Introduction:
Skeletal muscle is striated muscle tissue that can be voluntarily controlled and is composed of bundles of fascicles (Connaughton, 2014). Within each fascicle are sarcomeres that contain thick filaments called myosin and thin filaments called actin. Attached to actin is tropomyosin and troponin. During a contraction, calcium is released from the sarcoplasmic reticulum and binds to troponin. This causes a conformation change and exposes the myosin binding sites on the actin. The myosin heads then bind to the actin and shorten, causing the myosin head to move and pull the actin along with it. This pulling motion is what causes a muscle contraction. After the contraction, calcium ions are taken up into the sarcoplasmic reticulum, and the calcium that was bound to troponin is removed. This prevents more cross bridges from forming. Next, ATP attaches to the myosin head and caused it to detach from the actin. The troponin and tropomyosin move back to inhibit the myosin binding sites, and the contraction is finished. This entire process is known as the sliding filament theory (Connaughton, 2014). The strength of contraction of a muscle is dependent on the number of fibers that participate.
Electrical activity in muscles is generated due to changes in ion flow across cell membranes after activation by a nerve cell (Connaughton, 2014). This electrical activity can be recorded from the skin by using an electromyogram (EMG) (Milner-Brown and Stein, 1975). Surface electrodes are placed on the skin over the muscles, and the EMG records the electrical activity within. In this experiment, EMG readings will be collected to observe muscle contractions and functions. The first part of this experiment uses EMG to observe different levels of contractions in the masseter muscle that are generated by clenching one’s jaw and chewing different types of foods. The second part of the experiment uses EMG to examine differing speeds between reflex and voluntary muscle activation. The last part of the experiment uses a Hand Dynamometer in order to observe muscle fatigue using two different grip methods: continuous gripping and repetitive gripping. This part of the experiment also examines the effects of encouragement on muscle strength during fatigue. For the first part of the experiment it is hypothesized that muscles that generate stronger contractions will have a greater amount of electrical activity. This is believed because larger contractions involve more muscle fibers, which requires greater ion movement. This greater ion movement will therefore create a greater electrical activity. For the second experiment, it is hypothesized that reflexes will have a faster electrical signal than voluntary muscle contractions. This is due to the reflex arc having less cells and synapses in its signaling pathway. Also, its signaling pathway is much shorter than that of voluntary muscle contractions. A signal for reflexes only travels to the spinal cord and back while voluntary muscle signals travel all the way up to the brain and back down before signaling a motor neuron (Schieppati, 1987). For the last experiment, it is hypothesized that as the hand fatigues, the electrical activity will decrease. However, after receiving encouragement, it is predicted that the electrical activity will then rise. This prediction is based on the theory that as fatigue sets in, less calcium ions are available to bind troponin to cause muscle contractions. This decreased flow of ions causes a drop in the electrical activity of the muscle. It is believed that encouragement can cause an increase in this ion flow through neuronal activation of the muscles (Häkkinen, 1993).
Methods:
EMG of Conscious Clenching of the Jaw:
A Vernier computer interface was connected to an EKG sensor in order to measure conscious clenching of one’s jaw muscles. Three electrode tabs were placed on the subject. The first was placed on the upper cheek, the second was on the lower corner of the jaw, and the third was placed on the forearm. The third electrode was placed in order to serve as a ground electrode. Stable baseline data was collected for 5 seconds while the subject’s jaw was relaxed. Next, the subject clenched their jaw for 5 seconds then relaxed it for 5 seconds repeatedly for a period of 30 seconds. Data was collected at a rate of 10 samples a second. After the data was recorded, the minimum and maximum values were recorded for both the clenched and relaxed EMG readings.
EMG Comparison of Muscle Action in the Chewing of Different Foods:
EMGs were taken of the subjects when chewing a hard-boiled egg, a raw carrot, and a piece of gum. For each trial, another stable baseline was established for 5 seconds before the subject began chewing for 25 seconds. After the allotted chewing time, the subject swallowed the food or relaxed their jaws to obtain a final 5 second baseline reading. Data was collected at a rate of 10 samples per second. The minimum and maximum electrical reading was determined for each of the four trials, and they were used to calculate the change in electrical signal.
