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PART I FOUNDATION CONCEPTS


increases the passive force produced by the connective tissue. This passive force signifi cantly adds to the power produced by the muscle. The relationship between speed and intensity of the stretch is a concept that supports plyometric activities and exercises. During a plyometric exercise such as a vertical jump, the musculotendinous structures are stretched before a concentric contraction occurs. Before jumping, a person typically bends slightly at the hips and knees, a position that stretches the struc- tures associated with hip and knee extension and ankle plantar fl exion. This stretch of the connective tissues adds to the force generated by the muscles to produce a greater torque during the contractions that extend the hips and knees and move the ankle into plantar fl exion during the jump.


ACTIVE LENGTH-TENSION RELATIONSHIP


The optimal length of a muscle for an active muscle contraction is the length that allows the greatest number of cross-bridges to form in the sarcomeres. The active length-tension curve (Fig. 3.14) depicts the relationship between sarcomere length and isometric muscle contrac- tions. The beginning of the curve shows that a muscle and sarcomere length can be too short, causing an overlap in the cross-bridges and a decreased ability to generate active tension. As the sarcomere lengthens, there is an


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optimal length for actin and myosin binding. Once the length increases past this range, the actin and myosin are farther apart, and the muscle has less and less ability to produce active tension.


TOTAL LENGTH-TENSION RELATIONSHIP


The active tension produced by the contractile muscle tissue and the passive tension generated by the noncon- tractile connective tissue components produce a total system capable of producing forces throughout the lengthening of a muscle. At shorter lengths, the tension is generated by the contractile structures. Once the muscle is stretched beyond its resting length, the noncontractile tissues begin to contribute to the force as the contractile contribution decreases (Fig. 3.15). This active force and passive tension force allow muscles to generate a large range of force over a wide range of muscle length. In addition to muscle length, other factors, includ- ing the moment arm of the muscle, contribute to the total force generated. As discussed in Chapter 1, the torque generated is a product of the muscle force and the moment arm. There is also a relationship between the speed of shortening and the ability of the muscle to develop tension.


MUSCLE EXCURSION 50 A Total tension D 0 12 34 Length of sarcomere


Figure 3.14 The active length-tension curve. At point A, when sarcomeres are too short, the overlap between the actin and myosin reduces the number of cross-bridges that can form to produce muscle activation. At the optimal length of a muscle (points B and C), the actin and myosin are positioned to form the greatest number of cross-bridges. Between points B and C on the graph, the muscle has the greatest potential to generate an active force. At point D, as the sarcomere is lengthened, the actin becomes more out of range with the myosin, and the number of potential cross-bridge formations decreases. The sarcomere has far less potential to generate an active force.


Active tension Muscle length


Figure 3.15 The total length-tension curve. At shorter muscle lengths, the force is generated by contractile structures. Once a muscle is stretched beyond its resting length, both contractile and noncontractile tissues play a role in generating force. As the muscle is stretched further, passive forces increase, and active forces decrease.


Passive tension B C


For agonists and antagonists to work together during functional movement, each must be able to lengthen as its counterpart contracts and shortens. As the biceps contracts, shortens, and fl exes the elbow, the antago- nist triceps must lengthen for the movement to occur. Muscles can change length passively and actively. The


Active tension (percent)


Muscle tension


Resting length


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