Biomechanical Principles

Newton's First Law: Inertia

Newton's first law (the law of inertia) states that a body continues in its state of rest or uniform motion in a straight line unless an external force acts upon it – put simply, a force is required to change a body's state of motion (starting, stopping, accelerating, decelerating or changing direction).

• Inertia is the resistance a body has to a change in its state of motion, and increases with mass – a heavier object needs a larger force to change its motion. E.g. a heavier rugby forward is harder to stop running than a lighter winger, since the forward has greater inertia
• A high jumper illustrates the law directly: they move horizontally at a constant state until take-off, where an external force changes their motion to vertical to clear the bar

Newton's Second Law: Acceleration

Newton's second law (the law of acceleration) states that the acceleration of a body is proportional to the force causing it and takes place in the direction that force acts:

• For a given mass, acceleration is directly proportional to the force applied – e.g. a sprinter drives forward from the blocks by generating a large internal force through the gluteals, quadriceps and gastrocnemius, and a tennis player accelerates the ball over the net in the direction the racket force is applied
• Applied to football: the harder a ball is kicked (greater force), the greater its acceleration, so it will travel further and faster in the direction the force was applied

Newton's Third Law: Action and Reaction

Newton's third law states that for every action (force) there is an equal and opposite reaction (force) – when one body exerts a force on another, the second body exerts an equal and opposite force back.

• A ground reaction force (GRF) is the equal and opposite force the ground exerts back on a performer who applies a muscular force against it – e.g. a sprinter pushes back on the blocks (action) and the blocks push the athlete forward (reaction); a swimmer pushes back against the water with hands/feet (action) and is propelled forward by the water (reaction)
• Applied to football: when a player jumps to win a header, they push down on the ground (action) and the ground pushes back up on the player with an equal and opposite force (reaction), producing the jump

Measurements Used in Linear Motion: Speed and Distance

A scalar quantity describes only the size/magnitude of a measurement, with no direction taken into account – speed and distance are both scalar quantities.

• Distance is the total length of the path a body follows moving from one point to another (e.g. a completed 200m race covers a distance of 200m)
• Speed is the rate of change of position:

Units must be matched – e.g. distance in km and time in hours gives speed in km/h. On a distance/time graph, a horizontal line shows a stationary performer; a straight, angled line shows constant speed – the steeper the line, the greater the speed.

Centre of Mass and the Line of Gravity

• The centre of mass is a body's point of balance – the point where its mass is concentrated. Because the human body is an irregular, constantly moving shape, its centre of mass shifts continuously (e.g. raising the arms overhead raises the centre of mass to help maintain balance)
• In a standing position, centre of mass sits roughly around the hip region and differs by gender – typically slightly higher in males (more upper-body mass) and slightly lower in females (more mass concentrated at the hips)
• The line of gravity is the vertical line running straight down from the centre of mass to the ground

Factors Affecting Stability

Good balance/stability is essential in almost all sporting actions (e.g. an unbalanced kick or tackle is far less effective), and is influenced by several mechanical factors:

• Height of the centre of mass – a lower centre of mass increases stability (e.g. a low stance in rugby, judo or wrestling makes a performer harder to push over)
• Position of the line of gravity – stability is greatest when the line of gravity falls centrally over the base of support; moving the centre of mass toward the edge of the base causes over-balancing (deliberately used by sprinters in the blocks to generate forward momentum off the gun)
• Area of the base of support – more contact points create a larger base and greater stability (e.g. a headstand, with more contact points, is more stable than a handstand)
• Mass of the performer – greater mass generally increases stability, through greater inertia
• These principles can be applied strategically, e.g. the Fosbury flop in high jump allows a jumper's centre of mass to pass under the bar while the body passes over it, meaning less height needs to be gained overall compared with older techniques such as the scissor kick, where the centre of mass stays inside the body and must be lifted fully over the bar