EMG LAB I and II Electromyography (EMG)- recording of the electrical activity associated with skeletal muscle contraction. Dynamometry is the study of power output of muscles. Todays lab consists of two parts. 1)Design experiments in which your lab group will decide which muscles to record EMGs from, and what activities those muscles will perform. 2)Combine electromyography with dynamometry to examine motor unit recruitment and skeletal muscle fatigue.

Excitation-Contraction Coupling

Sliding Filament Theory

Muscle Contraction Tension – force muscle exerts on an object when contracted Responses are graded by:  Changing the frequency of stimulation  Changing the strength/intensity of the stimulus

Factors Affecting Tension Intensity of stimulus – number of motor units ▫Motor unit – a single motor neuron and all the muscle fibers it supplies ▫Recruitment – calling additional motor units, stimulating more fibers will increase muscle tension

Factors Effecting Tension Intensity of stimulus ▫Threshold stimulus – minimal stimulus needed to invoke visible muscle contraction ▫Maximal stimulus – all motor units are recruited, strongest contraction produced  Asychronous recruitment of motor units – alternates motor units

Factors Affecting Tension Frequency of stimulation ▫Twitch – single impulse, contraction followed by relaxation ▫Wave summation – when impulses are delivered in succession the second twitch will be stronger then the first ▫Complete tetanus – rapid stimulation results in sustained smooth contraction without periods of relaxation

Factors Effecting Tension Frequency of stimulation

Types of Contractions Isotonic – tension remains constant during contraction ▫Muscle length shortens or lengthens during contraction ▫Concentric vs. Eccentric

Isotonic Contractions Concentric - muscle shortens and does work ▫Examples: pick up pencil, kick soccer ball

Isotonic Contractions Eccentric – muscle contracts as it lengthens ▫Helps counter act gravity or prevent joint injury “muscle braking” ▫Example: squats – quadriceps stretch but are contracted to counter act gravity and control movement

Types of Contractions Isometric – Tension increases but muscle length remains the same ▫Muscle is unable to produce enough force to overcome the load ▫Example: pushing against a stationary wall

Fatigue Fatigue is a reduced ability to perform work. ▫Psychological fatigue results from reduced input from the CNS and usually occurs before the physical capabilities of the muscle are fully depleted. ▫Physiological fatigue results from the depletion of ATP at a rate faster than it can be replenished through respiration as well as a build up of lactic acid (from anaerobic metabolism).

Lab Equipment BioPacElectrodes

Lab Procedure Part 1 Design an experiment to measure how electrical activity changes when a muscle contracts at 4 different degrees of force. Design an experiment to demonstrate the actions of an agonist and its antagonist by recording from the two muscles simultaneously. Plug a second cable into Channel 2. Design an experiment to demonstrate concentric and eccentric contractions in the same muscle.

EMG Data To collect data select the I beam tool. Click and drag it through the first EMG cluster. Note the p-p value above the graph. This millivolt value can be used as an indication of the number of muscle fibers stimulated.

Dynamometer- hand held bar that is equipped with an electronic transducer to record the force exerted by the muscles on the transducer. Lab Equipment part 2

Lab Procedure Part 2 Select the subject’s dominant forearm to attach the electrodes to.  Perform three muscle contractions using increasing strengths  Record a maximal strength contraction and sustain your grip for as long as possible. Relax your grip only when you reach fatigue.

Lab Procedure Part 2 Measure data Record data on lab sheet Analyze results Answer questions

Muscle Twitch on a Myogram Myogram Calibration of a 50g weight