In plyometric exercise powerful muscle contractions are used in response to rapid, dynamic stretching. This is a type of training that uses the stretch reflex to facilitate recruitment of motor units. 4 The stretch reflex is facilitated by the muscle spindles and its primary purpose is to maintain posture and equilibrium. 8 This page will look at the role of the stretch reflex in plyometric exercise.

Functional Anatomy

The muscle spindle is a proprioceptive receptor that sense changes in skeletal muscle length, this information helps the brain to monitor where the body is in space. Muscle spindles run parallel to the extrafusal muscle fibers and they are thickest in the middle and taper out at the ends, they are 4-10 mm in length. 8 Muscle spindles contain three kinds of intrafusal muscle fibers: static and dynamic nuclear chain fibers and nuclear bag fibers. The nuclear chain fibers are skinnier and shorter than the nuclear bag fibers, this difference in shape is due to the fact that the nuclei in the chain fibers are even spread out in a line, or a chain, in the middle of the fiber, but in the nuclear bag fibers the nuclei are more clumped together in the
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Figure 2

Figure 1
middle of the fiber where the bulge, or bag, is seen (Figure 2). Nuclear chain and static nuclear bag fibers report information about the static length of the muscle. Dynamic nuclear bag fibers report information about the rate of change of muscle length or the velocity. A typical spindle has 1 static nuclear bag fiber, 1 dynamic nuclear bag fiber, and around 5 nuclear chain fibers.
There are three types of nerve fibers that synapse to intrafusal fibers. There are two sensory nerve fibers: group Ia fibers, group II fibers. 5,6
  • The group Ia fibers are also called primary afferent fibers and form annulospiral endings in the equatorial region of dynamic and static nuclear bag fibers and nuclear chain fibers. They detect small changes in muscle length and are also sensitive the rate of change in length. They have a tonic firing rate and increase their firing rate during stretch and then after the stretch continue firing at a tonic rate. The tonic firing rate after being stretched is higher than it was originally as the muscle has a new length (Figure 3). Similarly, if the intrafusal fiber goes slack than there will be a decrease in firing rate from the Ia fiber, it will actually stop firing for a brief period of time and then continue at an appropriate rate for the new length (Figure 4). This decrease in firing rates is known as the dynamic index. Ia fibers have thick axons, 1-20 μM in diameter, and very high conduction velocities ranging from 70-120 m/s. Usually there is a single Ia fiber per spindle that innervates each intrafusal fiber once. 5,8
  • The group II fibers are mostly found on the juxta-equatorial region found in between the equatorial and polar regions of the nuclear chain fibers where they form flower-spray endings. They are also found on the polar region of the static nuclear bag fibers. They are sensitive to the length of the muscle, but not the rate of the change in length. They fire at a tonic rate, the frequency of their firing rate changes as the muscle is stretched. The group II fibers synapse on interneurons the intermediate ventral horn. There are usually several group II fibers per muscle spindle. Group II axons are not as thick as Ia axons, 6-12 μM, and have slower conduction velocities, 30-70 m/s. 5,8
There is also a motor neuron that innervates the intrafusal fibers, the gamma motor neuron.
  • Gamma motor neurons originate in the anterior horn of the spinal cord and innervate the polar regions of the intrafusal muscle fibers. Gamma motor neurons are smaller and slower than alpha motor neurons. When they stimulate the polar regions of the intrafusal muscle fibers to contract it stretches the non-contractile equatorial region. This allows the gamma motor neurons to control how sensitive muscle spindles are to being stretched. The contraction of the intrafusal fibers doesn't cause a difference in the overall tension of the muscle. The gamma motor neuron can provide enough contraction in the intrafusal fibers to cause enough stretching of the equitorial region so that the Ia and II fibers receive their adequate stimulus. 5,8

