Across a range of different high-intensity sports like running, sprinting, changing direction, throwing and kicking, the body creates stability by cocontracting or co-activating muscles that surround joints or regions that are under stress. Cocontraction provides stability to some segments or body regions – so that they can be controlled – whilst others move.
Throughout a movement like a free kick in football, the body provides cocontractions around the ankle, knee and hip of the standing leg at more or less simultaneous moments. This creates a stable base to generate and transport energy for the kick. During a maximal sprint, subsequent cocontractions around the hip and trunk help in transmitting ground reaction forces from the legs, rotational forces and general coordination of the sprinting movement. During overhead movements (tennis serve, handball or water polo throw, baseball pitch), cocontractions around the shoulder provide a stable and robust base for energy transport from the pelvis and trunk to the elbow, wrist and hand which also protect the various joints. In agility movements, cocontractions around the ankles (for example) allow effective processing of ground reaction forces, thereby enhancing movement speed, balance and coordination to evade or pass an opponent
A good example of movement control through cocontractions can be seen in trunk control during sprinting and kicking/throwing. When looking at the trunk from the sagittal plane during running, as shown below, cocontractions between the abdominal muscles and spinal extensors create a ‘neutral’ position of the lumbar spine, providing stability in this region. This is considered the optimal position for the trunk to generate and transfer kinetic energy. If the body is not capable of maintaining this position (e.g. during excessive anterior tilt of the pelvis), this can potentially lead to suboptimal control of movement, energy leaks and risk of subsequent injuries like low back pain, hamstring strains or hip injuries.
The advantages and disadvantages of cocontractions
Cocontractions at these different regions contribute to successful coordination and performance of high intensity movements during sports. At the same time, they protect the specific body parts from incoming forces and perturbations and ensure that no energy is leaked by excessive motion of these parts. Also, the faster a movement, the more important cocontractions are as they can help any error signals sent by the central nervous system. By contracting together, they keep the joints at which they act upon more or less balanced and reduce the possibility of sensory errors from the central nervous system. Cocontractions also do not rely on proprioceptive feedback to the brain and subsequent ‘instructions’ from the brain about how to perform movement. One of the biggest advantages of not involving the brain in control of movement is that this costs very little time and the brain has more space available for other tasks.
However, all this comes with a cost. Cocontractions decrease movement speed because contraction of antagonist muscles limit the speed and intensity of agonist contractions. Cocontractions might thus reduce errors in movements, but they decrease and potentially limit intensity and speed of movement as well. A likely reason why the body limits movement in this way is to ensure the body is capable of dealing with the magnitude and intensity of the applied forces/perturbations and to keep movement controllable.
Cocontractions in context: training the trunk
Explaining how cocontractions work at a high level is helpful, but taking a look at specific examples can help coaches better understand how this works in practice and how it is trained. To help show this we’ll take a look at how the trunk functions during high speed movements and how it can be optimally trained.
Although there is no universally accepted definition, trunk stability in sports has previously been defined by Kibler et al. as “the ability to control the position and motion of the trunk over the pelvis to allow optimum production, transfer and control of force and motion to the terminal segment in integrated athletic activities.” If we look at high-intensity movement specifically, the trunk has been highlighted as an important body part for:
- providing flexibility and mobility in three planes of movement;
- providing postural control and stability in those same three planes;
- absorbing large external forces and perturbations;
- generating and dissipating kinetic energy;
- storing and releasing elastic energy (through isometric-elastic muscle function)
- enabling athletes to position their body in space to react to the environment.
The trunk muscles have a narrow force-length curve, as Frans Bosch described in his most recent book. This means that if either of these muscles is forced to function in a lengthened position, they cannot produce much force. If the trunk operates through larger ranges of motion (rotation, flexion or extension) with opposing and rotational forces, it can therefore be quite difficult for the trunk muscles to provide stability and control movement. The way the trunk corrects or limits this from happening is by finding the optimal position to produce force and provide stability: through a more or less neutral position of the trunk in relation to the pelvis. In this position, muscle length of the abdominal and spinal muscles is optimal to deal with large external or rotational forces in a short amount of time. Cocontractions of the trunk muscles are therefore a major stabilizing element and largely influences how the trunk is trained.
Examples of training trunk cocontractions
Here are some examples of how this might look in training, moving from utilizing cocontractions in more isolated movements to integrating the trunk cocontractions with high speed running:
The trunk should be trained in a context that is representative for the movement patterns the athlete performs during sport (contextual) and that stress and pressure is put on the coordination of the trunk within these high-intensity movement patterns (coordinative). The main objective of this way of training the trunk is developing preflexes and enhancing self-organization. This is performed through training cocontractions of the trunk in multiple-sport specific positions and stimulating the athlete to focus on the outcome of the movement (knowledge of result) rather than how he performs the movement. Exercises are designed according to the specific trunk function and muscle function identified above. In practice, this means paying attention to several elements in exercise design, as seen in the examples above:
- Specificity – Assuring similarity in internal and external structure of the movement, energy production, sensory patterns/sensory feedback and intention between the movement the athletes perform during sports and the exercises used in training.
- Overload – In order to strengthen the movement there must be overload, but be aware that the more specific the exercise is, the less likely this will lead to overload or increase of strength. Balancing specificity and overload is key.
- Constraints-Led Approach (CLA) – The search for specificity in exercise selection does not mean that every exercise that is used needs to replicate the exact sporting movement. Using a can assure the exercises overlap with the context of the sport, appropriate muscle function and contain sufficient context-related variability. Through regression or progression – limiting or adding degrees of freedom, involving few or multiple joints, varying movement speed and intensity – the challenge for the trunk muscles can be constantly shifted.
- Variation – By inducing variability in the environment (constraints), athlete (general or regional fatigue and cognitive demand) and task, a new, unique challenge can be constantly created. By getting the trunk to do multiple conflicting things throughout a workout, you allow the body to find the optimal position in which the trunk functions. This improves coordination, adaptable movement control under pressure and creates robust cocontractions to protect the body from large external forces.
- Speed – As Bosch describes, there is very little possibility for training of low speed, low-intensity movements to transfer to high speed, high-intensity movements. One of the most important considerations in designing training drills for the trunk is thus similarity in intensity and speed of rotational movements. For appropriate training of the trunk, it is therefore important that forces and perturbations are fast and vigorous to put the trunk under stress and force cocontractions to oppose forces through rotation and anti-rotation.
- Focus of attention: During performance and movement, the athlete is not focused on how the movement is performed (knowledge of performance), but on the effect of the movement (knowledge of result). Training and preparing the trunk should therefore aim to produce knowledge of result.