Human Locomotion BE522: Electromyography

Excitation-contraction Coupling

• motor neurone stimulated
• upper-motor neurone from brain travels down spinal cord
• lower motor neuron in the ventral horn of the spinal cord is activated
• action potential passes down lower motor neuron axon to muscle (K moves in, Na moves out)
• axon branches to supply a set of (10:eyeball -1000:gastroc) muscle fibers called a motor unit
• motor unit action potential (MUAP) is conveyed to motor end plate on each muscle fiber
• summation of all active motor unit activity is the EMG
• depolarization
• causes release of packets of acetylcholine into the synapse on surface of the muscle fiber
• electrical resting potential under the motor end plate changes, initiates an action potential along surface of muscle fiber
• action potential spreads inside the muscle fiber along transverse (or T-) tubules
• calcium released from sarcoplasmic reticulum
• sliding filament theory (Huxley 1960)
• calcium binds to troponin-tropomyosin (T-T system), changing its shape
• myosin (A bands – thick filaments) binding sites open on actin filaments (I bands – thin filaments)
• Myosin head with ADP+P binds to actin (cross-bridge)
• Power-stroke causes filaments to slide
• ADP+P moves off myosin head ("grab and swivel" ratchet mechanism)
• When mysoin is tilted forward a new binding site for ATP becomes exposed
• A new ATP molecule binds to myosin head, allowing it to release actin
• summation
• all or none effect - graded increase in contraction comes from
• recruitment of more motor units (spatial summation)
• increased firing rate of active motor units (temporal summation)
the sarcomere contractile unit

What is the smallest amount of muscle that contracts when a lower motor neurone fires ?one sarcomereone fibrilone fiberone motor unit

Which of these elements is NOT necessary for muscle contraction ?PhosphateCalciumMagnesiumPotassiumSodium

Types of Muscle Fiber

Which type of muscle do you think cardiac muscle closest to ?AnaerobicFast twitchIntermediateSlow

Recording EMG

Problem	Solution
Low amplitude (5 mV)	Common Mode Rejection (CMR) Amplification
Skin resistance (keratin, grease, oils)	Cleaning/abrasion/shaving of skin, gel, High impedance amp.
Movement of electrode (motion artefact)	High-pass Filtering (> 10-20 Hz)
Electrical noise	Pre-amplifier, Low-pass Filtering (< 200-500 Hz), Cage
Cumbersome cabling	Thin cable (multiplexing), telemetry
Some muscles deep	In-dwelling (transcutaneous) fine-wire electrodes
High frequency (risk of aliasing)	Sampling rate >> Nyquist (e.g. 1000 Hz)
Cross talk	Small electrodes, spectral analysis
Raw EMG is biphasic	Rectification (detection)
Quantification of varying signal	Root Mean Square (RMS)
EMG much higher frequency than muscle force	Linear envelope (low-pass filtering at 3 Hz after detection) - also called "integrated emg" ... produces the same shape as the muscle force or joint moment curve
Variability from stride to stride	Averaging - plot mean and standard deviation
Variations in electrode placement/skin from trial to trial	Nomalization to Maximal Voluntary Contraction (MVC)
Electromechanical delay	Difficult to interpret

Which is best for removing motion artefact ?low-pass filter at 20 Hzhigh-pass filter at 20 Hzlow-pass filter at 250 Hzhigh-pass filter at 250 Hz

Factors governing the relationship between EMG and muscle force (or moment)

• Length-tension Relationship (AV Hill, 1938)
• Isometric preparation (hold length constant, study force)
• Isotonic preparation (hold force constant, study length)
• Force-velocity relationship
• Recruitment of more motor units leads to non-linear increase in EMG
• As muscle contracts, its relationship to the elctrodes changes
• Eccentric (lengthening) contractions are associated with less EMG than concentric contractions
• Conclusion: EMG-Force relationship linear only under isometric conditions

Applications

• Compartmentalization - determining relative contribution of each muscle to moment
• Co-contraction - where nett moment may be zero
• Timing of muscle contraction - inappropriate or premature activity (spasticity)
• Fatigue assessment - from spectral analysis (FFT): Mean Power Frequency (MPF) falls with fatigue
• Diagnosis - muscle disorders such as Duchenne dystrophy, myaesthenia gravis, Lou Gherig's
• During selective dorsal rhizotomy - surgery to cut nerves causing spasticity

Noraxon Telemyo

• 8 channels standard, upgradeable to 16, 24, 32 or 40 channels
• No pre-amp - patented signal processing algorithm
• Minimal or no skin preparation
• Bandwidth (-3dB) 10Hz to 500Hz
• Lightweight & portable
• Raw and enveloped outputs
• Wireless (RF) telemetry - 300 yard range