BIOMECHANICS PSE4U

B IOMECHANICS Biomechanics: how physical forces affects human performance

Any discussion of biomechanics must begin with a tribute to Sir Isaac Newton and his three “Laws of motion”! Newton’s theories (and biomechanics) rests on two assumptions: physical equilibrium and the conservation of energy. Equilibrium: when more than one force acts on a body but the sum of the forces is zero, no change in velocity results. Conservation of energy: energy can never be created or destroyed, but can only be converted from one for to another.

NEWTON’ S 3 LAW S OF MOTION 1. The Law of Inertia: An object in motion will remain in that state of motion unless acted on by another force Ex. An athlete who is stationary will stay in that state until moved by his own muscles or another player Ex. A skier going down a hill will need something to act on her to slow her down 2. The Law of Acceleration (F=ma): A force applied to an object will cause it to accelerate in proportion to the force applied and inversely to the mass of the object being moved. Example: Force applied to a baseball being thrown vs. a shot put 3. The Law of Reaction: For every action there is an equal and opposite reaction. When one object exerts a force on a second object it will move in the opposite direction of the applied force. Example: A basketball player jumping. Pushes of the floor in a downward motion and moves inversely in an upward motion.

NEWTON’ S 3 LAW S OF MOTION

T YPES OF M OTION LINEAR MOTION: force is generated by the athlete’s muscles and the resulting motion is in a straight line. Motion usually involves acceleration Examples: Sprinter accelerates down a track or hockey player quickly veers to go around another player. Because human movements are not in a straight line, we need to introduce the notion of a force as a vector – in a particular direction. Take the example of a wide receiver running down the field. The forward movement of the wide receiver is a combination of a vertical force and a forward force, resulting in a vector force somewhere in between.

T YPES OF M OTION ROTATIONAL MOTION: the force does not act through the center of mass, but rather is “off-center” which results in movement about an axis. Angular Acceleration – acceleration around an axis (ex. A figure skater will increase angular velocity if they bring their arms closer to their body.) Moment of Inertia – Resistance to rotation. The larger the moment of inertia the larger the moment of force needed to maintain the same angular acceleration. EG. After leaving the high diving board, the diver curls tightly and then opens up just before entering the water. By opening up before entry, the diver increases the moment of inertia there by slowing down the angular velocity.

LEVER SYSTEM Every movable bone in the human body, acting alone or with others, is part of a lever system that facilitates movement. The HUMAN MUSCLE MACHINE: 3 classes of levers based on the location of the fulcrum in relation to the force.

LEVER SYSTEM Class I Lever: Ex. Teeter-toter The fulcrum (axis) is located between the force (effort) and the resistance (load) Class II Lever: Ex. Wheelbarrow The resistance is between the force and the fulcrum. Class III Lever: Ex. Snow Shoveling The force is between the fulcrum and resistance.

PRINCIPLES OF BIOMECHANICS There are seven main principles of biomechanics that can be broken into four broader categories. CATEGORY 1: STABILITY Principle 1: The lower the centre of mass, the larger the base of support, the closer the centre of mass to the base of support, and the great the mass, the more stability increases. This principle has 4 sub-components Height of the centre of mass: the point around which a persons mass is concentrated Line of gravity: an imaginary line that passes down through the centre of mass to ground Base of support: area between supporting limbs Mass: a measure of resistance to linear motion Ex. Sumo wrestling or football linemen

CATEGORY 2: M AXIMUM E FFORT Principle 2: The production of maximum effort requires the use of all possible joint movements that contribute to the objective. The more joints used, the more muscles will be contracted. Ex. Golf swing – need to rely on more than just your arms to swing

CATEGORY 2: M AXIMUM E FFORT Principle 3: The production of maximum velocity requires the use of joints in order – from largest to smallest. The force builds as each successive joint is put into motion Ex. Slap shot – reaching maximum velocity of movement at the exact moment of impact with the puck

C ATEGORY 3: L INEAR M OTION ( MOVEMENT IN A STRAIGHT LINE ) Principle 4: The greater the applied impulse, the greater the increase in velocity (also known as the principle of impulse) The more force needed, the longer range of motion the joints will go through. Can also work in reverse. To absorb force, the athlete uses a larger range of motion to control an object. EX. Basketball dunk – bend your knees to get height or soccer player stopping a ball with his foot.

C ATEGORY 3: L INEAR M OTION ( MOVEMENT IN A STRAIGHT LINE ) Principle 5: Movement generally occurs in the direction opposite that of the applied force (Newton’s third law). Every action has an equal an opposite reaction. When an athlete exerts a force, the surface pushes back with the same force Ex. High jump, swimming full body suits, drafting behind a cyclist

C ATEGORY 4: A NGULAR M OTION ( A CIRCULAR MOTION THAT OCCURS AROUND AN IMAGINARY LINE CALLED AN AXIS. Principle 6: Angular motion is produced by the application of a force acting at some distance from an axis, that is, by torque. Three types of torque: Entire body by an off center force (a force that does not go through the center of mass) – Ex. Hockey, rugby pushing someone so they lose their balance Body segment rotating around an axis – Ex. Pitch in baseball Imparting a spin on a projectile – Ex. Curve ball

C ATEGORY 4: A NGULAR M OTION ( A CIRCULAR MOTION THAT OCCURS AROUND AN IMAGINARY LINE CALLED AN AXIS. Principle 7: Angular momentum is constant when an athlete or object is free in the air Ex. Diver – angular motion will remain constant if the body position is constant. However, when the diver enters the water they open up increasing their moment of inertia and therefore decreasing their angular motion/velocity (tighter = increase angular velocity, looser= decrease in angular velocity)