CHAPTER 1 Principles of Kinesiology and Biomechanics 033


.ft 33 33 .lb = IMF Joint Position and Forces IMF EMA2 EMA1 ELF DW


Figure 1.24 The arm weight and the dumbbell weight with the external moment arm for each weight together increase the amount of muscle force required to maintain the arm in a static position.


When the torques are unequal, there is movement in the direction of the greater torque. When the biceps muscle contracts and brings the hand toward the shoul- der, the torque generated by the muscle is greater than the opposing torque generated by the weight of the limb segment. To understand the clinical application of adding weights to extremities in terms of increasing external forces, examine the same body diagram with the hand now holding a 5-lb dumbbell weight (Fig. 1.24). The external torque is calculated as follows:


ELF = Arm Weight = 5 lb EMA1 = 0.7 ft


DW = Dumbbell Weight = 5 lb EMA2


External Torque ELF EMA DW EMA lb


12 ×


=× + 5 = .- + . =11ft-lb


507 ft lb ×1 5 ft 35ft-lb 75ft-lb


..


If the external torque is now 11 ft-lb, the biceps will need to generate 33.33 lb of force to hold the weight in the static position shown in Figure 1.24.


External Torque ft-lb IMF IMA 11ft-lb IMF ft=× .0 33 11ft-lb IMF=


11 =×


of DW = External Moment Arm of DW = 1.5 ft =× +


The X and Y components of forces, whether internal forces produced by muscles or external forces produced from resistance, change in magnitude depending on the angle of the joint. The angle formed between the long axis of a bone and the tendon of the muscle where it inserts on the bone is called the angle of insertion. When the angle between the muscle insertion and the bone changes as the bone moves through a range of motion, it affects the components of the vector force produced by the muscle. Figure 1.25 depicts a vector representing the biceps muscle at the elbow and its component X and Y infl uences at various angles of insertion. At the extreme ends of range of motion of the elbow, whether near full extension or full fl exion (Fig. 1.25A), the angle of inser- tion is small, and the Y component that represents muscle force is minimal compared with the X component. As the angle of insertion approaches 45°, as seen in Figure 1.25B, the infl uences of the motion component Y and the joint compression component are equal. When the angle of insertion is at 90°, the Y component of the force and the total muscle resultant force are equal. This is the position where the biceps muscle is capable of generating the greatest amount of muscle torque (Fig. 1.25C). The infl uence of joint angle on the X and Y components of a force contributes to the understanding of how muscles can generate a greater torque (strength) at certain ranges of motion compared with other ranges.


FORCE COUPLES


A force couple occurs when two parallel forces produced by two or more muscles act simultaneously in opposite directions to produce a rotary motion of a body segment. Although the linear forces produced by the muscles act in different directions, the resultant torque is in the same rotary direction. A steering wheel is a good analogy for explaining a force couple. Place two hands on a steering wheel, one in the 1 o’clock position and the other hand in the 9 o’clock position. As one hand pulls down and the other pushes upward, the steering wheel turns in a circle (Fig. 1.26).


There are several examples of force couples in the human body when two or more muscle groups work together to rotate a body segment. Figure 1.27A illus- trates the force couple that the rectus abdominis, gluteus maximus, and hamstring muscles generate to produce a posterior tilt of the pelvis. As the rectus abdominis muscle contracts, it pulls the anterior portion of the pelvis supe- rior, while the gluteus maximus and hamstring muscles pull the posterior portion of the pelvis inferiorly, produc- ing a posterior rotation of the pelvis. Another example


23


IMA


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