Optimizing Propulsion in Swimming by Rotation of the Hands

Transcription

1 Optimizing Propulsion in Swimming by Rotation of the Hands B. Ungerechts The coach who is attempting to assist less talented swimmers is forced to increase his knowledge of the biomechanics of swimming because the average swimmer needs explicit instructions to approximate the proficiency of a champion. Biomechanics may be able to determine if the functional properties of the locomotor apparatus might fit the mechanical or the hydrodynamic demands. The hydrodynamic principles of competitive swimming have been recognized and listed (Reischle, 1976). The application of these principles in coaching or teaching, however, is in its early stages. Normally, it is hoped that swimljl~rs acquire a feeling for the details of highly skilled swimming through trfal~and error. In the early stages of learning, while obtaining the idea of the gross pattern this may be reasonable and sufficient. However, in the advanced stages this sort of learning will take a long time, perhaps too long for the swimmer to remain competitive. Knowledge of the functional properties of the locomotor system may help the swimmer to improve his performance. PURPOSE After the curvilinear stroke pattern made its way into competitive swimming the question arose as to whether the hand should be oriented normal to the flow or should be pitched. Hands, moved in a direction perpendicular to the oncoming flow, as in the paddles of a canoe, produce drag forces. Pitching th~ hand will create a lift force (Schieihauf, 1974), by which propelling th~tt in swimming direction is increased. The production of lift force is related to an optimal angle of attack {the angle between the axis of the hand and the actual oncoming direction of flow) and also to a correct hand position, with fully extended fingers that 55

2 56 Ungerechts coracobrachialis Figure 1. Muscles responsible for flexion of the humerus. are slightly opened (Lohr and Ungerechts, 1976). This rough explanation of lift force production holds true only when the flow velocity does not change. Because of the changing velocities of the hand (in magnitude or direction) during a stroke cycle, the angle of pitch (angle between the surface of the water and the axis of the hand) must be adjusted constantly. This fact has been derived from the study of twisted propeller blades of airplanes (Bilo and Nachtigall, 1977). Therefore, all phases of a stroke cycle must be considered, not only selected sequences that are oriented toward the "S" -pattern stroke. The latter fulfills a hydrodynamic law, which says that "efficient propulsion is obtained by pushing a large mass of water a short distance without much acceleration" (Counsilman, 1971). However, the "S" pattern itself does not lead to lift force production per se. It is also known that fishes control the angle of pitch in each phase of their stroke, but this seems natural because they are adapted to swimming. This article shows that a backstroke swimmer, although not adapted specifically to swimming, could do the same. ACTIVITY OF MUSCLES Descriptions of the activity of brachial muscles in relation to the various phases of the stroke cycle are mostly confined to semiempirical data because EMG studies during swimming are limited. The line of action of the force developed by an individual muscle follows from its loci of origin and insertion. In Figure 1 the line of action of the muscles during flexion of the shoulder joint is shown. The principle force is supplied by the anterior fibers of the deltoid, the supraspinatus, and the coracobrachialis. The pectoralis major with its da\'icular fibers and the long head of the biceps brachii are involved to a lesser extent.

3 Optimizing Propulsion 57 teres major triceps brachii Figure 2. Adductors of the arm. The adductors, which lower the lifted arm, are shown in Figure 2. The main adductors are the pectoralis major and the latissimus dorsi, which are strongly assisted by the teres major. The long head of the triceps brachii is also a strong adductor, in a lifted arm. The pectoralis major and the teres major are also involved in medial rotation of the arm. They support the action of the subscapularis, which is the most powerful muscle for this turning motion (Figure 3). The biceps brachii flexes the shoulder joint as well as the elbow joint. Its distal tendon is attached to the radius. The radius, carrying the hand with it, articulates with the ulna. Movements in the proximal radioulnar joint result in pronation or supination of the hand (Figure 4). Pronation is affected by the pronator quadratus and pronator teres. (In Figure 4 these muscles are prepared to twist the hand.) The radius is carried across the ulna, the thumb is moved toward the body, and the tendon of the biceps brachii is wound around the radius. In supination, the radius is situated lateral to and parallel with the ulna and the thumb is positioned laterally. subscapularis teres major Figure 3. Muscles responsible for medial rotation.

4 58 Ungerechts biceps~~ pronator teres ----+:11 radius-----~ ulna H Figure 4. Supination (left) and pronation (right) with position of the tendons of the biceps brachii. The most powerful supinator is the biceps brachii, which exerts its greatest flexing moment when the forearm is in supination. The only extensor of the elbow joint is the triceps brachii (Figure 2), which influences two joints. Its long head may assist in pulling a lifted arm backward and adducting it to the thorax. ANALYSIS OF MUSCLE ACTION DURING THE ARM CYCLE IN BACKSTROKE At the beginning of the "transitional phase" the arm is extended, and the palm of the hand points toward the bottom of the swimming pool. The specific muscles contributing to the lifting of the arm out of the water correspond to those of tlexion (Figure 1). When the hand is lifted out of the water, it is in a position between supination and pronation. It is then placed in "extreme pronation" by the pronator teres and pronator quadratus, and the tendon of the biceps brachii is wound around the radius. Having passed the "perpendicular position" above the body during the "recovery phase", the arm is under the influence of gravity. The following movement of the arm is controlled mostly by the muscles shown in Figure 1.

