The electrical activity produced by a motor unit during the process of contraction can be recorded through the skin by the means of an electromyogram (EMG)
An electromyogram, also referred to as an EMG, is a set of equipment which is able to measure the electrical activity found in contracting muscles. When a muscle is relaxed it is much harder to detect any electrical activity, whereas in a contracting muscle there can be a great deal of activity detected. The more forceful the contraction is the greater the activity that can be found within that muscle. An EMG is a very useful piece of apparatus as it can determine the reason behind paralysis or muscle weakness whether it being due to a malfunction in the actual muscle or nerves supplying the muscle.
Within this section of the practical the aim is to investigate the relationship in which EMG activity from the biceps brachii has in reference to different weight loads.
Proprioception and the stretch reflex
Proprioception refers to the awareness of the exact position of body parts, particularly the limbs due to nerve impulses which are generated by special receptors known as proprioreceptors. These receptors are located in muscles, tendons or joints and provide information about the body’s positioning and movements. This is achieved by transmitting action potentials to the central nervous system which then process the information and produces the sensory information needed for control of balance and posture as well as learned movements. These receptors indicate the degree in which muscles are contracting as well as the level of tension expressed on the tendons and the positioning of the joints. An example of this is the orientation of the head comparative to the ground and it’s positioning during movement which is the result of the hair cells located within the inner ear.
The sensations of proprioception allow you to recognize where the head and limbs of the body are located and the movement of these parts when you are not looking at them, for example when walking or getting dressed. The awareness of the body’s actual movements can be described by the term kinesthesia.
A contracting skeletal muscle occurs in reaction to the stretching of a muscle; this is known as the stretch reflex. The stretch reflex takes place via a monosynaptic reflex arc and is activated by a single sensory neuron that forms a synapse in the CNS with a single motor neuron. Within a muscle are sensory receptors known as muscle spindles which monitor changes in muscle length. The stretching of a muscle stimulates these receptors which in response produces one or more nerve impulses which are transmitted via a somatic sensory neuron to the spinal cord. Within the spinal cord the sensory neuron creates an excitatory synapse and activates a motor neuron in the anterior grey horn. As a result of this excitation nerve impulses occur in the motor neuron and are transmitted from the spinal cord through to the stimulated muscle. Neuromuscular junctions are formed by the axon terminals of the motor neuron and the skeletal muscle fibers of the stretched muscle. Muscle contraction results from acetylcholine being released by the nerve impulses at the NMJ which activates muscle action potentials in the stretched muscle. Therefore when a muscle has been stretched it is followed by contraction which relieves the stretching.
Within a muscle spindle are specialized muscle fibers known as intrafusal fibres which are sensitive to stretch. Motor neurons found located within the spindle called gamma motor neurons alter the tension in a muscle spindle to variations in the length of the muscle. When a muscle shortens these gamma motor neurons stimulate the ends of the intrafusal muscle fibers to slightly contract which keeps the fibers taut and sustain the sensitivity of the muscle spindle to stretching of the muscle. This produces a ‘pre load’ affect in the proprioreceptors in the muscle. It results in an increase in proprioreceptive sensitivity which causes a larger reflex muscle contraction when a muscle is stretched allowing more muscle power.
The objective of this section of the practical is to investigate the patellar stretch reflex and the effect in which the Jendrassik’s Maneuvers has on it.
Hypothesis
The hypothesis being tested for the EMG experiment is as follows:
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Alternative hypothesis – electrical activity will increase as workload increases
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Null hypothesis – electrical activity won’t increase as workload increases
The hypothesis being tested for the Proprioception and the stretch reflex is as follows:
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Alternate Hypothesis – there is an effect of the stretch reflex when the Jendrassik’s Maneuver is being employed
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Null Hypothesis – there is no affect on the stretch reflex when the JM is being employed
Materials & Method
Materials
The materials and equipment used within this practical were as follows:
- A variety of handheld weights
- EMG recording equipment
- Patellar hammer with computer switch
- Tape measure
Method
EMG Experiment
The subject needs to stand upright with their chosen arm bent at a right angle at the elbow with their palm facing upwards. The EMG electrodes then need to be positioned correctly with the positive (red) electrode just over the belly of the biceps brachii muscle, the negative (white) electrode a short distance towards the forearm jus below the positive electrode and finally the earth (black) electrode on the point of the elbow. Once the electrodes are in place, take a recording of the EMG activity. The amplitudes of the EMG traces then need to be measured. Using a number of different weights (1.5 kg, 2.5 kg, 3.5 kg and 4.5 kg) place each weight one at a time in the arm still positioned with the arm bent at the elbow and the palm facing upwards. Measure the EMG amplitude and investigate the EMG amplitude variation with increasing load.
