Rabia Mustafaking Edward Medical University

Fixed pulley

pulley

Definition: A pulley changes the  direction of the

force, making it easier to lift things.

A pulley is a simple mechanical machine and consists of a wheel that turns readily on an axle.

The wheel is usually grooved for a rope or a wire cable.

SIMPLE PULLEY SYSTEM

In the human body when the tendon of a skeletal muscle slides over a round bony surface, the “system” acts like a simple pulley. A simple pulley provides a change in the direction of the force or muscle pull. There is no change in the amount of force produced by the muscle.

For example, the knee acts as a simple pulley by which the quadriceps femoris M. extends the leg.

Sesamoid bones, such as the patella (kneecap), develop in tendons where pressure is applied to the tendon

Types of pulley

There are three types of pulleys: Fixed pulley Movable pulley Combined pulley

Anatomical pulleys In the human body, in most cases the pulley

is replaced by a bone, cartilage or ligament and the cord is replaced by a muscle tendon . The tendon is lubricated in a manner so that it may easily slide over the pulley.

Frequently, the fibers of a muscle or a muscle tendon wrap around a bone or are deflected by a bony prominence. When the direction of pull of a muscle is altered, the bone or bony prominence causing the deflection forms an anatomic pulley

Function of Anatomical pulley

The function of any pulley is to redirect a force to make a task easier. The “task” in human movement is to rotate a body segment.

Anatomic pulleys (in the majority of instances) make this task easier by deflecting the action line of the muscle away from the joint axis, thus increasing the MA of the muscle force.

By increasing the MA for a muscle force, a force of the same magnitude (with no extra energy expenditure) produces greater torque.

Fixed pulley These are used to alter the direction of

force. The pulley block is fixed and the rope which passes round the wheel is attached to the weight at one end and the effort is applied at the other .

It uses more effort than the  load to lift the load from the ground.

The fixed pulley when attached to an unmovable object e.g. a ceiling or wall, acts as a first class lever with the fulcrum being located at the axis but with a minor change, the bar becomes a rope. 

Single fixed pulley

A single fixed pulley: It changes the direction of the force acting on it and its magnitude remains the same on either sides of the pulley rope irrespectively to the angle of pull of the force.

It’s mechanical advantage is (1).

Single Fixed Pulley Two Fixed Pulley

Pulley like structures in human body Pulley-like arrangements in body

Patella at knee ▪ pulled between the condyles of femur ▪ patellar tendon - patella - quadriceps

arrangement quadriceps muscle pull proximally toward their bellies yet knee extension (from flexed position) cause leg to move in opposite direction

Lateral malleolus of ankle Epicondyle of femur

The patella as a fixed pulley:

  The classic example of fixed pulley is

that formed by the patella. The presence of the patella ( the pulley) improves the efficiency of the quadriceps muscle as the pulley will increase the angle of insertion of the patella ligament into the tibial tuberosity .

Biomechanics behind this action The action of the muscle at the joint is altered because

of the pulley. The quadriceps muscle belly lies parallel to the femur. The tendon of the muscle passes over the knee joint and attaches to the leg (tibia) via the patellar tendon at the tibial tubercle. For knee joint extension, the joint axis is considered to be located through the femoral condyles.

The MA for the quadriceps muscle force lies in space between the vector and the joint axis. Without the patella, the line of pull of the quadriceps muscle on the leg-foot segment would follow the patellar tendon at the tibial tubercle and would lie parallel to the leg-foot segment

Biomechanics behind this action

However, the patella lies between the quadriceps tendon and the femur, changing the angle that the patellar tendon makes with the leg (tibia) and changing the line of pull of the quadriceps muscle away from the knee joint axis . The effect of changing the line of pull of the quadriceps muscle on the tibia is to increase the MA. Give the increased MA, the same magnitude of force would produce greater torque (and a greater angular acceleration as the only force).

Ex. lateral malleolus as a fixed pulley Ex. lateral malleolus of the fibula acting as a pulley

around which tendon of peroneus longus runs. As peroneus longus contracts, it pulls toward it belly (toward the knee)

If it was not for the malleolus, this muscle instead of passing behind the lateral malleolus to be inserted in the base of the first metatarsal and to produce ankle plantar flexion and eversion, it would have produced ankle dorsiflexion and eversion because of its passage in front of the ankle joint.

Using the lateral malleolus as a pulley, force is transmitted to plantar aspect of foot resulting in eversion/plantar flexion.

Ex. lateral malleolus as a fixed pulley

Epicondyle of femur

The joint serve as the pulley . The size of the epicondyles of the femur gives the gracilis tendon a favourable angle of insertion as the tendon insert on the tibia .

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