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What Personal Trainers Should Know about Muscle Spindles and Golgi Tendon

This article is originally published in the NESTA Personal Fitness Trainer manual.

Muscle Spindles and Golgi Tendon Organs in Action

When a muscle is stretched too far or too quickly, the muscle spindle causes the muscle to contract as a protective mechanism. The muscle spindle is a sensory receptor that lies within the muscle fibers and detects muscle length and change in the rate of lengthening. The resulting muscular contraction created by the muscle spindle serves to protect the muscle from over-lengthening. It does so with an excessively quick contraction (more than the current musculotendinous conditioning will allow) and is known as the stretch or myotatic reflex.

In order to achieve significant benefit from stretching and avoid injury, it is imperative that the muscle spindle is overridden by gradually lengthening the muscle through a proper warm-up, or stretch, that gives the Golgi tendon organ (GTO – another sensory receptor) sufficient time to override the muscle spindle (20-30 seconds). The Golgi tendon organ protects the muscle by causing it to relax when the muscle develops too much tension or incurs a change in tension too quickly. The GTO monitors tendon length and is found in the tendons at the ends of the muscular fibers, where they attach to the bone.

The act of causing the muscle to relax in this manner is known as autogenic (self-generating) inhibition. An example of autogenic inhibition occurs by bringing the pectoralis major through its full range of motion (horizontal abduction) where it contracts to protect itself and is then allowed to relax when the GTO overrides the muscle spindle. Reciprocal (inverse) inhibition occurs when the antagonist relaxes due to contraction by the agonist. For example, the hip flexors will be reciprocally inhibited and forced to relax when their antagonist, the gluteus maxi- mus, contracts.

What is a Muscle Spindle?

The muscle spindle is a special sensory receptor that lies within skeletal muscle.

What does a Muscle Spindle do?

Muscle Spindles act as a simple motor reflex (sensory neuron terminates at the spinal cord resulting in an involuntary motor response to a stimulus). These sensory receptors senses the length of the muscle and rate of change of the length (i.e. they trigger a reflexive muscle action when the muscle spindle is stretched).

Why do we have Muscle Spindles?

Muscle Spindles act as a protective mechanism. If a muscle is quickly stretched, the muscle spindle causes the muscle to increase its force production to prevent potential damage.

Structure of the Muscle Spindle

A muscle spindle consists of 4-20 specialized muscle fibers (intrafusal) as well as the nerve endings (sensory and motor) associated with the fibers.

What Personal Trainers Should Know about Muscle spindles

Muscle spindles are constantly at work. Because muscle spindles respond to stretch or changes in length of the muscle, they are constantly firing throughout the day as we move. For example, when performing a bicep curl with free weights one must first pick up the dumbbell. As you pick up the weight and bring your arm to the starting position (fully extended) the muscle fibers are stretched. This movement will also activate the muscle spindles to be stretched which then send an impulse to the spinal cord activating the alpha motor neurons. This causes more force to be produced by the bicep which will allow the bicep to overcome the stretch of the muscle.

What is a Golgi Tendon Organ?

Golgi Tendon Organs are specialized sensory organs that are located at the muscle-tendon junction.

What does a Golgi Tendon Organ do?

Golgi Tendon Organs are proprioceptors or special sensory receptors that detect changes in muscle tension and force produced by muscle contractions. They are highly sensitive and capable of signaling even minute changes in muscle tension. GTOs have the ability to override muscle spindles.

Why do we have Golgi Tendon Organs?

GTOs tend to be inhibitory by nature mean- ing they allow the muscle to relax when being stretched. They are a protective mechanism and protect the muscles from overload by monitoring the force of muscle contractions. This protects the muscle from possible damage during activities requiring a great deal of force.

Structure

Golgi Tendon Organs are located next to where to tendon fibers attach to the muscle fibers. They contain Type Ib nerve fibers. The ratio of extrafusal fibers to GTOs is 10:1 in each skeletal muscle. They are encapsulated sensory receptors meaning they are surrounded by a protective layer. GTOs fire much slower than muscle spindles do.

Function

GTOs are critical in monitoring the force when there is an increased level of tension in the muscle. They are activated with active contraction or passive lengthening meaning their function increases when the stretch on the muscle spindles is released. GTOs are important in pro- tecting the muscle from damage by monitoring the intensity or force of a contraction.

Practical Application

The first step in the Five Levels of Fitness is neu- romuscular facilitation. Neuromuscular facilitation involves improving coordination and control of the neuromuscular system. Better neuromus- cular control leads to better control of the GTOs and the other proprioceptors. This can be seen when maximally lifting heavy weights. If your neuromuscular system is better trained and developed the GTO response will be less sensitive to greater amounts of force applied to the muscles (such as the forces during resis- tance training). This in turn will allow for heavier weights to be successfully lifted because the GTOs will be less sensitive to the force being lifted. Also, having better GTO control will allow for movement and coordination to be more fluid and in proper sequence.

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