Jumping is not as easy as it seems, especially when one starts to consider how this rather simple task, fundamental to most of us, is accomplished. Jumping is a coordinated sequence of joint movements where the muscles interact to optimize performance. When testing, training or working in rehabilitation, understanding these interactions will allow you to make better decisions in program design.
In order to make this analysis of the jump a little simpler, let us assume that we are to perform a vertical jump.
In order to get the basics straight, we know, based upon Newton´s third law (action and reaction), that the force a person is applying to the ground creates an equal and opposite directed force. If this force is greater than the weight of the person jumping there will be a net sum of force acting upon the person in the opposite direction.
Consequently, the person will accelerate according to Newton´s second law (SF=ma). That is, the acceleration of the jump will vary based on the weight and mass of the person jumping.
The question then becomes; how is this vertical force created?
This is where it starts to get complicated.
Based upon an observation of jumping, it becomes apparent that the linear displacement of a person is dependent upon the rotation of different joints. A bi-sagittal model is good and easy to use for this analysis, since it limits analysis to the sagittal plane.
Furthermore, if we simplify the analysis further to consist of only the lower extremity joints (hip, knee and ankle complex) it makes the analysis even “easier”.
If we then impose constraints on the system, in this case limiting joint rotations to the sagittal plane, we limit the possible solutions available to a person to solve the task of jumping.
Let us also assume that only a vertical displacement of the center of mass of a person is possible. This is where we have to define center of mass. This is not an anatomical structure, but rather a point where all the mass of a system, in this case a person, is located. Center of mass is used for calculations and is based upon the position of different body segments it will change position either within or outside the body. If the center of mass is to move vertically, all joints of the lower extremity have to move in a coordinated manner in order to accomplish the task. Isolated flexion and extension movements of one lower extremity joint, hip, knee or ankle, will move the center of mass not only vertically, but also anterior and posterior.
This means that if we are to effectively lower and elevate our center of mass we are dependent upon these three lower extremity joints.
Kinematic analysis of a vertical jump show a proximal to distal sagittal joint rotation sequence, which is also associated with a similar sequence of muscle activation patterns (EMG) (van Ingen Schenau, 1989). This proximal to distal sequence of joint rotation will translate the center of mass vertically. Van Ingen Schenau eloquently describes how this happens in his paper from 1989 (van Ingen Schenau, 1989). This is a brief and simple summary of his brilliant findings and descriptions. When hip rotation (extension) can not accelerate the center of mass vertically any more the force generated in the mono-articular hip extensor, such as the gluteus maximus, is transferred to the bi-articular rectus femoris. This happens as the muscle activation of hamstrings decrease and the activation of the rest of the quadriceps increases. Thus, the force generated proximally at the hip is transferred into knee extension. The knee extends and when further knee extension angular acceleration is useless, ankle plantarflexors are activated to control and transfer knee extensor output into a rapid plantarflexion.
Why is this important to know?
To sum up, human function is interconnected in both joint movements and muscle function for effective movement. This knowledge is important in both training and rehabilitation. Furthermore, one has to keep in mind that this finely coordinated movement pattern is executed by the central nervous system.
van Ingen Schenau, G. J. (1989). From rotation to translation: constraints on multi-joint movements and the unique action of biarticular muscles (Vol. 8, pp. 301-337). (Reprinted from: IN FILE).