Lever system lever is any elongated, rigid (bar) object that move or rotates around a fixed point called the fulcrum when force is applied to overcome resistance. Force (effort) Resistance (load) R Rigid bar Fulcrum
Force= effort Effort arm: perpendicular distance from the line of action of the force to the fulcrum. Resistance: load Lever system Resistance arm: perpendicular from the fulcrum to the point of resistance. Resistance (load) R Force (effort) Resistance arm Effort arm Fulcrum
Lever system Mechanical advantage (MA): is the way to measure the efficacy of this machine that means the ability of the machine to magnify force or to increase output in relation to the input. Mechanical advantage (MA) of a lever is the ratio of its output force to its input force. MA = Length of the effort arm(ea) Length of the resistance arm (RA)
Lever system If MA >1.0, the lever will produce more force, but less speed or distance, than the force exerted on it. If MA < 1.0, the lever will produce more speed or distance, but less force, than the input. If MA = 1.0, the lever function is to change the direction of motion or balance the lever. Resistance (load) Force (effort) R Rigid bar Resistance arm Fulcrum Effort arm
Human equivalents Bone rigid bar Joint axis fulcrum Muscles effort Resistance weight of segment + any weight. Effort arm MA of muscle force Resistance arm MA of Gravity line of action
Human equivalents: Lever system Biceps brachii R Radius E Fulcrum F
Classification of Lever Three classes of levers differ with respect to which component- the fulcrum (F), effort (E) (point of application of muscular force), or resistance (R) (point of application of resistance)- is in the middle.
Classification of Lever First-class lever: is one with the fulcrum in the middle. Resistance (load) Force (effort) R Resistance arm Effort arm
Classification of Lever Mechanical advantage of the first class: 1 MA 1 According position of fulcrum Resistance (load) Force (effort) Resistance (load) Force (effort) R MA more than 1 R MA less than 1 Resistance arm Effort arm Resistance arm Effort arm Resistance (load) Force (effort) R MA equal 1 Resistance arm Effort arm
Classification of Lever Functions of the 1 st class of lever: 1. Magnify force. 2. Gain high speed and ROM. 3. Balance of force. 4. Changing direction of forces.
Anatomical example Axial skeleton F Effort: Neck extensors Fulcrum: Atlanto-occipital joint Resistance: Wt. of head R E
Upper limb Effort: Elbow extensors (Triceps) Fulcrum: Axis of elbow joint Resistance: Wt. of forearm &hand E F R R
Lower limb (Non-weight bearing) Effort: Plantar flexors (calf ms) Fulcrum: Axis of ankle joint Resistance: Wt. of the foot
Classification of Lever Second class of lever: is one in which the resistance is in the middle. Force (effort) Resistance (load) Force (effort) Resistance (load) R Fulcrum Fulcrum Fulcrum Resistance Force (effort)
Classification of Lever Functions of the 2 nd class of lever: 1. Magnify force. 2. Converse energy: muscle can use less amount of force with minimum energy to produce big work.
Anatomical example Axial skeleton Effort: Masseter ms Fulcrum: Temporomandibular joint (TMJ). Resistance: Food located at the posterior aspect of the mouth F R E
Upper limb Effort: Brachioradialis ms Fulcrum: Elbow joint. Resistance: Wt. of forearm &hand F R E
Lower limb Weight bearing (stand on toes) Effort: Plantar flexors (calf ms) Fulcrum: Base of toes Resistance: Body wt.
Classification of Lever Third class of lever: the effort is applied between the fulcrum and resistance. Force (effort) Resistance (load) R Resistance R Fulcrum Fulcrum Effort
Classification of Lever Functions of the 3 rd class of lever: as the effort arm is always less than the resistance arm. So large effort is needed to overcome a moderate resistance: 1. Increase ROM 2. Increase speed.
Anatomical example Axial skeleton Effort: Masseter ms Fulcrum: TMJ Resistance: Food located in the front part of the mouth F E R
Upper limb Effort: Biceps brachii ms Fulcrum: Elbow Jt Rsistance: Wt. of forearm &hand& any wt
Lower limb Effort: Quadriceps ms Fulcrum: Knee Jt Resistance: Wt. of leg & foot F E R
Torque or Moment of Force Torque is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist. The moment of force about any point is equal to the magnitude of the force multiplied by the perpendicular distance from the action line of the force to that point: M=(F) X (D)
Moment Arm Perpendicular distance from the line of action of the force to the axis of rotation. Axis Moment arm Vector of force
Clinical application Active Resisted Exercises: applying resistance during exercises, the muscular contraction or effort depends on: 1. The distance from the point of application of the resistance to the fulcrum. The longer the resistance arm the greater will be its effect and the greater will be the effort produced by the muscle. 2. The angle: resistance applied at right angle is efficient than that applied at acute or obtuse angle.
