UNIT 3: Electromyography (EMG) Lab

Estimate the percentage of motor units active by dividing the EMG value for tonus by the EMG when all motor units are active at maximum contraction. Express as a percentage by multiplying by 100%

0.030181 mV/0.478173 mV x 100%= 6.31% motor units active

Define agonist (prime mover) and antagonist in the space below.

Agonist (prime mover)- Muscle that bears the major responsibility for effecting a particular movement; a prime mover. Antagonist- Muscle that reverses, or opposes, the action of another muscle.

Muscle twitch.

A single action potential produces a single muscle contraction known as a muscle twitch. Since action potentials are an all-or-none event, muscle twitches are also all-or-none.

What types of contractions are occurring in the right quadriceps femoris muscle as you walk upstairs (isotonic, isometric, concentric, eccentric)? For each of the 3 steps listed below, list the specific type of contraction that occurs. Concentrate on the actions at the knee joint, not the hip joint.

As you raise your right leg up to the next step: Eccentric As you raise your entire body up to the next step: Concentric As you stand on the right leg while the left leg is being raised up to the step: Isometric

Based on the results, was the decline in strength the result of psychological or physiological fatigue? Explain.

Based on our results, we concluded that the decline in strength was a result of psychological fatigue. The tension and the EMG curve went down at the same time. (curve started at the highest point then it went down until reached 50% then went down lower until reached the baseline- totally fatigued muscles)

Muscle in use? Action?

Biceps brachii Action: the biceps muscle in your arm is contracting concentrically when bringing up a weighted object. When you straighten your arm to return the weighted object back down, the isotonic contraction of your biceps is eccentric. Basically, eccentric contractions put the body in position to contract concentrically

Define concentric and eccentric contractions

Concentric Contraction: The type of muscle activation that causes tension as the muscle shortens. Eccentric Contraction: Muscle activation that occurs when the force applied to the muscle is more than force the muscle produces.

Experimental Protocol for Part 1

Design an experiment to measure how electrical activity changes when a muscle contracts at four different degrees of force. You must decide which muscle you will use and what activities your subject will perform to alter the force from light, medium, heavy, and overload (the fourth force needs to demonstrate isometric contraction). 1. Attach electrodes to the skin over the proximal and distal ends of the belly of the muscle to be measured, and a third ground electrode to the skin over the proximal or distal end of a nearby (not the same) muscle. 2. Before attaching the electrode pads, clean the skin areas with an alcohol swab and allow to dry. 3. firmly apply the gel-filled electrode pads to the three prepared areas. Please do not waste electrode pads. Once they are attached, do not remove them until the end of the lab. Plan to use each site as the ground or recording electrode for another experiment. 4. Clip the electrode wires to the electrodes. The red and white leads should be attached to the muscle being measured, and the black lead to the third site (this is the ground electrode). A clip only works on one side; if it doesn't attach, flip it over and clip it in place.

Dynamometry

Is the study of the power output of muscles. As muscle fibers contract, they produce force or tension. We will be using a hand dynamometer that is equipped with an electronic transducer to record the force exerted by the muscles on the transducer.

What is the difference between an isotonic and an isometric contraction?

Isotonic contraction- muscle length changes during contraction while tension remains constant Isometric contraction- tension increase but muscle length remains the same

As you reach physiological fatigue, the force exerted by your muscles decreases. What physiological process explains the decline in strength?

Results from the depletion of ATP at a rate faster than it can be replenished through respiration as well as a buildup of lactic acid

Why did the EMG show activity when the subject's hand and forearm were completely relaxed?

The EMG showed activity when the subject's forearm was relaxed because of tonus or muscle tone. Tonus or muscle tone is a small amount of tension produced by a resting muscle. This results from the periodic activation of a small number of motor units. This process cycles through all of the motor units, a few at a time, and functions to maintain the muscles in a state of readiness.

What is the EMG measuring?

The electrical activity associated with skeletal muscle contraction.

When you are holding an object for a short period of time does the number of motor units remain the same? Are the same motor units used throughout the duration of holding the object? Explain

The number of motor units does remain the same when holding an object for a short period of time.The same motor units are not used throughout the duration of holding the object. In motor unit recruitment, some are contracting while others are resting and recovering. This technique helps prolong a strong contraction by preventing or delaying fatigue.

overview

Today you will use physiological recording equipment interfaced with a computer to investigate the activity of different muscles. The lab consists of two parts: 1. You will design experiments in which your lab group will decide which muscles to record EMGs from, and what activities those muscles will perform. 2. You will also be combining electromyography with dynamometry to examine motor unit recruitment and skeletal muscle fatigue.

Explain any differences that you observe between the two types of contraction.

We observed that in a concentric contraction, your muscle must generate more force than the weight you are lifting to move it upward. Eccentric contractions involve generating tension as your muscles lengthen, as observed in our data above

Tension

force muscle exerts on an object when contracted

Wave summation

impulses are delivered in succession; the second twitch is stronger than the first

motor unit

is a single motor neuron and the group of muscle fibers it innervates. For weak contractions, only a few motor units need to be activated, while for stronger contractions, additional motor units can be activated. The activation of increasing numbers of motor units is known as motor unit recruitment. We can measure the relative amount of motor unit recruitment by the overall amplitude of the measured EMG.

Fatigue

is the decline in muscle tension as a result of previous contractile activity. Muscles are considered fatigued when contracting at 50% less than their maximum strength

Electromyography (EMG)

is the recording of the electrical activity associated with skeletal muscle contraction. When a nerve impulse travels down a motor neuron, it reaches all of the neuromuscular junctions supplied by that motor neuron. This generates impulses in the muscle cell membranes of all the muscle cells within the motor unit. The electrical activity from the muscle cells is conducted through body fluids to the skin, where it can be picked up by surface electrodes, amplified, and recorded. The recordings thus obtained are called electromyograms.

tonus (muscle tone)

is the small amount of tension produced by a resting muscle. This results from the periodic activation of a small number of motor units. This process cycles through all of the motor units, a few at a time, and functions to maintain the muscles in a state of readiness.

Eccentric

muscle contracts as it lengthens

Isotonic contraction

muscle length changes during a contraction while tension remains constant

Concentric

muscle shortens and does work

Complete tetanus

rapid stimulation results in sustained smooth contraction without relaxation

Psychological fatigue

results from reduced input from the central nervous system and usually occurs before the physical capabilities of the muscle are fully depleted. In psychological fatigue, we would expect that the EMG and muscle tension should both decline at equal rates

Physiological fatigue

results from the depletion of ATP at a rate faster than it can be replenished through respiration as well as a buildup of lactic acid (from anaerobic metabolism). In contrast, in physiological fatigue, the muscles are continuously being stimulated by the motor neurons so we would expect the EMG to remain high even as the tension declines.

Isometric contraction

tension increases but muscle length remains the same