Human locomotion consists of  movement patterns such as walking, running, jumping, lifting, throwing, striking and swimming. A skill involves movement patterns which are adapted to the constraints of a particular task or sport.  For example, (1) the skill of high jumping and long jumping fall under the movement pattern of jumping, (2) the skill of racquet ball, tennis serve, and a baseball hit fall under the movement pattern of striking, and (3) in swimming we find skills such as freestyle, breaststroke, backstroke, and butterfly. Technique makes reference to a particular type of skill. For example, there are some movement modifications for the skill of breaststroke swimming, and for high jump, and tennis serve.  The following handout presents basic mechanical principles applied to several movement patterns.
 

Practical Applications: mechanical principles applied to walking, running, aquatics, throwing, stricking and kicking.
 

WALKING AND RUNNING

A. General Points

1. Walking and running are both general motions characterized by the translation of the body’s center of gravity due to rotary motion occurring in the lower and upper extremities.

2.  Forces which control walking and running are weight, normal reaction, friction, air resistance and internal muscular force.  An interaction of all these forces determines the walking and running gait.

3.  The inertia of the body is changed by the horizontal component of the propulsive force (during the leg drive).  Since force applied over time (impulse) is non-uniform, the center of gravity experiences periods of speeding up and slowing down. Therefore the amount of inertia during the various phases of the walking and running motion is changing.  Ideally, you want to minimize the intracyclic velocity fluctuations of the center of gravity (a more even velocity of movement); the latter may minimize energy expenditure of motion.

4.  The force of the body acting through the foot (backward thrust of leg and foot on the ground) is counteracted by the vertical force (reactive force) of the ground.  The latter action results in forward movement. (Law of action and reaction, Newton’s 3rd Law).
 

B. Specific Points Applied to Running and Walking.

1. A body at rest will remain at rest unless acted upon by a force (Newton’s First Law of Linear Motion).  In running and walking the ground reaction force produced by extension of the lower extremities results in horizontal propulsion and therefore movement.  The momentum gained by the lower extremities is transferred to the upper body (trunk). The force required to change inertia is greatest at takeoff and least after acceleration has ceased.  Inertia is also great during the low points on the velocity-time curve of the center gravity. This means that you need to change a certain amount of inertia to accelerate the body forward.

2. Acceleration in walking or running is directly related to the force causing the acceleration and inversely related to body mass  (application of Newton’s Second Law of Linear Motion; acceleration = Force/mass; ).  Therefore, the greater the power of the leg drive, the greater the motion imparted to the trunk which means faster walking and running.

3.  According to the Law of Reaction, every action has an equal and opposite reaction (Newton’s Third Law of Linear Motion).  In walking and running, the force is provided through the upward and forward ground reaction force due to the downward, backward drive of the foot.  The smaller the vertical component of the force, the greater the horizontal force component.  If the vertical component of force is great, the runner or walker would have large intracyclic velocity fluctuations of the center of gravity (a bouncing action in the gait will be noticed) and would be inefficient.

4.  In running the foot should strike close to the line of gravity.  If the foot strikes far in front of the line of gravity, you will tend tohave a backward thrust and therefore slow down.  The latter is inefficient movement and would result in greater energy expenditure to maintain the same velocity of movement.

5.  The speed of running can be manipulated by increasing either the stride length or the stride rate, or both.  Walkers and runners tend to naturally select an optimum stride lenth and rate.

6.  The more completely the horizontal force is directed straight backward, the greater it’s contribution to forward motion.

7.  Along lever develops greater speed, therefore during the driving phase of running (and fast walking) the length of the leg should be as great as possible during the driving phase (especially when speed is an issue).  The latter is accomplished by a more complete extension of the leg.

8.  Efficiency of movement involves eliminating any unnessary forces.  Internal resistance such as tightness of tendons, ligaments, and high viscosity of muscle membrane can be reduced by proper warmup (light jogging plus stretching).   The resistive force of a wind can be altered by shifting the center of gravity forward.  In other words, a forward lean will work to counteract a head wind.

9.  It is important to have good mechanics during the walking and running motions.  Research has illustrated that “good” mechanics results in less energy expenditure for a given speed of movement.  In other words, good mechanics translates into good physiological economy.  The better your economy the faster you are able to run for a given metabolic power output.
 

THROWING, STRIKING AND KICKING

1. Muscles contract more forcefully if they are first put on stretch, provided they are not overstretched.  Windup in pitching, football pass, javelin throw, and volleyball serve are all important.

