The “Marriage” of Stretch Shortening Cycle (SSC) + Elastic Energy = Velocity
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Power and Performance Enhancement by Stored Elastic Energy in the Muscle-Tendon Complex and the Coupling Time
Throws are powered by rapid, sequential activation of many muscles, starting in the legs and progressing through the hips, torso, shoulder, elbow and wrist1. Torques generated at each joint accelerate segmental masses, creating rapid angular movements that accumulate kinetic energy in the projectile until its release. It has been shown that internal (medial) rotation around the long axis of the humerus is the largest contributor to projectile velocity. This rotation, which occurs in a few milliseconds and can exceed 9,000°/sec, is the fastest motion the human body produces. Although previous research has focused on the internal rotator muscles of the shoulder, these muscles alone cannot explain how humans generate so much internal rotational power. For one, calculations of the maximum power production capacity of all the shoulder’s internal rotator muscles indicate that these muscles can contribute, at most, half of the shoulder rotation power generated during the throwing motion. Peak internal rotation torque also occurs well before the humerus starts to rotate internally. Furthermore, variation in muscle fiber orientation in these muscles produce actions other than internal humeral rotation that reduce power output for this action.
Elastic energy storage has been shown to be an important source of power amplification for many high-powered movements. We propose that several evolutionarily novel features in the human shoulder help store and release elastic energy to generate much of the power needed for rapid humeral rotation during human throwing. According to this model, energy storage occurs during the arm-cocking phase, which begins with completion of a large step towards the target. As the foot hits the ground, the arm is already externally rotated, horizontally extended, and abducted nearly 90° at the shoulder, with forearm flexion approaching 90° at the elbow. As the cocking phase begins, large torques are generated by rapid rotation of the torso towards the target and by the activation of the major shoulder horizontal flexor, Pectoralis major. The positioning of the shoulder and elbow at this time increase the mass moment of inertia around the long axis of the humerus, causing the forearm and hand to lag behind the accelerating torso. Further, a flexed elbow during the cocking phase allows passive inertial forces to externally counter rotate the arm, stretching the short, parallel tendons, ligaments, and elastic components of muscles that cross the shoulder, potentially storing elastic energy in the large aggregate cross-sectional area of these structures. Then, when the biceps deactivate and elbow extension begins, the arm’s moment of inertia is reduced, allowing these stretched elements to recoil, releasing energy, and helping to power the extremely rapid internal rotation of the humerus.
However, it is well known that when muscle is actively stretched during or just prior to concentric contraction there is a notable increase in force production. Plyometric models propose that the pre-stretching of the shoulder internal rotator muscles during arm-cocking augment force enhancement during the subsequent acceleration phase of the throw. Accordingly, we also compared our average actual power values to force enhancement adjusted (1.75x 93) values of maximum muscle force, ranging from ~438-875 W kg-1. Even with this significant force enhancement, our measured average rotational power in the shoulder exceeds the maximum muscle values by 2-4 times. Thus, in order to achieve this joint power using muscular power alone, the individuals in this analysis would require internal rotator muscles at least 2-4 times larger than those used here. Given that the muscle masses used are the largest published 96, this is highly unlikely and suggests that elastic energy stored in the shoulder is used to enhance the internal rotation power output.
Next: Part 6 Dynamic Systems, Constraits and Butterflies…..