Types of Muscle Contraction

This week, instead of focusing on a specific muscle I'm going to talk about types of muscle contraction in the horse.
Types of Contraction:
Understanding muscle contraction is another concept that enables the rider to train more sympathetically. Muscles are signalled to contract via nerve impulses. Relaxation occurs when the nerve impulses cease. Very simply as muscles work they perform one of two actions.
These are:
1. Isotonic where the muscle contraction results in movement. This type of contraction can be subdivided into two categories although all movement uses a mixture of both. They are:
- Concentric where a muscle shortens to create movement and
- Eccentric where the muscle gradually lengthens to control movement, support and stabilise joints. This type of contraction also acts as a shock absorber during an abrupt movement such as coming to a sudden stop.
2. Isometric where the muscle is working hard but there is no change in the muscle length as it contracts to hold a position.
Isometric and Eccentric muscle contractions tend to create greater fatigue, tension and associated discomfort than concentric muscle contraction, particularly if the muscle is not appropriately conditioned. When relating these contractions to training it is particularly important to understand the significance of isometric contraction.
Horses commonly use isometric muscular contraction during dressage particularly when working in more advanced outlines. When top line muscles of the neck including the splenius muscle, contract concentrically (the muscle shortens in length) they extend the neck and raise the head. When the muscles work isometrically they contribute towards maintaining a flexed outline supporting the heavy weight of the head. When the horse is working in a more novice outline, or with the neck stretched long and low, the nuchal ligament plays a greater role in supporting the weight of the head and neck.
The horse uses isometric muscle contraction particularly in the abdominal muscles, deep back muscles and iliopsaos muscles to support the back when carrying a weight.
When the horse performs movements requiring high levels of collection and engagement the hamstring muscle group which are part of the extensor chain work isometrically both to support the joints in a more flexed position and carry a greater percentage of the weight.
Horses also use isometric muscle contraction to brace and support themselves whilst travelling.
To perform isometric contraction consistently and over a longer period of time requires strength and appropriate muscle conditioning. Imagine you were asked to hold a weight up high at arms length for several minutes or to stand with all your leg joints in a flexed position. You would soon begin to feel a burn or an ache in the muscle as a result of performing these isometric muscle contractions. To relieve this you would need to move the body part.
As isometric muscle contraction requires strength and appropriate conditioning, problems may start to arise if young or relatively unconditioned horses are asked to work in too advanced an outline for too long. The horse may begin to compensate by trying to maintain the outline through concentric muscle contraction rather than isometric. The rest of the dorsal chain muscles may also start to shorten (concentrically) as part of the evasion mechanism. This helps to explain why it is so important to allow horses freedom to stretch and move his neck at regular intervals during training.
There are many aspects to appreciating how muscles contribute to movement. Understanding the concept of muscle chains and how isotonic and isometric contraction affects training are only two very small contributory parts to a wide and complex subject.
Thanks to "Horses Inside Out" for use of this great article!