Voluntary Activation of the Quadriceps Muscle:
Three electrode tabs were placed on the subjects’ legs. The first electrode tab was placed 5 cm above the patella, the second was placed 13 cm above the patella, and the third was attached to the lower leg. The red and green leads were attached to the tabs that were placed above the knee, with the red electrode being closest. The black lead, which served as the ground lead, was placed on the tab on the lower leg. The subject then closed his/her eyes, and a 5 second baseline was recorded. A reflex hammer was then hit against a table, and the subject was to kick his/her leg out upon hearing the sound. This was repeated for 5 trials. Then, the time between hitting the table with the reflex hammer and the contraction of the quadriceps was determined by analyzing the peaks of the EMG.
The Patellar Reflex
The subject again sat with their eyes closed while a stable 5 second baseline was recorded. Then, the reflex hammer was used to strike the subject’s patellar tendon. This was repeated 5 times, and the time between the hammer strike and the reflex was calculated.
Hand Grip Strength:
A Hand Dynamometer was connected to a Vernier computer interface and was then held in the subjects’ right hand. The subject then closed his/her eyes, and a stable baseline was collected for 2 seconds. Next, the subject gripped the sensor with full strength for 8 seconds. This process was then repeated for the left hand. The data was collected at a rate of 10 samples per second. The maximum and mean force was then obtained through analysis of the electrical reading.
Muscle Strength with Continuous Grip
Three electrodes tabs were connected to the dominant arm of each subject. One tab was placed 5 cm from the medial epicondyle, the second was placed 10 cm from the medial epicondyle, and the third was placed on the on the upper arm. The red and green leads were attached to the tabs on the forearm, and the black ground lead was attached to the tab on the upper arm. The subject then gripped the sensor with their full strength for 100 seconds. After 80 seconds, the subject received encouragement for the remainder of the test. The mean grip strength, the maximum value, the minimum value, and the change in the electrical signal were then calculated using the values collected from the sensor.
Repetitive Grip Strength
The procedure used for the continuous grip strength was repeated for the repetitive grip strength experiment. However, instead of continuously gripping the sensor, the subject rapidly gripped and relaxed their hand. The data was again collected for 100 seconds, and after 80 seconds, the subject received encouragement. The mean grip strength, the maximum value, the minimum value, and the change in the electrical signal were again determined by using the values collected from the sensor.
Results:
During this experiment, the strength of jaw muscle contractions, the time difference between the response of voluntary and involuntary patellar reflexes, and the fatigue rate of hand grip strength were observed. Tables 1-6 show data collected during the testing of the strength of jaw muscle contractions. The data collected for this part of the experiment contained the maximum, minimum, and change in mV. The change in mV signals the strength of the contraction and the strength of the electrical signal. Subjects 1-4 and subject 6 all show that the strongest change in electrical signal was observed when the subjects were chewing gum, and the weakest signal was when the subjects were clenching their jaws (Tables 1-4 and 6). The second strongest change in electrical signal was seen when subjects were chewing carrots, and the third strongest electrical signal generated was produced when the subjects were chewing a hard-boiled egg. Subject 5 deviated from this pattern slightly (Table 5). Instead of having the greatest electrical signal for chewing gum, subject 5’s greatest signal came from chewing a carrot. However, the lower signals of clenching his/her jaw and chewing an egg remained true to the previous pattern.
The signals produced by the clenching of one’s jaw ranged from 0.155 mV as the lowest value (Table 2) to 0.48 mV (Tables 1 and 4). For the chewing of an egg, the range of the signals started at 0.21 mV (Table 5) and ended at 0.6 mV (Table 4). The strength of the range of signals increased to a lower limit of 0.31 mV (Table 5) and an upper limit of 0.74 mV (Table 1). Chewing gum produced a range of signals with the lowest value starting at 0.27 mV (Table 5) and the highest value ending at 1.02 mV (Table 3). These ranges of values illustrate which jaw muscle contraction produced the greatest electrical signal.
The comparison of the length of time between a stimulus and either the voluntary contraction of the quadriceps muscle or the involuntary patellar reflex can be observed in tables 7-19. The average length of time between the stimulus and the voluntary contraction of the quadriceps muscle for all subjects was calculated to be 0.5188 seconds (Table 12). The average length of time between the stimulus and the patellar reflex was found to be almost half of the time of the voluntary contraction. For all of the subjects, the average time between the stimulus and the patellar reflex was found to be 0.2692 seconds (Table 18). The averages of all of the subjects were amassed due to the averages of each individual having the same pattern between the voluntary and involuntary reflex time. This pattern can be viewed in tables 7-11 and tables 13-17. The data collected consistently shows that the patellar reflex has a much shorter length of time between the stimulus and the contraction than the voluntary reflex does.