Proprioception Experiment
The subject should be sat on a chair with their leg bent at 90º at the knee. The EMG electrodes should then be placed as follows; the positive (red) electrode on the centre of the muscle belly of the rectus femoris, the superficial central of the quadriceps group of the thigh muscles. The negative (white) electrode needs to be placed approximately 3cm distal to the positive electrode so that it is towards the knee. The earth (black) electrode finally needs to be placed firmly on the kneecap.
The PowerLab then needs to be set to ‘External Trigger’ to allow the recording to be started by the hammer switch. Before any recordings are taken then subject must be fully relaxed and calm. The stretch reflex can then be recorded by the means of simply tapping the hammer on the patellar tendon jest underneath the kneecap. Is it essential that the recording is from the strike of the hammer on the tendon and not of the waving of the hammer before it strikes the knee. Repeat a few times to ensure that enough results are obtained to receive an accurate average of these parameters.
Next, the same process should be repeated but this time applying the Jendrassik’s Maneuver where the subject needs to clasp their hands in front of their chest, and try to pull them apart. This maneuver will have an effect on the EMG response to stretch of the thigh muscle, as it will alter the latency and the amplitude.
Then using the tape measurer, the distance which the stretch reflex covers needs to be calculated. This should be double the distance between the centre of the rectus femoris and the lumbar vertebrae at the center of the lower region of the back. The speed of conduction will be able to be calculated using the data for the reflex latency.
Results
The results obtained from the EMG experiment were as follows:
The results obtained from the proprioception and the stretch reflex were as follows:
A T test was carried out for both sets of results in order to determine whether or not each of the hypotheses was accepted. The results are as follows:
EMG experiment
The T value found for this experiment was 0.21. The degrees of freedom used were 2 therefore 0.21 is not significantly different to 4.30. This value is not significantly different therefore supports the null hypothesis.
Proprioception and the Stretch reflex
The T value found for latency was 6.3. This shows that there is a significant difference and supports the alternate hypothesis.
The T value found for amplitude was 0.46. This doesn’t show a significant difference.
Figure 1: Graph showing EMG amplitude of muscle contraction using varying weight loads.
Figure 2: A scatter diagram of the relationship of amplitude against varying intensities of workload.
Figure 3: Graph showing the affects of the JM on the stretch reflex
Discussion
The results obtained from the EMG experiment, in which varying weight loads were lifted and the amplitude of the EMG traces recorded, shows that as the weight load increases in addition the amplitude becomes greater too. This demonstrates that electrical activity is taking place as the muscles are functioning harder to maintain the weight being upheld by the arm. To enable the muscle within the arm to contract and maintain its position whilst holding the weights, a series of events need to be instigated to manage the timing and force of the contraction. The α-motor neurons initiate this series of events by transmitting muscle action potentials from the spinal cord to the neuromuscular junction, which is the synapse between the motor α-motor neuron and the skeletal muscle fiber. This signifies that muscle action potentials are rapidly being transmitted through the sacrolemma of the skeletal muscle fibers and greater numbers of ATP is being synthesized in the sarcoplasm in order to maintain the energy needed to keep the arm uplifted.
The range in which the amplitude increases from when the subject has no weights at all but is just holding their arm up alone, to when they a holding the 4.5 kg weight is considerable. The average readings consists of0.146 without weights – 0.908 to holding the 4.5 kg weight. This demonstrates that whilst electrical activity is taking place the rate in which it occurs increases as the intensity of the weights increases.
During the proprioception and the stretch reflex section of the practical the latency and the amplitude should have changed when the Jendrassik’s Maneuver had been applied. As a result of this maneuver a reduction in latency, the time from which the hammer strikes and evokes a recording, should occur and an increase in amplitude should also be distinguished. From observing the results obtained from without using the Jendrassik’s Maneuver and by using it, it is clear to acknowledge that only the latency has been reduced, the amplitude however, remains higher when the Jendrassik’s Maneuver has not been employed. The Jendrassik’s Maneuver should invoke a change in the latency that is the time in which it took the reflex to occur as the neuron traveling to the area where the hammer strikes has a delayed affect put on it by the contractions occurring in the arm and hands of the maneuver. The mechanism utilized by the JM reduces the actions of the monosynaptic reflex arc in which the stretch reflex takes place, therefore the impulses traveling to the muscle aren’t initiated at the same speed causing a slow reaction and therefore response to the stretch reflex.
