Biomechanics describes human motion and its causes in a quantitative way. If a property is quantifiable, it means that certain aspects of such property are measurable and can be expressed by a number. Thus we observe in objects and phenomena their quantitative properties whose prevalence is expressed by a multiple of selected unit of this property.
Some of the properties and abilities used in biomechanics may be expressed by such concepts as velocity, inertia, power, momentum, force, mass, weight, distance, acceleration, etc.
Length
An elementary quantity we want to measure is length. Length is used to describe space in which motion happens. Length is a direct expression of sport performance in such sport events as shot put, long jump, high jump, etc. In certain sports length has an indirect impact on sport performance. For example length of running step is an important indicator of motion technique on both short and long distances. It can also be part of points evaluation of sport performance, as is the case in ski jumping. In the SI system3 the unit for length is metre (m). The symbol used is l.
Time
Time is another elementary measurable quantity. Time is an important factor in almost all sport events. Sports that include competition often use time as the measure of sport performance. Reaction time of goalkeepers in various collective sports determines success of the team as a whole. Similar situation is also in sports such as tennis or squash, where the reaction time of the player has a strong influence on returning the ball back to the opponent’s side. Time can be measured in seconds, minutes, hours, days, weeks, months or years. In the SI system the unit for time is second (s). The symbol used is t.
Now we are familiar with two elementary quantities for description of motion in space and time. Many quantities describing human motion during sport activities are derived from the knowledge of body’s position in relation to time. Good examples are velocity and acceleration.
Mass
Which property represents a difficult obstacle for players of American football when they try to change direction? And which property, on the other hand, helps them to break the opponent’s defence? It is inertia.
Inertia is the resistance of any physical object to a change to its state of motion.
Mass is the quantitative measure of inertia4.
How important is mass in sport and physical exercise? Athletes must change the direction of their equipment, such as a tennis racket, or the direction of somebody else’s body, for example in judo. Because mass is the measure of inertia, it decides about the amount of effort that athletes must make in order to change motion of either their body parts, or the equipment they use. In the SI system the unit of mass is kilogram (kg). The symbol used is m.
So far we have defined three elementary units: those of mass, time, and length. All other units of mechanical quantities may be derived from these three.
3 The SI system (Le Système International d'Unités) is an international system of units of physical quantities.Zpět
4 The concept of mass is often confused with the concept of weight. Weight will be defined in the next chapter.Zpět