They all use levers that have a mechanical advantage that is less than one.
The arrangement provides for greater speed of the limbs.
Nature likes speed to strength.
The speeds at the limbs are remarkable.
A skilled pitcher can throw a baseball at a high rate of speed.
This is the speed of his hand when he releases the ball.
The hip joint and simplified lever representation are typical of a male body.
The angle of this force with respect to the horizon is about 71.
The weight of the body is supported by this force.
The result is used in Eq.
The calculation shows that the force on the hip joint is more than the weight of the person.
The hip joint has a force of 1625 N.
Persons who have an injured hip limp by leaning toward the injured side as they step on that foot.
As a result, the center of gravity of the body shifts into a position more directly above the hip joint, decreasing the force on the injured area.
The forces applied during a one-legged stance have been reduced.
The spine pivots when the trunk is bent forward.
We will look at the forces involved when the trunk is bent at 60* from the vertical.
The effect of 1 can be seen by a weight suspended in the middle.
Two people are at the end of a lever arm.
The muscle has an angle between it and the spine.
320 N is 72 lbs and 160 N is 36 lbs.
The solution of the problem is an exercise.
A person is walking on an injured hip.
This example shows how large forces are exerted on the fifth lumbar vertebra.
It's not surprising that backaches are the most common at this point.
The recommended way of lifting a weight is not shown in the figure.
This lever is a Class 1 lever.
The balancing force is provided by the muscles on the heels.
The position of standing on tiptoe is strenuous.
The forces on the human body are static.
They are constant in time.
The human body is a dynamic system that responds to stimuli generated internally and by the external environment.
Because the center of gravity is less than the height of the soles of the feet, even a slight displacement can topple the body.
The simple act of standing upright requires the body to be in a constant back and forth, left and right swaying motion, keeping the center of gravity over the base of support.
In a typical experiment designed to study this aspect of posture, the person is instructed to stand, feet together, as still as possible, on a platform that records the forces applied by the soles of the feet.
To compensate for the shifting center of gravity, the center of pressure is constantly shifting over the area of the soles of the feet on a time scale of about half a second.
The ankle movements compensated for small back-and-forth perturbations of the center of mass.
Hip movements have to compensate for larger displacements.
As the support for the center of gravity shifts from one foot to the other, a series of compensating movements are needed to maintain balance.
Keeping the body upright is a very complex task for the nervous system.
The performance of this task is remarkable when we slip and the center of gravity is displaced.
An erect human body that looses its balance will hit the floor in about 1 second.
The whole muscular system is called into action by the "righting reflexes" to mobilize various parts of the body so as to shift the center of mass back over the base of support.
In the process of restoring balance, the body can perform amazing contortions.
The nervous system can get information from three sources: vision, the vestibular system and the somatosensory system.
An increasing number of injuries due to falls are caused by the decreasing efficiency of the functions required to keep a person upright.
The United States has a higher rate of accidental deaths due to falls for people over the age of 80 than it does for people under the age of 70.