Muscle Spindles and the Stretch Reflex
Muscle spindles are sensory receptors that are located in muscle. Their job is to detect changes in muscle length and the speed of change in muscle length. Below is an image of a muscle spindle.
When muscles lengthen, the spindles are stretched. This stretch activates the muscle spindle which in turn sends an impulse to the spinal cord. This impulse results in the activation of more motor neurons at spinal level that send an impulse back to the muscle.
This impulse tells the muscle to contract with greater force in order to decrease the speed at which the muscle is being stretched. This is known as the stretch reflex and is shown on the following diagram;
The strength or degree of the muscles response is determined by the speed at which the stretch occurs; where the stretch occurs more rapidly, the spindle stimulates a greater firing frequency of the motor neuron, and the more forceful the contraction of the muscle is in response.
This response is primarily protective, to avoid the potential damage that could occur when a muscle is rapidly stretched beyond its limit. For example, when you accidentally walk into a pothole and roll your ankle – there is a rapid stretch of the ankle and spindles within the stretched muscle(s). Because of the speed of stretch the spindles fire rapid impulses to the nervous system causing a rapid contraction in the stretched muscles. This response protects you from a completely torn muscle or broken ankle, and usually results in nothing worse than a minor sprain.
As this response needs to occur rapidly (virtually instantaneously), the impulses from the spindles only go as far as the spinal cord. For this reason the stretch reflex is known as a spinal cord reflex and is completely involuntary. If the reflex was mediated by the brain precious time could be lost before the protective impulse to contract reached a stretched muscle, resulting in….a potentially far more serious injury.
Muscle spindles are active with all exercise. 
If the squatter lowered themselves to the bottom position slowly the muscle spindle response would be minor, if they lowered themselves quickly the tension produced in the quadriceps in response to the muscle spindle would be far greater. Ultimately the muscle spindle is trying to keep the muscle at its resting length.
Stretching muscles to generate force
After reading the previous section on muscle spindles, you may think that stretching a muscle under load is very dangerous as they have their own protective reflex to increase force and decrease stretch. Well it is dangerous but stretching a muscle immediately before contracting it can also generate a lot of beneficial force, if done correctly.
Think about what you would do if I told you to jump as high as you could? Your first movement wouldn’t actually be to jump ‘up’ would it? Your first movement would actually be a brief flex of the knees and hips to lower yourself before jumping ‘up’. This brief lowering enables the muscles to generate more force for the jump. The following three points summarise how this occurs.
1. Loading the muscle spindle: When activated the muscle spindle causes the muscle being stretched to generate tension to resist the stretch, so by stretching a muscle immediately prior to a contraction you effectively prime the muscle to create more force for that contraction.
2. The stretch-shortening cycle: One of the properties of skeletal muscle is that it is elastic in nature. Just like a rubber band if you stretch it, it will want to return to its resting length. This is known as the stretch-shortening cycle. When you stretch a muscle you effectively store its ‘elastic energy’ which can then be released to create more force than would exist without the muscle being stretched.
3. Achieving optimal contraction length: The optimal length of a muscle to generate force is actually 1.2 x its resting length. This is the length that generates the most tension and primes the elastic properties of the muscle without causing damage to the muscle. Stretching a muscle beyond 1.2x its resting length increases the likelihood of damaging the muscle.
Below is a picture of a baseball pitcher. As he gears up to pitch the ball all three of the above principles are being applied to the muscles of his shoulder, allowing him to generate maximum force for the throw.
