Phyl 4518 Motor Learning F2024 Lecture Notes PDF

PHYL 4518 Motor Learning F2024 – Wk 5 Zoe Chan, PhD [email protected]  Reminders Online Quiz #1 grades posted – Question on constant error, graded manually  Summary and review Muscles contain both contractile and elastic components Conserve energy Amplify/t...

PHYL 4518 Motor Learning F2024 – Wk 5 Zoe Chan, PhD [email protected]  Reminders Online Quiz #1 grades posted – Question on constant error, graded manually  Summary and review Muscles contain both contractile and elastic components Conserve energy Amplify/transfer power Absorb power SE CE adjusts tone (tension) to act on PE the SE and then bones SE  Summary and review The contractile element produces the most active force at optimal length Force (max) Lo 0 Short Long Muscle Length  Summary and review The stretch-shortening cycle produces more force than an isolated concentric contraction Resistance training can increase stiffness Plyometric training can improve SSC Stretching may be counterproductive to SSC Vanegas et al. (2021) JMIR mhealth and uhealth Summary and review Direction of information Involuntary vs. voluntary  Summary and review Neurons Dendrites receive info Axons send info Sensory, interneuron, motor neuron Action potentials Neurons depolarize once they reach a threshold  Summary and review One motor unit (MU) = motor neuron + all innervated muscle fibres All-or-none principle: all contract or none contract within one MU  Describe a synapse and its connection Detail the 3 ways the nervous system regulates the MU to regulate force output Today’s Describe and distinguish between intramuscular and intermuscular learning coordination objectives  Synapse Site of connection Neuron to neuron (presynaptic neuron → postsynaptic neuron) Neuron to muscle, neuromuscular junction: Terminal end of a motor neuron Dobrowolny et al. (2021) Cells Muscle Types of Motor Units Type IIx Type IIa Type I Motoneuron soma size: Type I < IIa < IIx Motoneuron axon diameter: Type I < IIa < IIx # of muscle fibre within MU: Type I < IIa < IIx Types of Motor Units Type IIx Type IIa Type I Speed of activation Type II, faster activation Metabolic power Type IIx > IIa > I Type II: ATP produced quickly (Type II), more power Fatigue resistance Faster Slower Type I (oxidative phosphorylation), more fatigue resistance Muscle twitch Contractile response generated by single action potential Force Note: Ca2+ involved in both contraction and relaxation - contraction (Ca2+ release) is faster than relaxation (Ca 2+ re-uptake) Muscle twitch Contractile response generated by single action potential Single twitches rare in normal function Evoked artificially in research to study muscle function Type IIx Type IIa Type I Force MU behavior & force control The nervous system regulates 3 basic functions to control force output: 1. Rate coding of motor units (summation) 2. Recruitment of motor units 3. Coordination of motor units and muscles  1. Rate coding – twitch summation Muscle Force Twitch Twitch Twitch Low frequency = Full relaxation between twitches AP Summation Force Higher frequency AP Incomplete relaxation Stronger contraction AP Increased frequency, increased summation, increased force (to a point) 1. Rate coding – twitch summation Faster the firing rate of the action potentials, more tension can summate Rate coding: regulation of firing rate to modify force output Human muscle firing rates – Isometric contractions = 5 to 60 Hz (per second) – Ballistic contractions = up to 120 Hz Strongest contraction – fused tetanus – Artificial stimulation Fused Tetanus Force Time 1. Rate coding – twitch summation Not enough time to relax the elastic elements in the muscle → EE remains stretched → tension is increased Not enough time for Ca++ to be re-uptaken → abundance of Ca++ in the muscle cell → maximal number of cross- bridges forming Force  2. Recruitment of motor units Though experiment: Progressively harder squeezing of handgrip dynamometer MUs recruited according to size of soma Type IIx Force Type IIa Type I Time 2. Recruitment of motor units Type IIx Type IIa Type I Size principle of recruitment: From small to large  2. Recruitment of motor units 2 excitatory APs + Motor Action 1 excitatory AP Potential Soma Voltage - Temporal/spatial summation of EPSP  2. Recruitment of motor units Type II: larger cell body (soma) more difficult to “excite” and recruit higher activation from nervous system needed to reach threshold Membrane potential *Not recruitment threshold Depolarization Voluntary effort threshold