instead of shortening, muscle fibers were observed to isometrically stiffen, thereby tuning and pre-tensioning the springy tendons. One study showed that 66–76 percent of the work involved in jumping was accomplished by stored energy within the tendinous portion of the calf's muscle-tendon complex, with only 24–34 percent originating from muscle contraction itself.5 Other fibrous connective tissues, including aponeuroses and intermuscular septa, likely contribute to similar springlike functions. Hamstrings don't work alone. They function in concert with other myofascial and connective structures, both nearby and elsewhere in the body. The hamstrings are links in the long chains of fascial relationships that include the sacrotuberous ligament (which is aligned with the biceps femoris, sometimes sharing 4 the same collagen fibers, and acting as a continuation of the hamstrings' force vector). In a typical cross-stride in walking or running, the gluteus maximus, the lumbodorsal fascia, and the opposite-side latissimus dorsi continue this line of connection into the contralateral arm. The hamstrings work with other muscle groups in a variety of ways. In walking and running, the hamstrings decelerate and control the lower leg's kick-through, caused by the strong contraction of the quadriceps. One leg's hamstring muscles can all contract together, producing the powerful stride of hip extension combined with knee extension (Image 5, page 116). Or, the muscles can work individually, as they do when stabilizing and balancing It pays to be ABMP Certified: www.abmp.com/go/certifiedcentral 115