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Figure 3

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Figure 4

The Stretch Reflex

The stretch reflex begins when the The stretch reflex happens when the Ia fiber receives an adequate stimulus, the adequate stimulus for a Ia fibers is the stretching of the equitorial region. This will cause the annulospiral endings to sense the stretch causing the Ia fiber to fire at a higher rate. The Ia fibers enter the spinal cord through the dorsal root. In the spinal cord they have a monosynaptic connection to the homonymous alpha motor neuron, they also innervate an inhibitory interneuron the synapses with the antagonist alpha motor neuron - the process of activating the agonist muscle and inhibiting the antagonist muscle is called reciprocal inhibition. Thus the Ia fiber is activating the homonymous muscle while inhibiting the antagonist muscle when stimulated. As the muscle is stretched more rapidly the Ia fibers will fire more rapidly. Stretch reflexes often have groups of muscles involved at a time, not just a single agonist and antagonist muscle. This is because there are often groups of synergistic muscles working together and when one of them is stretched it is likely that others will be stretched as well since they are usually in similar locations on the same side of the joint. Another reason is the the monosynaptic connection to the homonymous alpha motor neuron will cause the antagonist muscle to stretch causing a stretch reflex in the antagonist muscles. This effect leads to stabilization of joints and helps to dampen some unwanted muscle activity which helps movement to be smoother and less jerky. When the CNS tells the alpha motor neuron to fire it will also signal the gamma motor neuron to fire so the spindle length will adjust along with the length of the extrafusal fibers, this mechanism is called alpha-gamma coactivation. 1,5,6,8,9,10

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Figure 5

The Role of The Stretch Reflex in Plyometrics

Plyometric exercise involves a lot of dynamic, fast movements such as box jumps.The basis for plyometric exercise is that these types of movements will allow for the stretch-shortening cycle to occur. This cycle uses mechanical as well as neurophysiological mechanisms to increase the output of a movement. The output, or force production in the muscle, is increased because in addition to the motor commands being sent to the muscle that occur in a normal movement there is also the added affect of the stretch reflex and elastic properties of the muscles. The stretch-shortening cycle has three main steps, first the stretched which stretches the spindles and briefly stores elastic energy in the muscle. Secondly, there is a brief pause as the Ia fibers of the spindle synapse to the alpha motor neuron. Finally, as the motor neurons are stimulated the muscle begins to contract, releasing the stored elastic energy as mechanical energy.
The impact of this cycle on the actual force production is highly variable. One reason is because the strength of the response to the stretch reflex can change based on the amount of stretch in the movement, which will affect the Ia firing rate, which in turn will affect its temporal summation effect on the alpha motor neuron. The Ia fiber could also be presynaptically inhibited. The level of response seen in the alpha motor neuron can also change because the muscle spindles are not the only structure sending input to the alpha motor neuron, other receptors along with higher levels of the CNS send input to the alpha motor neurons. This would affect the spatial summation of the alpha motor neuron. Another way that the output of the cycle can vary is because of the type of muscle that is involved. There is evidence that the stretch-shortening cycle effects fast-twitch and slow-twitch muscle fibers differently. There is an increase in the stretch-shortening cycle when doing small amplitude movements for people with more fast-twitch fibers, whereas people with more slow-twitch fibers are able to store elastic energy better. As previously stated the effect of the stretch reflex is to increase the amount of excitation input on the alpha motor neurons. This effect may increase the number of motor units recruited when compared to , or increase the rate code for an alpha motor neuron. A rate code is a way for the alpha motor neuron to increase its contractile force by increasing the number of action potentials it sends out. The more the alpha motor neuron is stimulated the more it will fire. 3,4,7


The muscle spindle, through the mechanism of the stretch reflex provides an effect during plyometric exercise that leads to an increase in muscle force output. In plyometrics the stretch reflex is a major component of the stretch-shortening cycle that is elicited through fast, dynamic movements which cause rapid stretch to a muscle. The overall effect of this cycle on plyometric exercise is variable and changes due to a number of conditions.

