Principles of Motion and jiu jitsu

Principle – Combining translatory (linear) and rotary motion.

Effective execution of a successful performance combines translatory and rotary motions.

Application. Since the serve involves synchronized motion of body levers, rotary motion is the most essential of the two motions.  The shift of weight from the back foot to the front foot as the movement begins requires rotary motion with only limited linear motion.  As the feet shift, the racket arm is extended at the side and slightly behind the hip.  The elbow then bends and the arm abducts.  As the elbow reaches the height of the shoulder, the wrist is abducted allowing the racket head to drop to a position behind the scapula.  The elbow is held high, allowing it to lead the motion of the forward swing which follows.This back swing motion must be executed in a fluent manner with the toss of the ball from the opposite arm (ball arm).  The ball arm begins its rotary motion to propel the ball upward into the air for striking.  This arm is fully extended in front of the server as the levers of the arm act to push and lift the ball.  This action gives the ball motion of its own, curvilinear motion and momentum.  Timing and the speed of the toss must be equal to that of the back swing for proper execution of the move.

No pause is experienced prior to the beginning of the forward swing.  The forward swing is begun by the racket elbow leading the upward motion, and flexion and lateral rotation of the glenoid cavity (shoulder) complete the upward move.  When the arm approaches a position above and forward of the shoulder, the forearm starts its rapid extension.  At a point where the racket arm is fully extended and the wrist is adducted, the ball is struck with the upper center strings of the racket.

Principle – Continuity of motion. When performing activities in which two or more consecutive motions contribute toward movement in the same direction, the performer should not pause between motions.

Application. Each motion of the back swing and ball toss should be done by successively moving complete body parts.  This same successive motion of body parts should be executed from the termination of the back swing to the forward swing carrying through the follow-through.  Unless one wishes to overcome inertia twice, there should be no sustained pauses or jerking in the motion.  Energy is conserved in the fluent motion of performing the serve.

Principle – Effects of momentum. When a moving object strikes another object, the greater its momentum at impact, the greater the force will be.

Application. Since momentum is the product of velocity and mass, increasing either factor will increase momentum.  In a serve it is possible to increase velocity only.  The racket may in fact be larger but is rarely significantly heavier, so mass is constant.  To increase velocity, the server can produce a harder swing by increasing muscular strength of the involved muscles and can more effectively hyperextend the back to generate more stretch of the involved muscles.  These work together to product greater velocity and a more forceful movement.

Principle – Transfer of momentum. Momentum developed in a body segment is transferred to the rest of the body only while the body is still in contact with the supporting surface.

Application. The momentum generated as the forward swing begins will pull the whole body off balance.  The racket pulled, as is the arm, into a position out of the center of gravity.

Principle – Maximum acceleration and efficiency of motion. All available forces should be applied sequentially with proper timing, and as directly in the intended line of motion as possible.  Body motions extraneous to the desired motion should be minimal.

Application. The major forces in the serve follow this essential sequence:
1.  Passive extension of the racket elbow.
2.  Arm abduction along with outward rotation of the arm.
3.  Scapular adduction and upward rotation of the glenoid cavity.
4.  Rotation of the thoracic spine.
5.  Elbow flexion of the ball arm.
6.  Elbow extension of the ball arm along with relaxation of the finger flexors.
7.  Elbow extension of the racket arm with hyperextension of the back.
8.  Adduction of the arm, inward rotation of shoulder and hyperextension of the racket elbow.
9.  Trunk flexion with hip flexion.The follow-through is a continuation of all of these.  Because of the strength and speed necessary for the serve, it must be executed in this sequence vigorously.  The ball arm is usually carried behind the server to maintain balance.

Principle – Counterforces in striking activities. The amount of force a striking implement imparts to an object depends upon the combined momentum of the implement and the object at the moment of impact.  Give in the implement reduces propulsive force.

Application. Strong muscular contractions of the involved muscles are essential for reducing the loss of force due to the give in both the racket and ball.  Grip on the racket should be firm, and the back foot must push strongly.  Tension increases on the strings of the racket may reduce loss of force as well.

Principle – Direction of the counterforce. The counterforce is directly opposite and equal to the applied force.

Application. The push of the back foot along with the strong adduction of the arm overhead produce the applied force of the swing.  The first force requires a strong push to propel the server upward and slightly outward leaving the ground.  The tendency to push too far forward reduces the advantage of the downward swing of the racket.  Full power is attained when hitting the ball at the top of the forward swing.  A broader starting stance insures more weight transfer is placed on the back foot.

Principle – Temporarily stored counterforce. If a surface, implement, or object used in a performance has elasticity, then an applied force produces bend or compression, which represents stored energy.

Application. Rackets may be strung at various tensions.  Some players prefer more tension on the strings to product more power to the serve.  A reduced tension adds energy to the striking force, but at the same time may reduce momentum of ball placement.

Principle – Leverage. By changing the amount and type of leverage, either speed (and distance) or force can be gained at the sacrifice of the other.

Application. The serve uses third class body levers.  These levers produce speed and range of motion (distance) at the expense of force.  Third class levers are especially effective in producing a fast striking force with a relatively light object.  By applying a total range of motion to each lever and utilizing great muscular force, the optimum effect can be obtained.

Motion and jiu jitsu