Photograph at the top shows arm model in neutral force configuration

Force applied by the biceps (lower left), pulling up with the hand: Apply 2.5 kg to the biceps cable to support the unloaded forearm. The forearm may be kept at equilibrium by the simultaneous addition of masses in the ratio of 10:1 at the biceps and at the hand. The torques are balanced almost independently of the angular position of the arm.

Force applied to the triceps (lower right), pushing down with the hand: Hang the spring scale between the top hook and the hand hook, and attach the hanger to the triceps cable. Add masses to the hanger to determine how much force in the triceps is necessary to push down with the force read on the scale.

A subject not sensitive about his or her weight stands very quietly on the scale. Observers will notice that the measurement is not extremely stable, but rather has a periodic short dip in the reading lasting a small fraction of the time between heartbeats.

The blood flows out of the heart in the upward direction. The blood to the head continues upward through the carotid artery. However, the blood servicing the lower part of the body flows upward out of the heart through the aorta, which the has a sharp bend that deflects the blood downward. The aorta exerts a downward flow on each burst of blood (called a "bolus"), which in turn exerts an upward reaction force on the aorta. This upward force is transferred to the body, which experiences a short period during which the body weight is decreased by the amount of force the bolus exerts on the aorta. This is seen in the practice as a series of small downward blips in the scale reading. Click your mouse on the link, below, to see a slow motion rendering of the weight as a function of time.

The study of this and related effects is known as "ballistocardiography." Ballistocardiography is a useful tool in determining the strength of heart muscles by directly measuring the blood flow; an electrocardiogram (ECG) measures the activity of the muscles in the heart less directly.

Since we cannot easily create a long duration microgravity environment in the classroom, this demonstration instead models this effect with the expansion being caused by a change in air pressure. A column of discs is separated by marshmallows, and the whole placed into a vacuum chamber. As the pressure is reduced, the gas in the marshmallows expands, forcing apart the spinal discs. When pressure returns, the spine collapses back, much like astronauts experience when they return to Earth.

Challenge your students to think of other body parts and processes that could be similarly affected by changes in gravity.

Force applied by the biceps , pulling up with the hand: Apply 2.5 kg to the biceps cable to support the unloaded forearm. The forearm may be kept at equilibrium by the simultaneous addition of masses in the ratio of 10:1 at the biceps and at the hand. The torques are balanced almost independently of the angular position of the arm.

Force applied to the triceps, pushing down with the hand: Hang the spring scale between the top hook and the hand hook, and attach the hanger to the triceps cable. Add masses to the hanger to determine how much force in the triceps is necessary to push down with the force read on the scale.