Transcript Lecture
Gradation of Muscle Force Two neural mechanisms responsible for force gradations: 1. Recruitment Spacial summation 2. Rate coding Temporal summation Recruitment Varying the number of motor units activated. Larger motor units Largest motor units Low stimulus threshold Higher stimulus threshold Highest stimulus threshold The Size Principle Amount of Force Required During Movement ↑ Number & Size of Motor Units Recruited ↓ Small motor units Rate Coding Rate coding refers to the motor unit firing rate. – Active motor units can discharge at higher frequencies to generate greater tensions. Recruitment vs. rate coding – – Smaller muscles (ex: first dorsal interosseous) rely more on rate coding Larger muscles of mixed fiber types (ex: deltiod) rely more on recruitment The firing of individual motor units occurs as a stochastic process Firing rate is a better term to describe the global changes in firing frequency (i.e., rate coding) Rate Coding Rate coding 100 Larger muscles Smaller muscles % Maximal Voluntary Motor Unit Recruitment 50 Motor unit firing frequency 0 0 50 % Maximal Voluntary Force Production 100 Rate Coding Rate coding occurs in two stages – Treppe (the treppe effect) A phenomenon in cardiac muscle first observed by H.P. Bowditch; If a number of stimuli of the same intensity are sent into the muscle after a quiescent period, the first few contractions of the series show a successive increase in amplitude (strength) – Tetanus A state of sustained muscular contraction without periods of relaxation Caused by repetitive stimulations of the α-motor neuron trunk (axon) at frequencies so high that individual muscle twitches are fused and cannot be distinguished from one another, also called tonic spasm and tetany Two forms of tetanus – Incomplete tetanus – occurs when there are relaxation phases allowed between twitches – Complete tetanus – occurs when the relaxation phases are completely eliminated between twitches Important! Smaller muscles (ex: first dorsal interosseous) rely more on rate coding Larger muscles of mixed fiber types (ex: deltiod) rely more on recruitment How do we measure motor unit recruitment and rate coding? Surface electromyogram and mechanomyogram Electromyography (EMG) Muscle force controlled by CNS Neural “signals” sent to muscle by CNS “Signals” are called action potentials Action potential that originates from the cell body of the primary motor neuron – Motor unit action potential (MUAP) Action potential that propagates along the sarcolemma (from the neuromuscular junction to ends of the fibers) – Muscle action potential (MAP) Electromyography (EMG) EMG is the recording of the MAP. EMG is defined as the algebraic sum of MAPs that pass within the recording area of the electrodes Electromyography (EMG) Used by clinicians Used by ergonomists Used by physiologists Used by biomechanists Recording EMG Signals Electrodes – Surface of the skin – Subcutaneous – Intramuscular Needle or fine-wire The size and location of the electrodes determine the composition of the EMG signal Recording EMG Signals Surface EMG provides a global measure of MAPs Fine-wire EMG records single action potentials in a few adjacent muscle fibers Surface EMG Bipolar electrode arrangement – Electrode pair over the muscle – A reference (ground) electrode over boney area – Signal = (E1 + ground) – (E2 + ground) Common mode rejection Placing Surface EMG Electrodes Identify the innervation zone Place bipolar arrangement distal to the innervation zone Use interelectrode distance (IED) of approximately 2 cm Position to avoid “cross-talk” Contaminating Factors Electrode and lead movements Volume conduction = cross talk Electromagnetic radiation of power sources Two common methods to reduce these: – Bipolar recording Common mode rejection – Signal filtering Isometric muscle action at 30% MVC Displacement Sensor Accelerometer Laser Beam Bipolar EMG Electrodes Force Transducer Orizio, C., Gobbo, M., Diemont, B., Esposito, F., Veicsteinas, A. Eur J Appl Physiol. 2003.