2.  Awkwardness and unnessary fatigue may be do to unnecessary movements in the performance of a motor skill.

3.  Adjustments in the neuromuscular mechanism is made through practice. Therefore, you obtain “skillful” and efficient performance.

4. Most efficient type of movement in throwing and striking skills is ballistic movement.

a.  Ballistic: Movements which are initiated by vigorous muscular contraction and  completed my momentum.

b.  Non-Ballistic: Constant muscular action throughout the movement.

General points on ballistic movement:

? Beginners tend to concentrate on non-ballistic types of movements, especially if they are concentrating on accuracy of aim rather than on a ballistic type of movement. This type of movement results in “moving fixation” or a slow, tense movement.

? A beginner should concentrate on form rather than accuracy if they are to master the skill of moving ballistically.

? Termination of ballistic movement: (1) contracting antagonistic muscles; (2) body part reaches the limit of motion (stopped by passive resistance of ligaments or other tissues); (3) interference of an obstacle.

5.  A lever appropriate for the task should be used. A long lever arm is desirable for movements requiring range or speed.  Object will move only if the force is of sufficient magnitude to change the object’s inertia and restraining forces.  Restraining forces include:  (1) friction; (2) wind or water drag; (3) internal resistance.   Warmup will help reduce internal resistance.

6.  Force exerted by the body will be transferred to an external object in proportion to the effectiveness of the counter of the feet against the ground.

7. Optimum summation of internal force is needed if maximum force is to be applied to move an object.  Heavier body segments typically move first and the lightest segments last.

8.  Force must be applied over time for a change in momentum to occur. If maximum momentum of a body segment and implement is desired, maximum impulse must be applied over a long period of time. This places importance on follow through.

9. Force applied in line with the center of gravity of the object will result in linear motion of the object.

10. If force is applied off-center to a freely movable object, the object will rotate (rotary motion).

11. The greater the velocity and mass (up to a point) of the striking implement, the greater the velocity of the struck ball. A good baseball player may use a heavier bat. Too heavy a bat, however, is inadvisable because it takes away from angular velocity.

12. The greater the coefficient of restitution (elasticity) of the ball and of the striking implement, the greater the speed of the struck ball.

13. Factors determining the direction taken by the struck ball: (1) direction of the striking implement at the moment of contact; (2) relation of the striking force to the ball’s center of gravity; (3) degree of firmness of grip and wrist at moment of impact; (4) principles affecting rebound (elasticity of striking implement and effects of spin)
 

AQUATIC LOCOMOTION

Compared to land based human motion, aquatic locomotion differs in the following ways, (1) buoyancy is the primary force versus gravity, (2) the substance (water medium) gives when pushed against, (3) water resistance is greater than air resistance, and (4) horizontal (swimming)  and vertical positions (shallow water exercise, deep water running) are assumed in the water.

Since water is 800 times more dense than air, the energy expenditure to move at a given velocity is great.  Water resistance encountered in swimming consists of (1) form drag,.. this is the greatest resistance encountered by the swimmer and is due to body frontal surface area exposed to the water,  (2) surface drag (skin friction),.. caused by the water coming in contact with the skin; swimmers do shave their bodies in order to reduce surface drag,..  and,  (3) wave drag,.. the body moving partially in water and in air creates waves which tend to offer resistance to the swimmer.  The amount of resistance the swimmer encounters depends on velocity of movement and type of movement being performed in the water (for example, freestyle swimming versus breaststroke swimming).

Basic mechanical principles as applied to swimming involve the following:

1. The initial mechanical challenge  is changing inertia.
2. Velocity of swimming is directely related to stroke length and stroke rate.
3.  Propulsive force in the water should be applied so that linear body velocity fluctuations are minimized.
4. The body will move in the opposite direction from that in which the propulsive force is applied.
5.  Both lift and drag forces  result in forward progression of the swimmer.
6.  The more streamlined the body, the less external resistance to movement.
7.  The resistance or drag on a body in any fluid increases approximately with the square of the velocity.
8.  Movement of any body part in the water results in movement of the rest of the body in the opposite direction (i.e., lifting the head results in the feet dropping,.. therefore more body surface area is exposed to the water, which results in poor efficiency.
9.  Minimizing drag and enhancing metabolic power delivery (energy output) will result in improved swimming performance