Plyometric+Training

__ Introduction __

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   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 There is also a motor neuron that innervates the intrafusal fibers, the gamma motor neur on.
 * The group Ia fibers are also called primary afferent fibers and form **annulospiral endings ** in the **<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%; line-height: 1.5;">equatorial region **<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%; line-height: 1.5;"> 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. <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%; line-height: 1.5; vertical-align: super;">5,8
 * <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">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. <span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; line-height: 1.5; vertical-align: super;">5,8
 * <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">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 **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;">adequate stimulus. ** 5,8



__ The Stretch Reflex __

<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">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**. <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%; vertical-align: super;">1,5,6,8,9,10

<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; line-height: 1.5;">

__ The Role of The Stretch Reflex in Plyometrics __

<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">Plyometric exercise involves a lot of dynamic, fast movements such as box jumps. <span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; line-height: 1.5;">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 <span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;">stretch-shortening <span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; line-height: 1.5;">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. <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%; line-height: 1.5;">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 <span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; line-height: 1.5;">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. <span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; line-height: 1.5; vertical-align: super;"> 3,4,7

__ Conclusion __

<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">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 <span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;">stretch-shortening cycle that is <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%; line-height: 1.5;"> 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.

__ Glossary __
 * **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; line-height: 1.5;">adequate stimulus **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; line-height: 1.5;">- the form of stimulus to which a receptor is most sensitive <span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; line-height: 1.5; vertical-align: super;">5
 * **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; line-height: 1.5;">alpha-gamma coactivation ** - the tendency for alpha and gamma motor neurons to be activated together 5
 * **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; line-height: 1.5;">annulospiral endings **- the termination of a group Ia afferent fiber in the equatorial region of a muscle spindle 5
 * **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;">antagonist muscles **- the muscle that acts in opposition to another 5
 * **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; line-height: 1.5;">dynamic index **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;">- the difference in rate of discharge of a spindle receptor during active stretching and maintained stretch <span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; vertical-align: super;">5
 * **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; line-height: 1.5;">equatorial region - **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; line-height: 1.5;">the center, non-contractile region, of an intrafusal muscle fiber <span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; line-height: 1.5; vertical-align: super;">5
 * **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;">extrafusal muscle fibers - ** the regular contractile fibers that produce the shortening of a muscle; they lie outside the muscle spindle 5
 * **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;">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
 * **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;">flower-spray endings **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;"> - the termination of group II afferent fibers on the nuclear chain fibers of a muscle spindle <span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; vertical-align: super;">5
 * **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;">homonymous muscles **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;"> - the muscle of origin of the nerve fibers being considered <span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; vertical-align: super;">5
 * **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;">intrafusal muscle fibers **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;"> - the contractile tissue of the muscle spindle; it lies inside the spindle <span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; vertical-align: super;">5
 * **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;">juxta-equatorial region ** - the region of the intrafusal fibers that is in between the equatorial and polar regions 5
 * **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;">monosynaptic ** - when there is only one synapse within the CNS 5
 * **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;">polar regions ** - the contractile ends of the intrafusal muscle fibers 5
 * **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;">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
 * **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;">proprioceptive receptors ** - A sensory receptor of the muscles, tendons, and joint capsules that detects muscle contractions and joint movements 8
 * **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;">rate coding **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;"> - 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. <span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; vertical-align: super;">4
 * **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;">reciprocal inhibition **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;"> - interconnections of neurons arranged so that pathways that excite one group of motor neurons inhibit the antagonist motor neurons <span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; vertical-align: super;">5
 * **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;">slow-twitch muscle fibers **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;"> - 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 <span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; vertical-align: super;">4
 * **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;">spatial summation **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;"> - algebraic addition of generator potentials elicited by stimuli at two different points <span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; vertical-align: super;">5
 * **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; line-height: 1.5;">synergistic muscles **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; line-height: 1.5;"> - a muscle that has the same action at the same joint as another muscle <span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; line-height: 1.5; vertical-align: super;">5
 * **<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;">temporal summation ** - algebraic addition of generator potentials elicited by two stimuli applied at the same point in rapid succession 5

__ Further Reading __

<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px; line-height: 21px;">These links provide additional information about plyometric training:

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A mini-lecture on the stretch reflex:

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__ Quiz __

True/False
 * 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

__ References __


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 * 2) <span style="background-color: #ffffff; font-family: 'Times New Roman',Times,serif; font-size: 16px;">Chen, Lu. "Spinal Reflexes." Lecture. //<span style="background-color: #ffffff; font-family: 'Times New Roman',Times,serif; font-size: 16px;">MCB Berkley //<span style="background-color: #ffffff; font-family: 'Times New Roman',Times,serif; font-size: 16px;">. UC Berkley, 2006. Web. 13 Dec. 2013. < <span style="background-color: #ffffff; font-family: 'Times New Roman',Times,serif; font-size: 16px; line-height: 1.5;">http://mcb.berkeley.edu/courses/mcb160/Fall2006/lslides/overhead-Spinal%20Reflexes.pdf>.
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 * 6) <span style="background-color: #ffffff; font-family: 'Times New Roman',Times,serif; font-size: 16px;">"Neuroscience Online." //<span style="background-color: #ffffff; font-family: 'Times New Roman',Times,serif; font-size: 16px;">Neuroscience Online: An Electronic Textbook for the Neurosciences //<span style="background-color: #ffffff; font-family: 'Times New Roman',Times,serif; font-size: 16px;">. The University of Texas Medical School at Houston, n.d. Web. 13 <span style="background-color: #ffffff; font-family: 'Times New Roman',Times,serif; font-size: 16px; line-height: 1.5;">Dec. 2013.
 * 7) <span style="background-color: #ffffff; font-family: 'Times New Roman',Times,serif; font-size: 16px;">Potach, David H., and Donald A. Chu. "Plyometric Training." Lecture. FAMU. Web. 17 Dec. 2013. <http://www.famu.edu/CampusRec/UserFiles/File/Trainer%20and%20Intern%20Resources/CSCS%20Track/Plyometrics%20CSCS%20(Ch%2016).pdf>.
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 * 9) <span style="background-color: #ffffff; font-family: 'Times New Roman',Times,serif; font-size: 16px;">Vilis, Tutis. "The Physiology of the Senses Transformations for Perception and Action Tutis Vilis." //<span style="background-color: #ffffff; font-family: 'Times New Roman',Times,serif; font-size: 16px;">The Physiology of the Senses Transformations for Perception and Action Tutis Vilis //<span style="background-color: #ffffff; font-family: 'Times New Roman',Times,serif; font-size: 16px;">. N.p., n.d. Web. 13 Dec. 2013.
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