A lever is a tool that consists of a rigid beam that rotates around a pivot called the fulcrum of the lever. Levers use the leverage concept, in which an input force is multiplied to provide a bigger output force and vice versa. The ratio of the output force to the input force is called the mechanical advantage.

For an ideal lever that does not cause energy loss due to friction or wear:

 The input force (The effort)

a The distance from the fulcrum to 

 The output force (The load)

b The distance from the fulcrum to

The mechanical advantage is given by:

For levers that have a mechanical advantage greater than one, a smaller force can be multiplied to move and lift massive objects.

An average human of 60 kg can walk, run, dance, or walk upside down on their hands with relatively little force compared to the force needed for an object of the same mass (60 kg). An average human can run at an average speed of up to 12 km/h and walk an average distance of up to 20 miles in one day. An average human of 60 kg can climb up to 300 meters, but if a mass like a rock of 20 kg is fixed to his body, the maximum height the human can climb reduces to several meters and the maximum distance he can walk reduces to tens of meters, even though we only add 20 kg. If the human is 60 kg, then the total mass he climbs with the rock of 20 kg is 80 kg, but removing this 20 kg changes the maximum height he can climb from several meters to 300 meters, even though we only removed 20/80 or a fourth of the weight [1,2].

Theory: A human has a built-in biological leverage system in which relatively smaller muscle forces can move and lift a massive human body.

The muscle forces that lift a human body are always less than the body weight after considering the mechanical levers of the human skeleton, so a human can lift his body up a short distance with his calf and foot muscles pushing with foot toes with force smaller than his weight after considering the foot as a mechanical lever. Here we must distinguish between the foot as a mechanical lever and the new biological leverage that add forces reduction to the human self-movement.

Experiment No.1

A Human Lifts His Body with Force Less Than His Weight

Each human muscle has an approximate maximum force that it can exert. If we measure the maximum force of calf and foot muscles for a single leg, we will find that this maximum force is smaller than the human weight, but it lifts the human. We measured the calf and foot muscles maximum force for a single leg for an individual by pressing as hard as he could on a scale by his toes, which turned out to be approximately 32 kg, and the individual weight is 63.4 kg. According to Newton's laws of motion, for the individual to lift his body up to a short height with his foot, the minimum force of the foot and calf muscles measured by the scale should at least equal 63.4 kgf. "The Achilles tendon will exert a bigger force, but the force measured by the scale should at least equal 63.4 kgf." However, the maximum force of the individual's muscles was 32 kg, which is below the minimum force needed to lift the individual (63.4 kg).

Measuring the Maximum Calf and Foot Muscles Force

A scale is fixed to a wall. A human is lying on the ground, pressing the scale with the toes of a single foot. The human head is on another wall, so the body will not move when the human presses the scale. The pushing is only with the foot and calf muscles; other muscles of the leg are not involved. The push movement is equivalent to the human lifting his body up to a short height and pressing the ground with his toes using a single foot; one movement is horizontal and the other is vertical, both of which are done by the same group of muscles. The human presses with his toes as hard as he can, and then the scale will measure the approximate maximum force exerted by the calf and foot muscles.

 Figure 1: measuring the maximum muscles force for a single leg.

Below are the calculations of the minimum tension by the Achilles tendon to lift the individual’s body and the maximum force by the Achilles tendon.

Assume the individual’s Achilles tendon pulls straight upwards on the heel bone while his tibia pushes straight downwards on the ankle joint.

Figure 2: Lifting the human body with a single foot.

The tension on the Achilles tendon T when the individual lifts his body can be obtained by the free-body diagram:

Figure 3: The free-body diagram of the foot.

By putting the pivot at the ankle. The two upward forces T and n have to be in rotational equilibrium, so:

T(5)=n(13)

For the individual body mass of 63.4 kg, the minimum tension on the Achilles tendon T to lift his body up with a single foot is:

T(5)=(63.4)(13)=164.84 kgf. He needs tension from the Achilles tendon, which equals at least 164.84 kgf, to lift his body.

Assume the force n is the maximum force measured by the scale previously, which was 32 kgf.

T(5)=(32)(13)

T=83.2 kgf. A single-leg Achilles tendon can exert a maximum of 83.2 kgf.

So the individual maximum Achilles tendon tension of a single leg he could exert was 83.2 kgf, but to lift his body 63.4 kgf, he needs a minimum tension of the Achilles tendon of 164.84 kgf. Despite this, the individual can lift his body with a single foot with this maximum Achilles tendon tension of 83.2 kgf or less. Usually, a human does not exert his maximum force to lift his body in this way.

