Ahmad Al-Obaidly
Abstract—In this experiment, a hand dynamometer was used to measure the grip strength of a subject. The forearm muscles were engaged in an activity, and the grip recorded. The electromyogram (EMG) activity was related to the grip strength by making a graph of the maximum grip strength against area under the EMG activity’s absolute integral during the contraction of muscle. Data was recorded from the dominant and non-dominant forearms of the subject and the electrical activity of each forearm compared to its diameter. Prolonged grip strength was also recordings and forearm EMG activity made to determine fatigue rate in both the dominant and non-dominant forearms. The dominant forearm has a greater grip strength and wider diameter as compared to the non-dominant forearm. The increase in tension is more rapid in the non-dominant forearm than it is in the dominant forearm and dominant forearm does not fatigue as quickly as the non-dominant forearm. The steady increase in tension, great grip strength and low fatigue are contributed to the member and the size of muscle fibers in the dominant forearm.
INTRODUCTION
A
motor unit refers to a motor neuron together with the muscle fibers that it innervates (Seeley, Stephens, & Tate, 2004). In a persistent muscle contraction, there is a repetitive firing by multiple motor units throughout muscle contraction process. The strength developed by the contracting muscles is related to the number of active motor units at the moment. Most of the mature extrafusal fibers of the skeletal muscle in mammals have only a single α motor neuron that innervates them. Within the muscles, individual motor axons make branches in order to synapse on a lot of different fibers distributed over a huge area in the muscle. This is due to the fact that the number of muscle fibers exceeds the number of motor neurons by far. The branching is presumed to make sure that the force of contraction by the motor unit is evenly distributed (Purves, Augustine, & Fitzpatrick, 2001). The technique that is used in the evaluation and recording of the electrical activities from a skeletal muscle is known as electromyography. The technique is done using electromyograph, to produce a record known as an electromyogram (EMG) (Robertson, Caldwell, Hamill, Kamen, & Whittlesey, 2001).
ATP is the molecule that universally offers energy supply for the contraction of muscles. During exercise no matter how less intense, the energy for the contraction of muscles is given by the breakdown of a molecule of ATP stored in the muscles. The composition of ATP molecule is an adenosine and three molecules of phosphate. ATP in the body is mainly generated from nutrients such as fat, proteins, and carbohydrates. However, the most source of ATP that is mostly preferred is carbohydrate. The three phosphate molecules in ATP are joined together through chemical energy.
When a nerve impulse gets to the muscle fiber, there is stimulation of a contraction process where one of the phosphates in ATP is split. The splitting of the phosphate molecule releases energy that is then used to power the contraction process. This results in the production of an ADP molecule, which is an adenosine with two molecules of phosphate. There is an accumulation of the free inorganic phosphate in the muscle fiber. There are other chemical sources that the muscle use to power contraction, although they are first converted into ATP before they can be used.
Muscle fatigue refers to the reduction in the ability of a muscle to produce force. Muscle fatigue may result from vigorous exercise although abnormal fatigue does occur due to barriers or interference to the contraction of muscles. Muscle fatigue may result due to the reduced ability for the nerve to generate sustainable signals or through a reduction in the ability of calcium ions to stimulate contraction. When there is a repeated and intense muscle use it leads to a decline in the performance of the muscles. This decline in the muscle performance is referred to as muscle fatigue. There are numerous properties that change when the muscle is undergoing fatigue such as the action potential, intracellular and extracellular ions, as well as, the intracellular metabolites. Different mechanisms have been suggested that lead the decline in the muscle performance. One of the traditional explanations is the accumulation of intracellular hydrogen ions and lactate leading to the marred functioning of the contractile proteins. Others include change in the ionic concentration and thus affecting the action potential as well as the failure of the sarcoplasmic reticulum to release calcium ions. There is also an impact from the increased reactive oxygen species in the muscles
The EMG that is recorded during the muscle contraction is viewed as a burst of spike-like signals, and the burst duration is almost equal to the duration taken by muscle contraction. The raw data given by EMG have to be transformed mathematically in order to quantify the amount of electrical activity in a muscle. The most common method of transformation is through the integration of the absolute values of the EMG spikes amplitudes. This integration results to a linear proportion between the area under the curve and the strength of the muscle contraction.
In this experiment, a hand dynamometer was used in the measurement of the grip strength of a subject. The forearm muscles were engaged in an activity, and the grip recorded. The EMG activity was related to the grip strength by making a graph of the maximum grip strength against area under the EMG activity’s absolute integral during the contraction of muscle. Data was recorded from the dominant and non-dominant forearms of the subject and the electrical activity of each forearm compared to its diameter. Prolonged grip strength was also recordings and forearm EMG activity made to determine fatigue rate in the dominant, as well as non-dominant forearms.
