Final answer:
The extensor muscles in the leg are considered anti-gravity because they combat the force of gravity, helping maintain an upright posture. They are essential both on Earth for movement and posture and in space to counteract the effects of weightlessness on the body's musculature and bone density.
Step-by-step explanation:
The extensor muscles in the leg are considered anti-gravity because they help in maintaining an upright posture and counteracting the force of gravity.
For example, when standing on one's toes, the extensor muscles located at the back of the leg contract and pull on the Achilles tendon, which in turn, raises the heel off the ground. This action is akin to operating a lever system, where contraction of the extensor muscles contributes to the lifting of the body against gravity's pull.
Astronauts experience weightlessness in space, highlighting how crucial gravity is for maintaining bone and muscle health.
Without the normal force of gravity, bones can lose mass, and muscles can weaken. On Earth, our muscles, especially the extensors, work continuously to combat gravity to keep us upright.
This is also why specialized fitness equipment is necessary in space, to replace the anti-gravity effects of the leg extensors and maintain muscle tone and bone density.
The skeletal muscle pump mechanism is another example of the body's anti-gravity design. The leg muscles, including the extensors, help to move blood from the lower pressure veins in the legs back to the heart, working against gravity.
If these muscles did not contract, as happens when standing still for too long, blood can pool in the legs leading to fainting. Thus, military recruits are trained to flex their leg muscles slightly to prevent this.