5 Optimizing Propulsion 59 y Figure 5. Proposed hand position during the backstroke ("pull phase") L, lift; D, drag; T, thrust. During the "introductory phase," in which "essential preliminary movements for the production of propulsion are created," (Wiegand, Wuensch, and Jaehnig, 1975), the arm is lowered deeper into the water. The elbow joint remains extended. The swimmer starts to roll about a longitudinal axis with the hand still in a pronated position. At the beginning of the "propulsive phase," the hand starts to twist as a result of the action of the biceps brachii and the supinator. The innervation of the biceps brachii causes three effects; it: 1) supinates the forearm, 2) adducts the arm, and 3) flexes the elbow. Having passed its deepest position in the water, the hand, still supinated, is turned back in pronation by the action of the pronator teres. Moreover, the pronator teres flexes the elbow joint, assisting the biceps brachii. The hand should be rotated according to the flow pattern. Bending of the elbow brings the hand near the surface. In its highest position, the palm is not oriented normal to the flow but is pitched. The elbow joint is flexed to an extent but not to its maximum. The arm is perpendicular to the body. This part of the propulsive phase is known as the "puil-phase" (Figure 5). While adductive motion is maintained, the flexed elbow is extended. The velocity of the hand is increased beyond that of the elbow. The elbow joint is extended by the contraction of the triceps brachii. The shortening of the adductor muscles (Figure 2), which have been contracted all the time, leads to medial rotation of the arm. Because of their morphology and line of action the adductor muscles create an increasing rotary component when the arm is close to the body. By vectorial addition of the three movements (extension, adduction, and medial rotation), the hand is forced toward the feet on a curved line. The subscapularis completes the medial rotation of the arm, and intiates the raising of the shoulder out of the water to make the f'mal

6 60 Ungerechts recovery of the arm much easier. At the end of the "propulsive phase," the palm of the hand is directed toward the bottom of the swimming pool. CONCLUSION The analysis of the specific muscles contributing to actual performance in the arm movements of the backstroke shows a useful interplay of the action of the muscles and hydrodynamic demands, especially rotation of the hand. At the beginning of the "propulsive phase" the stretched arm is flexed. The muscle initiating this flexion-the biceps brachii-generates its best mechanical efficiency when the forearm and the hand are in a position of supination. Because the hand is first pronated, it is necessary to place the hand thereafter in supination. This occurs just before the hand reaches its deepest position in the water. Twisting of the hand facilitates the action of the biceps brachii. Moreover, the pronator teres is tensed and therefore is better prepared to assist in flexion of the elbow. Its contraction pronates the forearm, and the hand assumes a position, wherein lift force may be produced (Figure 4). This hand pattern is different from the popular opinion that the palmar surface should be oriented normal to the flow and should be moved normal to the direction of body motion. In the latter case propulsion is accomplished by drag. As Counsilman (1971) has shown, this is an ineffective way of producing propulsion, and will require more energy to obtain the same swimming speed compared with a pitched hand. The mechanical efficiency of the thrust-producing muscles is increased because the lift forces dominate the drag forces. Therefore, lift production should predominate not only during the "pull phase" but also during the "push phase" of the arm stroke. A swimmer has to alter slightly the coordination of his arm segments because he never experiences constant velocity since there are definite fluctuations in both magnitude and direction in each phase of the stroke. Conditions are also changed at higher swimming speeds. Assumptions about the magnitude of the angle of attack cannot be made because little evidence is available about hand speed relative to a three-dimensional reference frame. REFERENCES Bilo, D. and Nachtigall, W Biophysics of bird flight: Question and results. Fortschr. Zooi. 24(2/3): Counsilman, J. E The application of Bernoulli's principle to human propulsion in water. In: L. Lewillie and J. P. Clarys (eds.), First International Symposium on Biomechanics in Swimming, pp Universitie Libre de Bruxelles, Brussels.

7 Optimizing Propulsion 61 Uihr, R., and Ungerechts, B Experimentelle Bestimmung der optimalen Fingerstellung beim Kraulschwimmen (Experimental determination of the optimal finger position in crawl swimming). Leistungssport 4: Reischle, K Das Antriebsproblem beim Schwimmen (The propulsive problem in swimming). Leistungssport 4: Schleihauf, R. E A biomechanical analysis of freestyle. Swimming Tech. 11:89. Wiegand, D., Wuensch, D., and Jaehnig, W The division of swimming strokes into phases, based upon kinematic parameters. In: L. Lewillie and J. P. Clarys (eds.), Swimming II, pp University Park Press, Baltimore.