Applied anatomy and kinesiology; the mechanism of muscular movement . 10 Fig. 15.—Conditions of action of a muscle acting on the elbow-joint to lift a weightin the hand: <S, shoulder; E, elbow, M, muscle; H, hand; L, lever. To illustrate fully how the muscular requirement is influencedby these elements of leverage and how to attack such problems,let us inquire with what force a muscle acting on the elbow-jointmust pull to lift 10 pounds in the hand when the forearm is 45degrees above the horizontal, the muscle-arm being 2 inches, theweight-arm 12 inches, and the angle of pull of the muscle
Applied anatomy and kinesiology; the mechanism of muscular movement . 10 Fig. 15.—Conditions of action of a muscle acting on the elbow-joint to lift a weightin the hand: <S, shoulder; E, elbow, M, muscle; H, hand; L, lever. To illustrate fully how the muscular requirement is influencedby these elements of leverage and how to attack such problems,let us inquire with what force a muscle acting on the elbow-jointmust pull to lift 10 pounds in the hand when the forearm is 45degrees above the horizontal, the muscle-arm being 2 inches, theweight-arm 12 inches, and the angle of pull of the muscle 75 degrees. The conditions of this problem are illustrated by Fig. 15. Evi-dently the weight will act upon the lever so as to resist the action ofthe muscle with a force equal to 10 pounds multiplied by the sineof 45 degrees, or pounds. This multiplied by its lever-arm( X 12) gives inch-pounds to be overcome by the actionof the muscle. From the law of levers we have / X 2 = , or 38 THE BONES AS LEVERS. f = pounds. This is the effective force that must be producedby the action of the muscle at an angle of 75 degrees (sine = ).We wish to find F, so in the formula / = F X s we substitute the known quantities, giving the formula, = F X , or i^ = -^, from which F or the whole forceof contraction is pounds. In applying the general principles ofleverage to bones it is necessary to bearin mind that the two arms of a lever aretwo straight lines drawn from the twoother points to the axis; in some casesthese two may form one and the samestraight line, but usually not. In case ofthe humerus, for example, the point ofcontact with the scapula that serves asthe axis of the shoulder-joint is an inchor more to one side of the shaft of thebone; as a result the two lever armsmeetat a rather large angle, as shown in In most cases we have one principalresistance, and therefore one resistance-arm, with several muscles acting, e
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