Small soma Time Membrane potential Small excitatory input Large soma threshold Smaller change in voltage Time 2. Recruitment of motor units Size principle of motor unit recruitment From small to large motor unit Orderly recruitment Small motor units are activated first and de-recruited last Fatigue resistant (type I fibres) Most active during prolonged exercise  2. Recruitment of motor units Larger soma motoneurons innervating more fatigable fibres Force (type IIa and IIx) reserved for high force/speed (or type I finally fatigue) Low intensity (force) exercise Small soma motoneurons innervating fatigue resistant (type I fibres) are preferentially recruited Time  2. Recruitment of motor units Motor Unit Properties and Characteristics Small Medium Large Neuron/axon size Small Medium Large Number of muscle fibers Few Medium Many Type of muscle fiber Slow twitch Either/ Fast twitch intermediate Energy needed for activation Least Moderate Most Recruitment order First Second Third Function Endurance Mixed Force/power precision 3. Neuromuscular coordination Intermuscular coordination Inter- = ‘between’ 2 or more groups Coordination of muscle groups and body segments Intramuscular coordination Intra- = ‘within’ the same group Patterning and use of MUs within a muscle or across a muscle task groups 3.1 Intermuscular coordination Observed in the patterning and role-playing of different muscles The muscle’s role may change from one moment to the next Sometimes a muscle may function as an agonist Sometimes as a stabilizer Sometimes as a neutralizer E.g. Glute max Agonist: extension during running Stabilizer: holds hip in place when standing Neutralizer: external rotation counteracts glut min’s internal rotation during most movements 3.1 Intermuscular coordination Outcome often measured as EMG (electromyography) and biomechanical efficiency Muscle electric activity (muscle action potential) are detected by the electrodes inserted into the muscle or placed on the skin surface above the muscle Muscle active (electric potential) Potential read and measured as EMG signals 3.1 Intermuscular coordination Outcome often measured as EMG (electromyography) and biomechanical efficiency Agonist Antagonist EMG magnitude (mV) EMG magnitude (mV) Untrained (uncoordinated) EMG magnitude (mV) EMG magnitude (mV) Trained (coordinated) 3.2 Intramuscular coordination Intramuscular coordination is expressed in 3 fundamental ways: Coordination between motor unit firing rate and recruitment Discharge patterning (synchronization) Compartmental coordination 3.2 Intramuscular coordination Rate coding & recruitment Both mechanisms used throughout the force range Recruitment the predominant mechanism in the low force range Firing rate predominant mechanism in the high force range Factors affecting the mix and match: Muscle and type of movement Small muscle (hand): full MU recruitment at 30% MC Large muscle: rate coding early and not recruit all unit until 80-90% MC 3.2 Intramuscular coordination 100 ~90% of MUs recruited % of motor units recruited Only the highest threshold MUs still left to be recruited 50 Recruitment Firing Rate predominant mechanism predominant mechanism 0 0 50 100 % Maximal Contraction 3.2 Intramuscular coordination Discharge patterning – manipulating firing rate of MUs to meet task demands EMG magnitude (mV) EMG magnitude (mV) MU 1 Smooth contracted required (most of the time) MU alternate contraction MU 2 3.2 Intramuscular coordination Discharge patterning – manipulating firing rate of MUs to meet task demands EMG magnitude (mV) EMG magnitude (mV) MU 1 When maximal force is required MU fire at the same time MU 2  3.2 Intramuscular coordination Compartmentalization Smaller and independently controlled groups of muscle fibers within a single muscle or group of muscles Compartments based on: Muscle morphology Fast (Type II) vs slow (Type 1) twitch Neural recruitment E.g. some compartments only active during some tasks Biomechanical functions E.g. angle of pull  3.2 Intramuscular coordination Compartmentalization of the deltoid Medial Anterior Posterior Adduction Flexion  MU behavior adaptations to training Strength and power training have been shown to: Increase the maximal level of motor unit activation to 100% if not previously maximal. Increase average or maximal firing rates. Cause an earlier onset of high threshold units Faster rate of Enable high firing rates to be reached sooner or force contractions started at a higher firing rate. development Most MUs active All MUs active 50 Hz 60 Hz