Electromyography Introduction .
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| Electromyogram during alternate contraction and relaxation of biceps muscle . |
[1]. Electromyography is the study of electrical activity of the muscle.
[2]. Electromyogram (EMG) is the graphical registration of the electrical activity of the muscle.
Electromyographic Technique .
[1]. Cathode ray oscilloscope or a polygraph is used to record the electromyogram.
[2]. Two types of electrodes are used for recording the electrical activities of the muscle:
1. Surface electrode or skin electrode for studying the activity of a muscle.
2. Needle electrodes for studying the electrical activity of a single motor unit.
Electromyogram .
[1]. Structural basis for electromyogram is the motor unit.
[2]. Electrical potential developed by the activation of one motor unit is called motor unit potential.
[3]. It lasts for 5 to 8 milliseconds and has an amplitude of 0.5 mV. Mostly it is monophasic .
[4]. Electrical potential recorded from the whole muscle shows smaller potentials if the force of contraction is less.
[5]. When the force increases, larger potentials are obtained due to the recruitment of more and more number of motor neurons.
Uses of Electromyogram .
Electromyogram is useful in the diagnosis of neuromuscular diseases such as motor neuron lesions, peripheral nerve injury and myopathies.
Muscle Endurance .
[1]. Three factors are essential for the contraction of skeletal muscle:
1. Strength of the muscle
2. Power of the muscle
3. Endurance of the muscle.
[2]. Strength and power of the muscle are the two factors which determine the endurance of the muscle. [3]. Power of the muscle is developed by strength of the muscle
Strength of the Muscle .
[1]. Maximum force that can be developed during contraction is known as strength of the muscle.
[2]. It is defined as the maximal contractile force produced per square centimeter of the cross-sectional area of a skeletal muscle.
[3]. The normal force produced by a muscle is about 3 to 4 kg/cm2 area of muscle.
[4]. If the size of the muscle is more, the strength developed also will be more.
[5]. The size of the muscle can be increased either by exercise or by some hormones like androgens.
[6]. For example, weight lifters will have the quadriceps muscle with cross-sectional area of about 150 cm2 . So, the total strength of the quadriceps muscles is between 500 and 550 kg/cm2 .
Types of Muscle Strength .
Strength of the muscle is of two types:
1. Contractile strength
2. Holding strength.
1. Contractile Strength .
[1]. Contractile strength is the strength of the muscle during the actual contraction or shortening of muscle fibers.
[2]. For example, while jumping, when a person takes his body off the ground, there is contraction of the leg muscles. This is called the contractile strength.
2. Holding Strength .
[1]. Holding strength is the force produced while stretching the contracted muscles.
[2]. For example, while landing after jumping, the leg muscles are stretched.
[3]. The force developed by the muscles at that time is called the holding strength.
[4]. The holding strength is greater than the contractile strength.
Power of The Muscle .
[1]. Amount of work done by the muscle in a given unit of time is called the power.
[2]. Power of the muscle depends upon three factors.
[3]. Muscle power is directly proportional to these factors:
1. Strength of the muscle.
2. Force of contraction.
3. Frequency of contraction.
[4]. Muscle power is generally expressed in kilogram-meter per min (kg-m/min), i.e. the weight lifted by a muscle to a height of 1 meter for one minute.
[5]. The maximum power achieved by all the muscles in the body of a highly trained athlete, with all the muscles working together is approximately,
First 8 to 10 seconds : 7,000 kg-m/min
Next 1 minute : 4,000 kg-m/min
Next 30 minute : 1,700 kg-m/min
[6]. This shows that the maximum power is developed only for a short period of time.
Endurance of the Muscle or Muscle Endurance .
[1]. Capacity of the muscle to withstand the power produced during activity is called endurance.
[2]. It depends mostly on the supply of nutrition to the muscle.
[3]. Most important nutritive substance for the muscle is glycogen. This is actually stored in the muscle before the beginning of the activity.
[4]. More amount of glycogen can be stored in the muscles if a person takes diet containing more carbohydrates than the diet containing fat or a mixed diet.
[5]. Following is the amount of glycogen stored in the muscle in persons taking different diets.
High carbohydrate diet : 40 gm/kg muscle .
Mixed diet : 20 gm/kg muscle .
High fat diet : 6 gm/kg muscle.
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