The last part of the experiment tested overall hand grip strength and whether continuous grip or repetitive grip and release would cause greater fatigue in one’s hand. For overall hand grip strength, the population tested contained 2 male and 9 female subjects. The strongest hand grip strength came from the 6’3 male, and had a max force of 304.2 N and a mean force of 267.8 N (Table 20). These forces were measured for this subject’s right hand. The weakest hand grip strength came from a 5’3.5 female’s left hand. The max force generated by this subject was 67 N, while the mean force was 52.89 N (Table 20). The same female also had the lowest grip strength in the female right hand category as well. The maximum force created was 99.98 N, and the mean force was 52.89 N (Table 21). The highest force registered by a female’s right hand was measured to have a maximum force of 223.8 N, while the mean force was 201.6 N (Table 21). The greatest force from a female subject’s left hand had a maximum force of 187.4 N and a mean force of 167.8 N (Table 21).
All but one of the overall hand grip strength subjects appear to have right hand dominance. The subject that is left hand dominant had the lowest hand grip strength for both hands out of the two males in the population. The maximum and mean grip strength generated by his right hand were 131.5 N and 105.9 N respectively (Table 21). This subject’s left hand was slightly stronger than his right hand, and created a maximum force of 153.4 N and a mean force of 116.9 N (Table 21). For the overall hand grip force, the gender and heights were included in the data in order to see if any correlations exist between these three factors. Data was also included for both hands in order to observe the difference in grip strength between the dominant and non-dominant hands of the subjects tested.
For both the continuous grip trial and the repetitive grip trial, the grip strength first started out strong, but gradually decreased until the subjects began to receive encouragement. During the period of encouragement, the hand grip strength increased until the end of data collection (Graphs 1 and 2). It appears as though the continuous grip strength generates a greater force than the repetitive grip. However, the declining slope of the repetitive grip trial appears to be less severe than that of the continuous grip trial (Graphs 1 and 2). The data for these two trials was organized into three time periods in order to see the effects of fatigue and encouragement. Tables 24 and 29 appear to have incorrect data entered. It is highly unlikely that subject 3 had a hand grip strength of over 100 mV for both the continuous and repetitive hand grip strength.
Discussion:
It was thought that in the jaw muscles, greater amount of electrical activity followed stronger contractions. The data collected supports this prediction. The clenching of the jaw required a greater muscle contraction than holding the jaw slack a baseline did. In the data, the clenching of the jaw exhibited a jump in electrical activity that was well above baseline levels. Greater electrical activity was also observed in the chewing of different foods. It was predicted that chewing gum would require the greatest muscular contraction to chew, followed by carrots, and then by the hard-boiled egg. The chewing gum was expected to require the greatest force to chew because it is sticky. The ingredients that compose gum stick together to form a “homogeneous mass” that is not broken down during mastication like other foods (Yang, 1992). The carrots were expected to have the second greatest contraction when chewing due to the hardness of the food, and the egg was expected to have the lowest contraction out of the chewed food due to its softness. These predictions were supported by the data collected. The chewing gum produced the greatest electrical change from baseline, indicating that it required the greatest contraction. The second greatest signal produced came from the chewing of the carrots, followed by the chewing of the egg, with the clenching of the jaw producing the lowest electrical signal. These results are important to this experiment because they support the first hypothesis that muscles with stronger contractions produce a greater amount of electrical activity. The results could also allow for this experiment to branch further. For example, this kind of test can be used on any muscle of the body, and could potentially be used to test for reduced function of muscles or muscle recovery. Based on the data collected, it can be concluded that the greatest electrical signal produced on the ECG was the product of the strongest contraction of the jaw muscle.
For the voluntary contraction and reflex experiment, it was predicted that the patellar reflex would have a faster electrical signal in response to a stimulus than would a voluntary contraction. This prediction was based on the two different pathways a stimulus would have to take in order to cause quadriceps muscle to contract. During a voluntary contraction, the stimulus would first have to travel up to the brain to be processed before it would be sent back down to a motor neuron to cause a contraction. During a reflex, the sensory neurons only synapse to the spinal cord before being sensed by a motor neuron and causing a contraction (Schieppati, 1987). This reflex pathway contains fewer cells, less synapses, and is a much shorter and faster pathway. The average time gap between the stimulus and the response for the reflex was almost half of that of the voluntary response. The data that was collected was important because it supported the hypothesis that reflexes would have a faster electrical signal. In conclusion, based on the correlation of the data and the hypothesis, reflexes generate faster electrical signals based on them having a shorter sensory pathway.