Experiment No.2

The pressure on the human body by the body weight force is relatively small

A human is lying on the belly of a concrete block. The concrete block does not touch any bones, just the belly, and the rest of the body is free in the air. The human body weight of 60 kg will not damage the belly; however, the block of 20 kg put on the belly will damage it. The block of 20 kg of weight will exert more pressure on the belly "damage it" than the pressure on the belly by the human weight of 60 kgf even though the human weight is 60 kgf three times the concrete block weight of 20 kgf. The areas of contact in both cases are equal. On the other hand, if a rock of 60 kgf weight is put on the belly of an average human, it will damage it severely compared to the human of 60 kgf lying on his belly on the rock, which will not cause any damage to the average human (the 60 kg rock touches only the belly of the human so that the areas are equal in both cases). The individual with a mass of 63.4 kg was able to lie on his belly on the concrete block for one minute without any damage to his belly. We cannot put a 63.4 kg block on the individual's belly for his safety, so this experiment can be conducted on an animal.

The Biological Leverage

Below is a demonstration of how the human foot supports a load less than the weight. When the human stands on a scale, the force on the foot will be smaller than the weight, but the scale weighs the actual weight. The foot will lift the body via biological leverage. The foot supports a load less than the human weight, the belly in the experiment above, as well as other parts of the body such as the spine.

Figure 4: The concept of the biological leverage.

Observations of Experiment 1

63. The individual of 63.4 kg mass was able to lift his body up to a short height with a single foot; however, when the individual was lying on the ground with his legs up, he could not lift a rock of 63.4 kg with an equivalent movement by pressing on the rock with his toes.
64. The individual of 63.4 kg could lift his body up holding a bar with his arms several times, but he could hardly lift a rock of 63.4 kg with an equivalent movement by pulling a rope through a pulley.

Observations of Experiment 2

1. An average human of 60 kg can lie on a bar on his belly for several seconds without pain or damage, but the human cannot bear the same bar put on the belly with smaller weights than 60 kgf.
2. The average human body mass of the trunk, the arms, and the head is approximately 40 kg. Even though the human's trunk, arms, and head weigh a massive 40 kg, the human spine can bear it for years without spine cartilage damage. This is a result of small pressure on the spine. A rock of mass 20 kg fixed equally on the human trunk for one week of everyday life will damage the spine cartilage severely. So it is one week vs. 2571 weeks (50 years) and 40 kg vs. 60 kg. Removing only a third of the weight (the rock, 20 kg) multiplies the time the spine cartilage can bear by thousands.

As in Experiment No. 1, the human is able to lift his body with a force less than his weight. The tension on the Achilles tendon to lift the human with the minimum force, which is equal to his weight, must be a minimum of 164.84 kgf, as in the previous free-body diagram, but the maximum tension by the Achilles tendon of a single leg was only 83.2 kgf, and the human actually does not exert this maximum force to lift his body with a single foot. And in experiment No. 2, the individual could support his massive body weight of 63.4 kgf on his belly without belly damage, so the pressure on the belly when the individual lies on it on a concrete block is relatively smaller than the pressure of a same-mass 63.4 kgf concrete block put on the belly. As experiment No. 2 and the observations suggest, the belly can support relatively massive human weight and the spine cartilage as well, but the spine and the belly cannot support the same mass put on them as in experiment No. 2 and the observations. If the individual of 63.4 kg is going to lift his body with the smallest force that is greater than his weight, as current physics suggests, then he should exert tension by the Achilles tendon greater than 164.84 kgf, as was shown, but the maximum Achilles tendon tension was 83.2 kgf, so the individual was able to lift his body with force less than his weight by the biological leverage system. The Achilles tendon in experiment No. 1 and the human belly in experiment No. 2 both support forces smaller than the human weight due to the biological leverage. A human body uses this concept to move the massive body by walking, dancing, running, walking upside down, etc. with relatively smaller force. Compared to an equal-mass object, a human can run at speeds up to 12 km/h and climb up to 300 meters with this massive average human mass of 60 kg. The human uses a biological leverage in his self-movement; the force exerted is less than the human weight, as in the experiments and observations after considering the foot as a mechanical lever. This force is multiplied by this biological leverage so that it is equal to or greater than his weight. The work done is the energy consumed by the muscles. This theory applies to any other living being. Animals undergo the same concept. So an average horse of 400 kg mass can support this mass by its belly when it lies on its belly on a concrete block, but a rock of the same mass, 400 kg, put on its belly will damage its belly severely.

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