Essay on Electromyogram: Muscle Fibers

Sarah Shidban
Lab 1
Ali Izadpanah
July 12, 2013

Electromyogram
Experiment 1: (i) See Figure 1 for trace. Figure 1 is measuring the activity (force and time of contraction) of the entire muscle, which consists of several types of muscle fibers (Type I or slow twitch, Type IIa or intermediate fast twitch, and Type IIb or fast, fatigable/anaerobic) whereas a concentric needle electrode only detects and records the activity of one type of muscle fiber. A typical triphasic waveform is observed, in contrast to the monophasic shape of the intracellular action potential whereas the EMGs using surface electrodes are biphasic waveform. (ii) See Table 1 (iii) See Figure 2 and Table 2. The latency for a hard tap is slower than a hard tap because the reflex is not a deep tendon reflex it is a muscle reflex, it takes longer to transmit the more intense stimulus to the spinal cord. (iv) When the reflex hammer hits the tendons, receptors in the biceps brachii, called muscle spindles, detect the stretch. The muscle spindle then sends this information through type 1a sensory afferent neurons to the spinal cord, which then directly synapses onto an alpha motor neuron. The alpha motor neuron then innervates the biceps brachii causing it to flex.

* * * * * * * Experiment 2: (i) See figure 3 (ii) See Figure 4 (iii) Recruitment is defined as the ‘successive activation of the same and additional motor units with increasing strength of voluntary contraction’. The size principle is the concept that motor units are not only recruited in increasing number, but also increasing size. For example, slow twitch, fatigue-resistant are recruited first with light-to-moderate activity, like walking or lifting a small amount of weight. If the load or the intensity of the activity increases in effort (jogging) then fast, fatigue-resistant muscle fibers are recruited. This is lastly followed by fast, fatigable muscle fibers, which are recruited during strenuous, high-intensity activity. This general trend of increasing activity in the biceps brachii is shown in the EMG in figure 3. Figure 4 displays the integrated EMG values of eccentric and concentric contractions separately and better shows that concentric contractions have greater EMG values, because concentric contractions occur around the muscle’s optimal length, which is where actin and myosin overlap the most. Additionally, the slope of the line (rate of rise in iEMG as a function of load) is higher at higher intensities, because fatigue occurs more quickly. (iv) In figure 4 there is a discrepancy at a load of 9lbs. The eccentric iEMG is greater than the concentric iEMG value. This could be due to the timing error when the bicep curls were being recorded (the subject was still undergoing concentric contraction while the investigator had already switched to timing the eccentric contraction). This could also be due to the fact that skeletal muscles are voluntarily controlled meaning the subject may sometimes be tensed up when carrying out the experiment and relaxed at other times, which affects the pattern. * Experiment 3 (i) See figure 5 (ii) See figure 6 (iii) Figure 5 shows that the triceps had much smaller EMG signals during both concentric and eccentric contractions (mV). Bicep EMGs were about double the amplitude of the triceps EMG signals. Figure 6 Triceps Integrated Emg values for concentric and eccentric contractions were very similar in value, with the eccentric contractions being slightly smaller than their concentric counterpart. This is the same case for the Biceps Integrated EMG values. However the biceps brachii had a greatly greater iEMG value for both concentric (0.00049 mV*s) and eccentric (0.00048 mV*s) than the triceps concentric (0.00018 mV*s) and eccentric (0.00015 mV*s) contractions. (iv) These results are explained by the concept that the biceps are undergoing isometric

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