|
|
|
|
|
Elasticity
When a limb is rotated around a joint by an agonist muscle, the antagonist muscle stretches. Muscles have elastic properties so they resist the stretch and store elastic energy. When the agonist muscle(s) stop their work the elastic energy in the antagonist muscle(s) is released and the limb returns to a neutral position. The neutral position is usually half way between the limb's extreme positions but can be temporarily affected by external forces such as gravity or an imbalance in agonist/antagonist stretch distance to resistance ratio.
Returns and unwanted tension
When an individual is playing in a relaxed way (without unwanted tension) the returning of the limb to a neutral position (hereafter called a return) gives the visual appearance of a bounce from a limb's extreme position back towards the neutral position. The return is a gentle motion even in high speed movements, although it does increase its amplitude with the motion speed.
In a movement it is normal to see agonist action followed by antagonist action. The antagonist action is there to offer control and to protect the system from high speeds at extreme joint positions. There can be a degree of co-activation of both agonist muscles and antagonists muscles. This appears to be less in expert performances and it is reasonable to assume that minimising co-activation is a positive move in producing higher speeds. Furthermore, what athletes and coaches describe as 'unwanted tension' or 'feeling tense' (especially at critical moments) is probably over activation of antagonist muscles and therefore increased co-activation and reduce movement speeds.
We have theorised that unwanted tension can be diagnosed by a reduction of the amplitude, or disruption in the form, of returns. When this is combined with the principal of continuity as described in the next section, this makes returns a useful diagnostic tool in detecting unwanted tension.
Stretch shortening cycles
A stretch shortening cycle (SSC) describes a movement that begins in the opposite direction to the goal, thus stretching the goal agonist muscles, followed rapidly by the goal movement (in the opposite direction). The importance of the stretch shortening cycle is that it has been shown to produce higher speeds than concentric muscle action on its own. The exact reasons for this are still debated but probably include: utilisation of stored elasticity, greater range of movement and therefore time to ramp up force and pre-activation of muscle tension. What is not debated however, is that the stretch shortening cycle is beneficial.
The timing of voluntary muscle activation is important so that opposing muscle groups are not strongly activated at the same time. Also, the change of movement from away to towards the goal must happen quickly. Examples of where SSC's can be used are in back swings and jumping (using a counter movement jump) . The extra energy provided by the stretch shortening cycle gives more power to the shot or height to the jump. Note that body parts can undergo independent SSCs that can be sequenced at different times. More details of this will be presented in the specific technique sections.
| Prev: Muscle characteristics | Next: Principal of Continuity |