1Electromyography(Chapter 8)EXS 587Dr. Moranhttp://www.delsys.com/images/library/teresa.jpgOutline• Review of Muscle Contraction Physiology• Physiological Basis and Concepts of EMG (Alwin Luttmann)• Methods of EMG Collection• Electromyograhy in Ergonomics (Shrawan Kumar)• Limitations & Uses• Journal of Electromyography and Kinesiology (full-text in ScienceDirect)2Physiological Basis• Muscle contraction due to a change in the relative sliding of thread-like molecules or filaments• Actin and Myosin• Filament sliding triggered by electrical phenomenon • The recording of muscle APs is called electromyography•What can be learned from an EMG?• Time course of muscle contraction•• Coordination of several muscles in a movement sequence•• Field of Ergonomics: from the EMG conclusions about ___________ and the ____________ can be derived as well3Excitable Membranes• Cell membrane separates intracellular from extracellular space•• Cell Membrane Structure– Double layer of phospholipids (both surfaces covered in proteins)•• Hydrophobic Tail– Role of Proteins• Transport–• Receptor– Transfer informationhttp://www.longevity.ca/images/cell_membrane3.gifFluid Distribution• Concentration of ions different inside vs. outside of cell membrane• This results in an electrical potential difference known as a • Typical magnitude of membrane potential is -60 and -90 mV (interior of cell is negatively charged) • This potential can change within ______________to +20 to +50 mV• This rapid change is called an4Ion Concentration• Intracellular Fluid• High concentration of Potassium cations (K+) and Protein anions (A-)• Extracellular Space• High concentrations of Sodium cations (Na+) and chloride anions (Cl-) ------155A-1:301204Cl-40:14155K+1:1214512Na+Ratio:Inside/outside[Extracellular]mmol/l[Intracellular]mmol/lIonsUneven distribution the work of active transport that pushes Na+ from inside to outside and K+ from outside to inside (ION PUMP, requires ATP)OUTSIDELow [K+]INSIDEHigh [K+] CELLMEMBRANE(permeable to K+)- +FCONFEL5Nernst Equation• Used to determine resting membrane potential» Vm= • Nernst Extension (Goldman 1943): considered the effect of only , , and • Based on their permeabilities and [ ] values in preceding slide the resting membrane potential is -75 mVR Tz Fln (ci/co)Action Potential•• AP abide by the all-or-none principle• If MP reaches threshold voltage then Na+channels open at first (Which direction will Na+flow?)• Na+channels only open for 1 ms, this causes repolarization(K+channels also open during this time to speed up return of resting membrane potential)6Action Potential(continued)http://upload.wikimedia.org/wikipedia/en/thumb/7/78/Apshoot.jpg/300px-Apshoot.jpgRelease of Action Potentials• AP occur at muscle fibers from two processes:(1) AP propagation along muscle fibers(2)• AP propagation velocity dependent upon:(1) (faster for thick – fast twitch)(2) [K+] in extracellular fluid (KÖssler et lal., 1990)7Motor Unit Action Potential• Typically, each motorneuron innervates several hundred muscle fibers (innervation ratio)• Motor Unit Action Potential (MUAP) = summed electrical activity of all muscle fibers activated within the motor unit• Muscle force increased through higher recruitment and increased __________Physiological Basis of EMG“The technique of electromyography is based on the phenomenon of electromechanical coupling in muscle”Shrawan Kumar1.) 2.) Travel INTO muscle cells through invaginations (T-tubules)3.) AP trigger release of Ca2+ions from sarcoplasmic reticulum into muscle cytoplasm4.) Ca ions start the cascade of filament sliding*this is a EXTREMELY brief synopsis of the excitation-contraction coupling (ECC)Movie on Muscle AP Propagation8Recording Methodology• Sweep of AP  similar to a wave• Height of wave and the density of the wave can be recorded• Represented graphically Recording Methodology(continued)• Electrical potential difference measured between two points bipolar electrode configuration used• Bipolar Electrode Types•• Needle•– Most common, less invasive– Silver-silver chloride electrodes• Electrode Placement• Overlying the muscle of interest in the direction of predominant fiber direction• Subject is GROUNDED by placing an electrode in an inactive region of bodyhttp://www.hhdev.psu.edu/atlab/EMG.jpg9Fine Wirehttp://educ.ubc.ca/faculty/sanderson/EMG/Index.htmFactors Influencing Signal Measured• Merletti et al. (2001)• Geometrical & Anatomical Factors– Electrode size– Electrode shape– Electrode separation distance with respect to muscle tendon junctions– Thickness of skin and subcutaneous fat– Misalignment between electrodes and fiber alignment• Physiological Factors– Blood flow and temperature– Type and level of contraction– Muscle fiber conduction velocity– Number of motor units (MU)– Degree of MU synchronization10Factors Influencing Signal Measured(continued)• Merletti et al. (2001)• Conclusions:»» Muscles with parallel fiber type» Electrode arrays should be used to determine the most appropriate single-pair placement»EMG Amplitude vs Muscle Contraction Intensity• Amplitude increases with increased contraction intensity• BUT it is not a linear relationship• Non-linear relationship between EMG amplitude and contraction intensity11EMG Uses• Types of questions EMG can answer (maybe):1.) Whether a muscle is active or not during a movement activity2.) When the muscle turns ON/OFF during a movement activity*sometimes categories of activity are used to classify EMG signal, such as: none, slight, less than slight, more than slight, strong3.) Phasic relationship between muscles during a movement activity4.) Does the activation pattern indicate skill aquistion5.) Does an increased EMG magnitude imply a higher muscular stress?6.) Is the muscle fatigued?Analyzing the EMG Signal• Amplitude & Frequency•  more spikes and turns in signal• Change in firing rate • Major variables: » peak-to-peak amplitude (p-p)» average rectified amplitude» root-mean-square (RMS) amplitude» linear envelope» integrated EMG12Peak-to-Peak Amplitude•• M-wave = synchronous electrical activity of all muscle fibers following an electrical stimulus– Calculated from the negative-peak to positive-peak amplitudeAverage Rectified Amplitude• EMG contains a varying