  • adequate stimulus - the form of stimulus to which a receptor is most sensitive 5
  • alpha-gamma coactivation - the tendency for alpha and gamma motor neurons to be activated together 5
  • annulospiral endings - the termination of a group Ia afferent fiber in the equatorial region of a muscle spindle 5
  • antagonist muscles - the muscle that acts in opposition to another 5
  • dynamic index - the difference in rate of discharge of a spindle receptor during active stretching and maintained stretch 5
  • equatorial region - the center, non-contractile region, of an intrafusal muscle fiber 5
  • extrafusal muscle fibers - the regular contractile fibers that produce the shortening of a muscle; they lie outside the muscle spindle 5
  • fast-twitch muscle fibers - also called type 2 muscle fibers. A type of muscle fiber with a low oxidative capacity and a high glycolytic capacity;associated with speed or power activities 4
  • flower-spray endings - the termination of group II afferent fibers on the nuclear chain fibers of a muscle spindle 5
  • homonymous muscles - the muscle of origin of the nerve fibers being considered 5
  • intrafusal muscle fibers - the contractile tissue of the muscle spindle; it lies inside the spindle 5
  • juxta-equatorial region - the region of the intrafusal fibers that is in between the equatorial and polar regions 5
  • monosynaptic - when there is only one synapse within the CNS 5
  • polar regions - the contractile ends of the intrafusal muscle fibers 5
  • presynaptic inhibition - decrease in the probability of firing of a neuron due to decrease in transmission to it that results from hypopolarization of afferent terminals on it 5
  • proprioceptive receptors - A sensory receptor of the muscles, tendons, and joint capsules that detects muscle contractions and joint movements 8
  • rate coding - refers to the frequency of impulses sent to a muscle. Increased force can be generated through increase in either the number of muscle fibers recruited or the rate at which the impulses are sent. Also called frequency coding. 4
  • reciprocal inhibition - interconnections of neurons arranged so that pathways that excite one group of motor neurons inhibit the antagonist motor neurons 5
  • slow-twitch muscle fibers - also called type 1 muscle fibers. A type of muscle fiber that has a high oxidative and a low glycolyitic capacity, associated with endurance-type activities 4
  • spatial summation - algebraic addition of generator potentials elicited by stimuli at two different points 5
  • synergistic muscles - a muscle that has the same action at the same joint as another muscle 5
  • temporal summation - algebraic addition of generator potentials elicited by two stimuli applied at the same point in rapid succession 5

Further Reading

These links provide additional information about plyometric training:

A mini-lecture on the stretch reflex:


  1. Muscle spindles are located in the extrafusal muscle fibers and contain intrafusal muscle fibers. T
  2. During the stretch reflex reciprocal inhibition causes the agonist muscle to be inhibited. F, antagonist
  3. Muscle spindles can be up to 1cm in length. T
  4. There are two kinds of nuclear chain fibers, dynamic and static. F, two kinds of nuclear bag fibers
  5. Group II fibers create flower-spray endings on the juxta-equatorial region of the nuclear chain fibers. T
  6. Group Ia fibers create annulospiral endings on the equatorial region of nuclear bag and nuclear chain fibers. T
  7. The adequate stimulus for the Ia fibers is vibration. F, stretch
  8. When the intrafusal fibers are stretched the Ia fibers will fire at a higher frequency. T
  9. The gamma motor neurons help set the sensitivity of the muscle spindle. T
  10. Gamma motor neurons innervate smaller muscles and alpha motor neurons innervate larger muscles. F, alpha motor neurons innervate all skeletal muscles, gamma motor neurons innervate the intrafusal muscle fibers

Short Answer
  1. Describe the stretch reflex in detail.
  2. What is the role of the stretch reflex in plyometric exercise.
  3. Briefly describe the stretch-shortening cycle.

True/False Answers: T,F antagonist,T,F there are two kinds of nuclear bag fibers,T,T,F stretch, T,T,F alpha motor neurons innervate all skeletal muscles, gamma motor neurons innervate the intrafusal muscle fibers


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