Our bodies can absorb great stress. The brain, muscles and skeleton all help to reduce the forces in the body. Our camera shows these forces in action during a vertical jump.
Zero gravity isn't good for the body. To perform the same standing jump requires less work. In fact doing anything requires less work which means muscles begin to break down. Without any load bones stop making new cells and lose calcium at an increased rate. This even affects the bone marrow with the number of red blood cells also falling.
Without gravity acting on the skeleton the spine can actually lengthen by as much as 8mm. This growth can lead to blocked nerves and back pain, which can also lead to decreased touch sensitivity. In order to run longer space missions NASA is researching how to stop bone loss. If successful it could help not only astronauts heading to Mars, but also patients with limbs in plaster and those with the condition osteoporosis.
While jumping on the spot may seem easy to most of us, researchers in biomechanics can show that there are many things happening together, all coordinated by our brain.
Firstly we need to generate the power to leave the ground, this means overcoming gravity. Watch how you start the jump, you probably dropped into a crouch. Crouching puts tension into your tendons and muscles (think of them like pieces of strong elastic), this tension is the potential energy that can be converted into the jump. Remember that tendons attach muscles to the bone, while ligaments hold bones to other bones. We also tend to use our arms which does make us jump higher by changing the amount of work carried out by the lower joints.
Upon landing the body must then cushion itself against the forces. Gravity will have accelerated the body towards the ground so there is work to bring it to a halt. The springy arch of the foot will absorb some of the force and the hips and knees will bend.