Muscle Tension Lab Report

Abstract
The purpose of this experiment was to use the muscle tension and the electrical activity in the dominant and non-dominant forearm muscle, the flexor digitorum superficialis, to analyze the determinants of muscle tension and fatigue, and the reasons as to why differences may occur between the dominant and non-dominant arm. The generation of tension in a muscle is determined by the major type of motor unit being recruited, as well as the rate in which action potentials are being fired. A subject performed two tests, one to display the correlation between tension and electrical activity in the muscle, in which a sequence of four increasing-strength grips were performed, and the second test studied the effects of fatigue in which a continuous
grip at maximum tension was performed. Upon analysis of the increasing grip strengths, from the first grip to the fourth grip, the results indicated an 87% increase in tension, a 99.5% increase in EMG amplitude, and a 144.5% increase in frequency.
In the fatigue test, the tension had decreased by 72%, the EMG amplitude had declined by 133.3%, and the frequency declined by 89% in the time to half maximum tension. In comparison of the dominant and the non-dominant forearm, the dominant arm demonstrated a higher fatigue resistance and higher tension and EMG activity. Experimental data suggested a correlation between how muscle tension corresponds proportionally to the EMG activity positively, as well as the proportional correlation between the negative effects of fatigue on tension and EMG activity.

Introduction
Through this experiment, muscle tension was measured by analyzing the electrical activity in both the dominant and non-dominant forearm muscle, the flexor digitorum superficialis, during an
increasing grip strength and fatigue tests. Strength is relatively proportional to the relative number of motor units available for use by that muscle and can be activated all at once to become summated and provide forceful contractions. A motor unit is composed of a single motor neuron and all the skeletal muscle fibers innervated by the axon terminal of the motor neuron. Often times multiple groups of motor units work together to form the contractions of a muscle in which the signal first comes through the motor cortex, a neuron of the brain and spinal cord that conveys the motor plan to the motor units. For a continuous muscle contraction, multiple motor units are stimulated by the motor cortex to fire action potentials repetitively in order to maintain the strength and tension of the muscle. The summated intensity of the action potentials are recorded through an electromyogram (EMG) as a stacked signal of motor unit firings which is related to the force of a muscle contraction. The EMG values recorded are visualized in the Labscribe software as a series of spike-like signals in which the duration of the bursts of peaks is equal to the duration of the muscle contraction. This relationship is directly proportional to the amount of electrical activity occurring in the specific muscle. Through understanding these relationships, clinical scenarios such as its effect on sporting performance, recovery from surgery, and how multiple sclerosis affects the motor unit’s function can be understood better that can ultimately lead to improvement of techniques and cures.
Research led by Van Cutsen, Duenateau, and Hainaut (1998) suggest that strength training not only produces adaptations in the muscular system but also the nervous system which can be seen by the increase in strength that is greater than the changes in muscle diameter can account for, as seen through enhanced EMG levels of activity. In athletes looking to perfect their sporting performance endurance, building muscle weight is not as effective as increasing neuronal activity. Through using dynamic training, a method of training where small loads at maximal movement velocity involve ballistic contractions that cause an increase in maximum force and rate of tension development. This shortens the twitch time to peak of single motor units without changing the order of recruitments. The results of their experiment had indicated that with training, the motor units had increased their force while still respecting the size principal rule in that the type of motor units recruited match the action potential output, simultaneously causing an increase in the maximal firing frequency.
For those recovering from a surgery, the effects of motor unit firing can be seen. Research led by Delitto, Rose, McKowen, Lehman, Thomas, and Shively (1988) tested the effectiveness of electrical stimulation versus voluntary exercise as post-operative muscle strengthening protocols. The results of their experiment showed that the patients using electrical stimulation had finished the training recovery period with higher percentages of flexion and extension torques than those who used voluntary exercise, indicating that patients using an early post-operative rehabilitation of electrical stimulation regiment can achieve high muscular strength than exercise alone after surgery. An explanation for these results is that in disuse, type II muscle fibers waste away which becomes responsible for the loss in strength after surgery if no post-operative rehabilitation occurs. Electrical stimulation however has shown benefits of being more effective than exercise in eliciting muscle fiber activity. Knowing the relationship between strength and electrical activity while also knowing how muscle fiber atrophy can occur without any intervention of exercise or electrical stimulation allows physicians to prevent muscle degeneration and the loss of strength and tension by providing physical therapy as well as electrical stimulation for those that are unable to participate in exercise.
In patients with multiple sclerosis, a degenerative neurological disorder that causes demylinizations of the central nervous system pathways, results in neuromuscular dysfunction due to a massive loss of lower motor neurons as well as dying spinal motor neurons.
Research by Gelhsen, Grigsby, and Winant (1984) suggests that multiple sclerosis patients are able to overcome some neuromuscular deficits through physical activity. Multiple sclerosis causes neuropathological influences not only on skeletal muscle but also on physiological alteration, such as the impact seen on the EMG-force relations. As a result of multiple sclerosis, the inability to produce peak torque at higher velocities may be due to the disuse of the high threshold motor units that produce a fast twitch, causing the overuse of low threshold motor units to occur which produces a slow twitch response in muscle fibers. These ultimately result in atrophy of the muscles and a compromise of their strength, fatigue resistance, and
function.
The electrical activity in skeletal muscle is dependent upon a particular type of motor unit. The three major motor units are Type 1 (Slow), Type IIa (Fatigue Resistant), and Type IIb (Fast Fatigable). Each type of motor unit displays various characteristics that are unique and are provided in summary in the table below.