Introduction to Levers

Introduction to Levers ● Definition of a Lever: ○ A lever is a simple machine consisting of a rigid bar that rotates around a fixed point called the fulcrum when a force (effort) is applied to overcome resistance (load). ○ Effort Arm: The perpendicular distance from the line of action of the force to the fulcrum. ○ Resistance Arm: The perpendicular distance from the line of action of the resistance to the fulcrum. ● Components of a Lever System: ○ Fulcrum: The pivot point (joint in the human body). ○ Effort: The force applied by muscles. ○ Load: The resistance that needs to be moved (e.g., weight of a body part or external load). Lever System in the Human Body ● Bone: Acts as the rigid bar. ● Fulcrum: The joint around which the bone rotates. ● Load: The resistance, which could be the weight of a body part or an external load. ● Effort: The force generated by muscles at their insertion points. ● Effort Arm: The moment arm of the muscular force. ● Resistance Arm: The moment arm of the gravitational force. Mechanical Advantage (MA) ● Definition: ○ Mechanical advantage is a measure of the efficiency of a lever system. It is the ratio of the effort arm to the resistance arm. ○ Formula: ○ MA = Effort Arm / Resistance Arm ​ ●​ When MA = 1: ○​ The effort arm equals the resistance arm. ○​ The lever changes the direction of movement or balances the lever without magnifying force or speed. ●​ When MA > 1: ○​ The effort arm is longer than the resistance arm. ○​ The lever magnifies the force, making it easier to move heavy loads with less effort. ●​ When MA < 1: ○​ The effort arm is shorter than the resistance arm. ○​ The lever magnifies speed and range of motion but requires more effort to move the load. Classification of Levers Levers are classified into three types based on the relative positions of the fulcrum, effort, and resistance: 1.​ First-Class Lever: ○​ Fulcrum is located between the effort and resistance. ○​ Examples: Seesaw, scissors, crowbar. ○​ Functions: ■​ Balance when the fulcrum is in the middle (MA = 1). ■​ Strength when the fulcrum is closer to the resistance (MA > 1). ■​ Speed and range of motion when the fulcrum is closer to the effort (MA < 1). ○​ Examples in the Human Body: ■​ Head and Neck: During neck extension, the atlanto-occipital joint acts as the fulcrum, the weight of the head is the load, and the neck extensor muscles provide the effort. ■​ Triceps Brachii: During elbow extension, the elbow joint is the fulcrum, the weight of the forearm is the load, and the triceps provide the effort. ■​ Supraspinatus: During shoulder abduction, the shoulder joint is the fulcrum, the weight of the humerus is the load, and the supraspinatus muscle provides the effort. ■​ Plantar Flexion (non-weight bearing position): During plantar flexion in non-weight bearing position, the ankle joint is the falcrum, the weight of the foot is the load, and the effort is the soleus pulling upward on the heel. 2.​ Second-Class Lever: ○​ Resistance is located between the fulcrum and effort. ○​ Examples: Wheelbarrow, nutcracker, nail clippers. ○​ Functions: ■​ Magnifies force and conserves energy. ■​ Sacrifices speed for strength. ○​ Examples in the Human Body: ■​ Masseter Muscle: During chewing, the temporomandibular joint is the fulcrum, the food is the resistance, and the masseter muscle provides the effort. ■​ Plantar Flexion (weight-bearing position): During ankle plantar flexion, the ball of the foot is the fulcrum, the body weight is the load, and the gastrocnemius and soleus muscles provide the effort. 3.​ Third-Class Lever: ○​ Effort is located between the fulcrum and resistance. ○​ Examples: Manual broom, tweezers, fishing rod. ○​ Functions: ■​ Designed for speed and range of motion at the expense of force (MA < 1). ■​ Requires a large effort to move a relatively small load. ○​ Examples in the Human Body: ■​ Biceps Brachii: During elbow flexion, the elbow joint is the fulcrum, the weight of the forearm is the load, and the biceps provide the effort. ■​ Hamstrings: During knee flexion, the knee joint is the fulcrum, the weight of the leg is the load, and the hamstrings provide the effort. Clinical Applications of Levers ●​ Mechanical Advantage in Physiotherapy: ○​ The relationship between lever arm distance, angular displacement, and torque production is crucial in assessing movement and prescribing interventions. ○​ Shorter Lever Arm: Increases angular displacement and velocity but requires more force. ○​ Longer Lever Arm: Increases torque production but reduces speed. ●​ Lever Arm Dysfunction (LAD): ○​ Disruption in the moment generation of a muscle-joint complex due to an ineffective lever or moment arm, despite normal muscle force. ○​ Example: Forward neck posture can lead to LAD, affecting the efficiency of neck muscles. Levers in Sports ●​ First-Class Lever in Rowing: The oarlock acts as the fulcrum, the rower's muscular force is the effort, and the water resistance is the load. ●​ Second-Class Lever in Basketball: When a player jumps, the ball of the foot is the fulcrum, the body weight is the load, and the calf muscles provide the effort. ●​ Third-Class Lever in Football: During a penalty kick, the quadriceps contract (effort) to straighten the knee (fulcrum) and kick the ball (load). Torque and Lever Arms ●​ Torque is the rotational force produced by a lever system. ●​ Formula: Torque=Force×Lever Arm ●​ Torque=Force×Lever Arm ●​ ●​ Examples: ○​ Biceps Brachii: The force required to lift a load depends on the length of the lever arm. Increasing the insertion point of the biceps reduces the force needed. ○​ Long Levers in Sports: Longer levers (e.g., straight-arm drive in tennis) produce more linear force and better performance in sports like baseball, hockey, and golf.