LeversBy: Dr. M. Abbas Jamil

Lever:

A lever is a rigid bar that can rotate about a fixed point when a force is applied to overcome resistance.

Parts of Lever:Levers has 3 parts:

Force (effort)Resistance (load or weight)Axis (fulcrum)

The fixed point about which the lever rotates is the axis (A), sometimes referred to as the fulcrum.

The resistance (R) that must be overcome for motion to occur can include the weight of the part being moved, gravity, or an external weight.

The force arm (FA) or effort’s arm (EA) is the perpendicular distance, or length, between the line of force and the axis while the resistance arm (RA) or weight’s arm (WA) is the perpendicular distance, or length, between the line of resistance and the axis.

Classes or orders of Lever:

First-class lever: In a first-class lever, the axis is located between the force and the

resistance.Most efficient class of lever BALANCE

A good example of this would be a playground seesaw. The seesaw (lever arm) rotates on a crossbar (axis), which is located somewhere between a child sitting on one end of the board and pushing down against the ground (force) and the weight of the child sitting at the other end (resistance).

Examples in human body

Skull pivoting on the atlas vertebrae of the spine, with weight of the head held by the trapezius and sternocleidomastoid muscle of the muscle of the neck.

A second-class lever has the axis at one end, the resistance in the middle, and the force at the other end.

The wheelbarrow is a second-class lever. The wheel at the front end is the axis, the contents of the wheelbarrow are the resistance, and the person pushing the wheelbarrow is the force.

Power

Examples in human body

Action of the ball of the foot with the gastrocnemius & soleus muscles of the calf lifting the weight of the body, which acting through foot during plantar flexion

Push ups

A third-class lever has the axis at one end with the force in the middle and resistance at the opposite end:

Third-class leverMost common in the body ROM / Speed

An example of this type of lever would be a screen door that has a spring attachment . The axis is the door hinges, the force is the spring that closes the door, and the resistance is the door itself.

Baseball bat

Examples in human body

Action of biceps muscles as it lifts a load in the hand whilst pivoting elbow

Action of hamstring muscle

Mechanical Advantage

The efficacy of force in relation to a Lever is dependant upon two factors:i. Weight or Effortii. Weight’s arm or effort’s arm

Moment of Force: Product of Force exerted (W) or effort (E) and its perpendicular distance

from the Fulcrum (weight’s arm or effort’s arm) is called moment of force (torque).

Ratio between weight to the effort. ORRatio between force arm and resistance arm.

MA=W/EMA = FA/RA

Force(effort) Arm > Resistance(weight) arm= MA is >1Force(effort) Arm < Resistance(weight) Arm = MA is <1

The Greater the Mechanical Advantage, the less force is needed to cause motion.

Mechanical advantage is always obtained in levers of 1st order and 2nd order when fulcrum is nearer to the weight than to the effort.

It is not obtained in levers of 3rd order.

Mechanical disadvantage is obtained in 1st order lever when fulcrum is nearer to the effort than to the weight and in all levers of 3rd order.

When the force arm (FA) is greater than the resistance arm (RA), as with a second-class lever, the mechanical advantage (MA) is greater than 1. For example, if a force arm were 2 feet and the resistance arm were 1 foot, the mechanical advantage would be:

MA= FA/ RA MA= 2 / 1 MA = 2This means that the force arm has twice the length as the resistance arm.

Therefore, it has twice the torque (rotary force).

If, however, the force arm were shorter (1 foot) and the resistance arm were longer (2 feet) as with a third class lever, the mechanical advantage would be:

MA = FA / RA MA= 1 / 2 MA = 1/2 In other words, the force arm has half the length of the resistance arm.

Therefore, it has half the torque (rotary force).

An example in therapeutic exercise is the application of force to a patient’s lower leg while the patient tries to keep the knee extended. It takes less force on your part if you place your hand distally versus proximally.

In this case the axis (A) is the knee joint, the resistance (R) is the insertion of the quadriceps muscle, and the force (F) is your hand on the lower leg. the resistance arm is 2 inches and the force arm is 4 inches. The mechanical advantage can be calculated as follows:

MA =FA /RA MA = 4 / 2 MA = 2

The RA remains at 2 inches and the FA is 10 inches. Its mechanical advantage is calculated as follows:

MA = FA/ RA MA =10 / 2 MA = 5Simply stated, less force is needed to cause motion when the mechanical

advantage is greater.

If the force arm and resistance arm are the same length, and the amount of force and resistance are equal, then the system is balanced and no motion will occur. Using the seesaw example, if child A and child B each weigh 40 pounds, and are both sitting 5 feet from the crossbar, the seesaw will be balanced. If child A moves one foot closer to the crossbar, the system is no longer balanced. Child A would go up, and child B would go down. Child A needs to either add weight (more force) or move back (lengthen FA) to balance the seesaw again. Remember, with a longer force arm and a shorter resistance arm, less force is needed to cause motion.