Methods
The experiment was conducted using PC Computer, IWX/214 data acquisition unit, C-AAMI-504 ECG cable and electrode lead wires and the FT-325 Hand Dynamometer as the major equipments. All the equipments were set up according to the manual (Iworx, 2013). The EMG cables, as well as the hand dynamometer, were connected to an IWX/214 as illustrated in Figure 1 below. The EMG electrodes were on the forearm as illustrated in the Figure below.
Figure 1: Illustration on how to set up the equipments. 1a is the EMG cables and hand dynamometer connection to an IWX/214 and 1b is the EMG electrodes attachment on the forearm.
EMG Intensity and Force in Dominant Arm
This was done with an aim of determining the relationship existing between the force of contraction and the intensity of EMG activity in the dominant arm. The subject was allowed to sit quietly with the dominant forearm resting on the table top and the experimental procedure explained. The subject was allowed to squeeze the fist around the hand dynamometer four times. Each of the contractions was two seconds long and was followed by two seconds of relaxation. Each of the successive contraction was made to be approximately stronger than the first contraction by two, three, and four times. Using a piece of string and a metric ruler, the circumference of the dominant forearm was measured at approximately 3 centimeters below the elbow and the data recorded.
EMG Intensity and Fatigue in Dominant Arm
This was done with an aim of observing the relationship existing between the length of muscle contraction, strength of a muscle contraction and the EMG activity in the dominant forearm. The subject was allowed to sit quietly with the dominant forearm on the table top, and the experimental procedure explained. The subject squeezed the bulb of the hand dynamometer as tightly for as long as possible in order to fatigue the muscles of the forearm. When the muscle strength of the subject dropped to a level that was below half the maximum muscle force at the beginning, the recording was stopped.
EMG Intensity and Force in the Non-Dominant Arm
This was done with an aim of determining the relationship existing between the EMG activity intensity and the muscle contraction force in the subject’s non-dominant arm. The subject was allowed to sit quietly with the non-dominant forearm resting on the table top and the experimental procedure explained. The subject was allowed to squeeze the fist around the hand dynamometer four times. Each of the contractions was two seconds long and was followed by two seconds of relaxation. Each successive contraction was made to be approximately stronger than the first contraction by two, three, and four times. Using a piece of string and a metric ruler, the circumference of the forearm at approximately 3 centimeters below the elbow was measured and the data recorded.
EMG Intensity and Fatigue in Non-Dominant Arm
This was done with an aim of observing the relationship existing between the length of muscle contraction, muscle contraction strength and EMG activity in the non-dominant forearm. The subject was allowed to sit quietly with the non-dominant forearm on the table top, and the experimental procedure explained. The subject squeezed tightly the bulb of the hand dynamometer for as long as possible in order to fatigue the muscles. When the muscle of the subject dropped to a level that was below half the maximum force of the muscle at the beginning, the recording was stopped.
Results
The results for the EMG intensity and force in the dominant arm are shown in below (Table 1). Both the values for the absolute area of EMG and that of absolute area under force curve increased as the contraction intensity increased. For the dominant forearm, during the first contraction, the value of the absolute area of the EMG activity was recorded as 0.002 while at the fourth contraction, the value was 0.722. For the non-dominant forearm, the value of absolute area of EMG activity was 3.102 during the first contraction while during the fourth contraction the value was 37.899. The absolute area under force curve for the dominant forearm was 0.114 for the first contraction and 0.594 for the fourth contraction. For the non-dominant forearm, the absolute area under force curve during the first contraction was 2.528 and 36.619 for the fourth contraction. This trend is clearly shown by a graph of absolute area of the contraction of the muscle as a function of the absolute area of the EMG signal (Figure 2).
Figure 2: A graph of absolute area of muscle contraction against the absolute area of the EMG signals
The graph of absorbance area of the muscle contraction versus the absolute area of the EMG signals shows a linear relationship between the two variables. The relationship exists in both the dominant and non-dominant arms. The time that is needed by nerve fiber to restore its resting potential is referred to as the refractory period. The refractory period is mainly observed in nerve fibers. The muscles fibers also have a refractory period just like the nerve fibers. Every fiber of the muscle has got myofibrils arrays that are stacked all through the fiber length numerous mitochondria to provide the necessary energy, smooth endoplasmic reticulum and a lot of nuclei.
