G. KINETIC LINK PRINCIPLE

The kinetic link principle (KLP) refers to the method in which the body produces momentous force. A summation of forces is produced through the body via a  ‘linking’ process of a number of segments together (37). Diagram 5 produced by Kreighbaum et al. (37) describes a sequential process by which a force is produced. It describes force starting from the proximal segments of the body and progressively transferring momentum to the distal segments. This is possible via the proximal segments being larger in mass and having a greater inertia. An upper body throwing action may be used as an example. The initial segments (i.e. trunk rotators) accelerate eliciting a force, where the next body segment (i.e. shoulder horizontal adductors / internal rotators) then initiates an accelerating force, which then refers on to the upper arm (i.e. elbow extensors), and the end point (wrist and finger flexors). The key to this process is that each segment is smaller than the previous and contains less inertia. As one segment accelerates it allows the following segment to initiate movement at its top speed as it passes on momentum to the next segment before it decelerates.  The speed of the consecutive segments gradually increases, with the result allowing the end point velocity of the final segment to be much greater than the initial. This biomechanical process occurs through the “serape / functional line effect” as mentioned previously, and can be applicable to not only a throwing action, but a range of different functional skills.

Subijana et al. (57) performed a biomechanical study on kinetic energy transfer during the tennis serve. They analysed two tennis player’s service actions via a system referred to as 3D photogrammetry, where a proximal to distal segment kinetic energy transfer was discovered by a mean correlation analysis. The analysis found that for one of the players it could successfully predict the quality of the serve with 100% accuracy and the other player 76%, based on the kinetic transfer properties seen during the action.   

Roetert et al. (52) referred to a tennis serving action. He reported the following findings in the literature:

• The largest portions of kinetic energy or force generated in the serving stroke are developed in the legs and trunk, where 51% of the kinetic energy and 54-60% of the total force are produced.

• As seen in diagram 5 and 6, each segment has a cocking or stabilization phase, and an acceleration phase.

• Segment ‘drop out’ or kinetic chain breakage decreases the ultimate force and energy available to produce a forceful movement, and also puts excessive unnecessary strain on the surrounding segments. 

• A 10% reduction in energy transferal from the hip or trunk requires a 14% increase in shoulder rotation velocity or a 22% increase in shoulder mass to create the same amount of kinetic energy for the movement.

The KLP is a very trainable aspect of functional development where analysis of the process one uses to efficiently accelerate at different joints can be identified, and exercise prescription can be applied to enhance the method of force transferal through the body. This process is common in a number of sports such as baseball, golf and tennis (32, 57), where biomechanical feedback is given to the individual athlete so alterations can be made to decrease the potential for injury occurrence and improve the power profile through one or more specific sporting actions.

Diagram 5. The Kinetic Link Principle. Kreighbaum et al. (37)

Diagram 6. The Kinetic Link Principle In Action. www.timhartwig.com

(Transferal of momentum / summation of forces from proximal to distal)