Chapter 12, Problem 53AP

When a circus performer performing on the rings executes the iron cross, he maintains the position at rest shown in Figure P12.37a. In this maneuver, the gymnast’s feet (not shown) are off the floor. The primary muscles involved in supporting this position are the latissimus dorsi (“lats”) and the pectoralis major (“pecs”). One of the rings exerts an upward force ${\stackrel{\to }{F}}_k$ on a hand as show n in Figure P12.37b. The force ${\stackrel{\to }{F}}_s$, is exerted by the shoulder joint on the arm. The latissimus dorsi and pectoralis major muscles exert a total force ${\stackrel{\to }{F}}_m$ on the arm. (a) Using the information in the figure, find the magnitude of the force ${\stackrel{\to }{F}}_m$ for an athlete of weight 750 N. (b) Suppose a performer in training cannot perform the iron cross but can hold a position similar to the figure in which the arms make a 45° angle with the horizontal rather than being horizontal. Why is this position easier for the performer?

Figure P12.37