Final answer:
Weak core, abdominal, and low-back muscles are causes of poor posture, leading to back strain and muscle fatigue. These muscles are responsible for stabilizing the body's center of gravity, ensuring proper alignment, and when weak, can lead to debilitating discomfort and potential injury.
Step-by-step explanation:
Weak core, abdominal, and low-back muscles are primary causes of poor posture. Good posture, as depicted in Figure 9.26 (a) and Figure 9.29 (a), requires minimal muscular exertion because the center of gravity (cg) is aligned directly over the hips and base of support, meaning no torque is produced at the hips. In contrast, poor posture shown in Figures 9.26 (b) and 9.29 (b) demands significant muscle action.
Poor posture results in a clockwise torque about the hips due to the upper body's cg being located in front of the pivot point in the hips, which in turn requires the back muscles to exert a sizable force to counteract this torque, leading to back strain.
Furthermore, maintaining a balanced posture is a complex task, which requires the muscles of the vertebral column, thorax, and abdominal wall to extend, flex, and stabilize various parts of the body's trunk. If these muscles are weak, they might fail to function effectively, compromising body posture and potentially contributing to muscle strain and discomfort. Ergonomic solutions, like specially designed chairs, can help locate the body's cg above the base of support to alleviate back pain caused by prolonged poor posture.