Figure 1.27 Force couples in the human body. (A) The gluteus maximus, hamstrings and rectus abdominis muscles work together with forces in the opposite direction to produce a posterior tilt of the pelvis. (B) The upper and lower trapezius and the serratus anterior muscles produce a force couple to rotate the scapula in an upward direction during arm elevation.
Forces are measured in terms of their magnitude and direction. The composition of vector forces can deter- mine the total result of several forces. A specifi c force can be broken down into its components through resolution of vector forces. Motion in the human body is usually produced by forces causing rotary motion around a joint axis. This motion represents a torque force. When two parallel forces acting together but in opposite directions
produce a rotary motion of a body segment, they form a force couple. The concepts presented in this chapter lay the foundation for the study of human movement and how forces can produce movement and increase func- tion or decrease movement while providing stability. The study of forces also contributes to an understanding of how excessive or inappropriate forces can play a role in tissue and structural damage that results in dysfunction.