Class of Lever in Two-joint Muscles Biarticular muscles: a muscle which pass over two joint at the same time. Biarticular muscles are: 2 nd class of lever over the proximal joint and 3 rd class over the distal joint.
Examples Rectus femoris: pass over the hip and knee (act: hip flexion{2 nd class} and knee extension {3 rd class}). Gastrocnemius: pass over the knee and ankle (act: knee flexion {2 nd class} and ankle planter flexion {3 rd class}). Biceps brachii : pass over the shoulder and the elbow (act: shoulder {2 nd class} and elbow flexion {3 rd class}).
Characteristics of Two Joint Muscles 1. Act on more than one joint at the same time so they are more efficient in performing patterns of movement. 2. Active insufficiency: when a two joint muscles shortened over the two joints at the same time, it will lose its tension after a certain limit. It means that muscle cannot produce its complete tension at both joints at the same time. So the muscle loses its tension over one joint to complete the ROM over the other joint.
Characteristics of Two Joint Muscles Advantage of active insufficiency: Tendon muscle action (tenodesis): is the movement that occur in one joint due to passive tension of the muscle in the other joint when muscle passes over two joints at the same time. Tenodesis can be used to correct pattern of spasticity in patients with Upper Motor Neuron Lesion {UMNL} (facilitation of the fingers extensors).
Rectus femoris: pass over the hip and knee (act: hip flexion and knee extension ). To complete full ROM of hip flexion, the rectus femoris will loose the knee extension firstly.
Characteristics of Two Joint Muscles 3. Passive insufficiency: lengthening of the two joint muscle at the same time, it will cause pain after certain limit.
Pulley Is a simple machine that performs a mechanical job. It consists of a grooved wheel and a cord or rope running over it.
Function of the pulleys: Pulley 1. Change the direction of a force. 2. Balance force as the first class of lever. 3. Modify the effect of force. 4. Magnify forces via increasing the mechanical advantages.
Types of Pulleys: Pulley 1. Fixed pulleys: has a fixed axle (the only example of pulleys in the human body). 2. Mobile pulleys: has a free axle (used in exercises). 3. Mixed pulleys: combination between fixed and mobile pulleys. It is used in exercises.
Pulley system The pulley is represented in the body as: Wheel : bony component External bone: sesamoid bone as the patella. Bony prominence: condyles, trochanters and tuberosity. Rope: muscles or tendon Patella acts as an external pulley for the tendon of the quadriceps
Patella as a Pulley Patella acts as an external pulley for the tendon of the quadriceps?????
Function Patella as a Pulley Move the muscle away from the joint axis. Increase the moment arm of the muscle up to 4.9cm. Increase the torque of the quadriceps (F m x d). Increase the angle of pull of the muscle. Increase the magnitude of the vertical or rotatory component of the quadriceps muscle (F sin ɵ). Increase the efficiency of the quadriceps during extension of the knee. Improve mechanical advantage of tibia by improving line of action of the muscle.
Pisiform bone as a Pulley Act as a pulley for the tendon of the flexor carpi ulnaris. It increases the angle of pull of the muscle so increase its efficiency. Insertion: Pisiform, hook of hamate bone & 5 th metacarpal bone
Lateral Malleolus as a Pulley Part of the Mortise (lower end of the tibia and fibula with the upper surface of the talus). It acts as a pulley for the tendon of the peroneus longus muscle as it passes behind the lateral Malleolus. If this muscle does not pass behind the lateral Malleolus, its action will be eversion and dorsiflexion. So its force direction is changed and improve its performance.
Lateral Malleolus
Condyles acting as pulleys Medial femoral condyle: act as pulley for the gracilis muscle Dorsal tubercle of the radius: extensor pollicis longus (improve function of the thumb in extension and aids in extension of the wrist. Condyles of the interphalangeal joint: flexor digitorum superficialis and profundus.
Condyles acting as pulleys Gracilis Extensor pollicis longus