Final answer:
Good posture involves aligning the body's center of gravity over the pivot at the hips to minimize muscle strain, whereas poor posture creates additional torque that muscles must counteract, leading to muscle fatigue and strain. These concepts are important when discussing posture during physical activities, including curl-ups and weightlifting, and can explain cases of extreme fatigue and muscle strain.
Step-by-step explanation:
Understanding the mechanics of good posture and muscle strain due to poor posture can explain why certain positions, like incorrect posture during a curl-up, lead to excessive muscle fatigue. For good posture, as described in Figures 9.26 and 9.29, the center of gravity (cg) of the upper body is aligned above the pivot point at the hips, minimizing the need for muscle exertion to maintain balance. This condition exerts no torque about the hips.
In contrast, poor posture shown in Figures 9.26 and 9.29 occurs when the upper body's cg is positioned in front of the hips, generating a clockwise torque that must be counteracted by the muscles in the lower back. These muscles are at a mechanical disadvantage due to a smaller effective perpendicular lever arm compared to the center of gravity, forcing them to exert significant force to maintain balance. This prolonged and excessive force leads to back strain and muscle fatigue.
When considering actions like lifting weights, as in a biceps curl described in section 9.6, the forces exerted by the muscles are not equivalent to the weight of the object being lifted, further highlighting complexities in muscular and skeletal mechanics. Case studies, like the tired runner, illustrate the impact of muscle fatigue and strain in real-life scenarios, leading to symptoms such as extreme fatigue, muscle cramps, and spasms, which can significantly impair physical performance.