Understanding how the Muscle Spindle Apparatus (MSA) and Golgi Tendon Organs (GTO) work is helpful in developing an understanding of the physiology behind Plyometrics and P. N. F. stretching, which may be needed to answer examination questions in PHED 3. Both MSA and GTO are proprioceptors.
Proprioception means “sense of self”. In our limbs, the proprioceptors are sense organs that provide information about joint angle, muscle length, and muscle tension, which is used to give information about the position of our limbs in space. The MSA is one type of proprioceptor that provides information about changes in muscle length. The GTO is another type of proprioceptor that provides information about changes in muscle tension.
Each MSA consists of three modified muscle fibres that lie within the normal muscle fibres, and are supplied with both motor and sensory neurones. By sending nerve impulses to the brain about changes in muscle length, they allow the brain to ‘know’ how contracted a muscle fibre is. They are also involved in the anticipation of how much of a contraction the muscle is going to need.
Whenever a muscle is stretched or shortened, the pull felt by the muscle fibres is transferred to the MSA and it is similarly stretched or shortened. Whenever muscles change their length, the sensory nerve endings within the muscle spindle report the changing condition of that muscle back to the brain, so that appropriate adjustments in muscle contraction can be made.
This can be easily demonstrated using a set of three boxes – I find the boxes in which reams of printer paper arrive the perfect size. Before the lesson, and out of sight of the students, fill the boxes (they should all look the same) with different amounts of paper; one with about 1 cms depth of paper; one with about 3 cms depth of paper and one with no paper. Seal the boxes and label them A, B and C respectively.
Decide on a (un)willing volunteer; who stands in front of the group, but side on to the class. Get them to flex both elbows to about 90° and hold their hands supinated (face up!). Explain to them that you are going to stand in front of them and drop one of these boxes from a height of about 10 cms, into their open hands. Tell them that the weight of the box will cause their hands to drop slightly, but they should try and return their hands to their original position as quickly as possible. Use box A.
Now discuss with the class which sense organs the volunteer used to make the adjustment of their hand position. Invariably the students will end up suggesting that they use their eyes – seeing the box drop and making the necessary adjustment.
So repeat the box drop with box B, but this time blindfold the volunteer, removing their sense of sight. Don’t tell them box B is heavier, let them find out themselves!
Another discussion should follow, and again, invariably the students decide that it must be the sense of touch that’s sensing the dropping box. It’s at this point that it’s best to explain that it is the MSA that is detecting the changes in muscle length which is the result of the box dropping. Sensory nerve impulses are sent from the biceps muscle to the spinal cord from the MSA as the box lands, and a reflex (protective) contraction of the biceps follows immediately to restore the biceps to its original length and the hands to their original position.
Muscle spindles are also involved in the anticipation of how much strength a muscle is going to need for a certain action (muscle loading). The muscle pre-sets the tension within itself, based on information held in memory. The MSA can then adjust the tension required, by feeding back sensory information to the brain and causing immediate adjustments.
This idea can be shown with the third box, but requires some acting ability. It’s probably best to suggest to the volunteer that the reflex occurs whatever the weight of the box and that there is no need to wear the blindfold. You need to act out that box C is much heavier than the others. The volunteer with be expecting a heavy weight and his MSA will pre-set the tension required to hold the hands/arms/box in position. Drop the box and invariably the poor volunteer will have pre-set too much tension and the box will fly upwards.
One very important aspect of training and competition is the need to warm-up, and at the same time increase flexibility. If a muscle is suddenly stretched during such exercises, the muscle involved almost instantly contracts to resist the stretch. This is the stretch reflex.
The commonest demonstration of this phenomenon is the knee-jerk reflex, where, if the legs are crossed (stretching the quadriceps muscle), and the tendon attached to the muscle (patella tendon) tapped with a suitable object, the quadriceps muscle is rapidly stretched.
The MSA detect the sudden stretch and send nerve impulses to the spinal cord, and a corresponding nerve impulse is sent muscle fibres, causing the quadriceps muscle to contract, and the lower leg is seen to suddenly jerk. The stretch reflex is a protective mechanism.
Plyometrics is a type of training used by elite athletes to specifically develop their power. Plyometrics involves high-intensity, explosive muscular contractions that engage the stretch reflex (stretching the muscle before it contracts so that it contracts with greater force). An analogy is the stretching of an elastic band; the more it is stretched, the more force the corresponding release generates.
A plyometric contraction involves an initial rapid eccentric movement, followed by an explosive concentric movement. A muscle that is eccentrically stretched before a concentric contraction, will contract more forcefully and more rapidly. The shorter the time involved in the changeover from eccentric to concentric contractions (amortisation phase) the greater the effect. This sequence of three phases is called the stretch-shortening cycle.
A classic example is the “squat” just prior to a vertical jump. By lowering the body quickly, the muscles involved in the jump are momentarily stretched producing a more powerful movement.
A simple practical demonstrates the effect. Ask students to jump as high as possible using a vertical jump in two different ways. Either by the plyometric method described above, or by jumping from a position where the squat position is held (isometric contraction) before jumping. In variably the former produces the greatest vertical jump.
Plyometrics works as a form of power training because when a quick stretch is detected in the muscles, an involuntary, protective stretch reflex occurs to prevent overstretching and injury. This stretch reflex increases the activity in the muscle undergoing the stretch or eccentric muscle action, allowing it to act much more forcefully.
The result is a powerful braking effect of the eccentric contraction, and the potential for a powerful concentric muscle action. If the concentric muscle action does not occur immediately after the pre-stretch, the potential energy produced by the stretch reflex response is lost. In other words, if there is a delay between dropping down and then jumping up, the effect the is lost.
Stretching exercises are used to increase flexibility. Passive stretching is where the performer’s stretched position is held in place by some other part of their body, or by a partner or some other apparatus. Proprioceptive neuromuscular facilitation (PNF) is a type of passive stretching that uses alternating contraction and relaxation movements to increase flexibility.
The reason why PNF is better than other forms of flexibility training is that it helps the body’s muscular inhibition. For the elite performer, the most useful type of PNF is the CRAC (contract-relax, antagonist-contract) technique, which uses isometric muscle contractions as its basis.
PNF stretching is best performed with a partner, but some stretches can be performed alone. The partner must avoid over-stretching the muscles.
In a hamstring stretch, the performer lies on their back. The stretch begins with the performer lifting one leg straight up until a comfortable stretch is felt through the hamstring. Their partner holds this leg in this extended position, but should not push. This passive stretch is held for 15 seconds.
Then the performer tries to push their leg back to the floor, with the partner resisting. This isometric contraction of the hamstrings, should be held for six seconds. Then, the performer contracts their hip flexors and quadriceps and pulls their leg further back, increasing the stretch on the hamstrings. Again, the partner can help by holding the leg in the new position for another 15 second stretch, and then repeat the isometric and increased stretch routine. In practice, the isometric contraction is often referred to as ‘hold’ and the concentric muscle contraction is referred to as ‘contract’.
The isometric muscle contractions completed immediately before a passive stretch help to achieve autogenic inhibition. This involves the MSA, which when activated by a stretching muscle, produce the stretch reflex which causes the stretched muscle to contract and prevents overstretching. The GTOs, found at the junction of muscles and tendons, are also involved. These detect how much tension is being placed on a tendon. However, the GTO differs from the MSA because when they are activated by an increase in muscle tension they send this sensory information to the brain, they relax the antagonistic muscle, allowing it to stretch to prevent damage.
Getting students to attempt PNF stretching exercises while explaining the physiology involved is a good revision tool.