All motor neurons that feed the skeletal muscles contain branching axons. Each of these axons terminates in a junction known as neuromuscular junction with one muscle fiber. One a nerve impulses has been created it goes down the single motor neuron and is capable of triggering contraction process in all the muscle fibers where the axon branches terminate. One unit of contraction containing a motor neuron and the muscle fibers that it innervates is referred to as the motor unit. The smaller the motor unit size, the higher the control precision. Although the motor unit responds through an all or none response, the strength of the response is dependent on the number of motor units that have been activated.
The recruitment of motor unit refers to the progressive muscle activation through successive recruitment of the contractile units also called motor units. This is usually done aiming at increasing the contractile strength gradually. Muscles have got a several motor unit with the fibers that belong to a motor unit may be distributed intermingling with the other fibers belonging to the other units. In some instances, a single motor unit may spread almost through the entire muscle. This usually depends on the fiber number and the muscle size.
After a motor neuron has undergone activation, all the muscle fibers that are receiving nerve impulse from the motor neuron are stimulated and contraction takes place. When only one motor neuron is innervated, there is a weak but well distributed muscle contraction that is created. When more motor neurons are activated, there are more muscle fibers that are activated and hence a stronger muscle contraction results. The recruitment of motor unit is a measure of the extent at which muscle fibers of the muscle are activated. As the recruitment increases, the muscle contraction becomes stronger. The recruitment of the motor units follows an order of smallest to the largest as the contraction increased. This phenomenon is referred to as Henneman's Size Principle.
When a progressive increased tension is required, the smallest motor units are the ones that are usually recruited. When the load on the muscle is increased, the larger motor units are recruited, and the maximum contraction is attained when all the motor units have been recruited. The increase is also dependent on the number of muscle fibers that are firing.
The results for the EMG intensity and fatigue in dominant and non-dominant forearms are shown below (Table 2).
The muscles attained a maximum contraction force of about 140 units and the time taken to get to half-max fatigue time was around 10.7 seconds for the dominant forearm. For the non-dominant forearm, the maximum force was 114 units and the time taken to get to half-max fatigue time was around 9.5 seconds for the dominant forearm. This shows that the dominant forearm is stronger than the non-dominant one with the percent difference in maximum grip strength between the two arms being 18%.
The weaker forearm has a higher ratio of average maximum grip strength to the EMG absolute integral area than the dominant or the stronger forearm. The gradient of the dominant forearm was 44.6 while the non-dominant forearm had a gradient of 67.44. This gives a percentage difference of 33.84%. This big difference may be contributed to the fact that the dominant forearm has a diameter of 260 mm and hence more motor units as compared to a diameter of 240 mm for the non-dominant one with a percentage difference of 7.6%. The big motor units in the non-dominant arm are, therefore, recruited sooner than those in the dominant arm and thence the sharp rise in force.
Difference in the diameter of the arms also contributes to the difference in the maximum force attained by each arm. This is because of the number of motor units in each arm with the dominant arm having more units than the non-dominant one. The circumference difference is mainly caused by the difference in the diameter of each muscle fiber. The dominant arm has fibers that are of large diameter and thus they have large muscle fibers. On the other hands, the non-dominant forearm has muscle fibers that have smaller diameters and hence a less total arm diameter. The dominant forearm is mostly used than the non-dominant forearm. The difference in diameter may, therefore, result from the frequent use of the dominant arm since exercise enhances the size of muscle fibers.
The dominant arm took more time to fatigue as compared to the non-dominant one. This results from the big muscle fibers in the dominant forearm in which the myofibrils, as well as, the contractile proteins are many. This enables the muscle fibers to contact for a longer time than when these proteins are less. The ability to fatigue is also determined by the number of fast muscle fibers and slow muscle fibers are in the muscle. The muscle fibers that have more fast muscle fibers than the slow muscle fibers tend to fatigue faster than those that have few fast fibers and many slow fibers. The slow fibers use oxygen slowly and are therefore able to endure for long. The fast muscle fiber use oxygen at a faster rate and therefore fatigue easily.
conclusion
In conclusion, the experiment provided a number of finding. First, the dominant forearm has a greater grip strength and wider diameter as compared to the non-dominant forearm. Secondly, the increase in tension is more rapid in the non-dominant forearm than it is in the dominant forearm. Finally, dominant forearm does not fatigue as quickly as the non-dominant forearm. The steady increase in tension, great grip strength and low fatigue are contributed to the member and the size of muscle fibers in the dominant forearm.
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