Exercise as a Method of Improving Force Production PDF

Exercise as a Method of Improving Force Production Santos et al. (2023). Long-term neurophysiological adaptations to strength training: A systematic review with cross-sectional studies. Journal of Strength and Conditioning Research.  Classic Digby Sale tr...

Exercise as a Method of Improving Force Production Santos et al. (2023). Long-term neurophysiological adaptations to strength training: A systematic review with cross-sectional studies. Journal of Strength and Conditioning Research.  Classic Digby Sale training response and update DOI: 10.1007/s00421-021-04730-4 Medicine & Science in Sports & Exercise Force can change based on the drive (neural signaling) of the muscle Maximal Activation Motor unit recruitment Rate coding Spinal Cord Connections Bilateral limb facilitation? Motor unit synchronization Neural Excitability Coordination Co-activation of antagonist muscle Improved temporal and spatial motor recruitment How Can we Measure Changes in the Neural System Muscular Electromyography (EMG) Surface Isolated motor neurons Transcranial Brain Stimulation Interpolated Twitch Technique EMG Transcranial Brain Stimulation Interpolated Twitch Technique What Have We Learned From These Tools? Remembering That all Movement is Based Upon Muscle Force: What factors in the neural system might be susceptible to change leading to increased force? Muscle Activation Maximal Activation Hartman MJ, et al. (2011). The effects of Knight CA & Kamen G (2001). Adaptations fatigue of the plantar flexors on peak in muscular activation of the knee extensor torque and voluntary activation in muscles with strength training in young untrained and resistance trained men. and older adults. Journal of Journal of Strength and Conditioning electromyography and Kinesiology. 11. Research. 25(2). 527-532. 405-412. How does this occur? Motor Unit Recruitment Following training Increase activation of previously inactive motor units Rate Coding Single motor unit Fire frequency initially ↑as force requirement ↑ to some maximal firing rate Allows for 2-4 fold increase in force With Training ↑ in rate coding Motor unit can fire with a greater frequency, particularly in large motor units, than prior to training Power and speed training appear to be the most effective means of increasing rate coding Discharge Rate Endurance vs Strength Vila-Cha, et al. (2010). Motor unit behaviour during submaximal contractions following six weeks of either endurance or strength training. Journal of Applied Physiology. Spinal Cord Connections Bilateral Limb Deficit Concept: When both limbs contract together, the neural system divides resources to engage both units. When single limb is contracted, all resources are allocated to the single limb Force of combined single limbs is higher than force of both limbs contracting together With Training A reduction in bilateral limb deficit, some suggest a bilateral facilitation in resistance trained individuals Motor Unit Synchronization Concept: When motor units fire to contract a muscle, there is a level of variability in the temporal sequence of that firing With Training: The level of variability in the temporal sequence decreases leading to more motor units firing at the same time Does it really make a difference? Computer simulations suggest that the increased synchronization does not play a large role in increased force development Motor Neuron Excitability Some evidence to suggest that motor unit recruitment thresholds (tibialis anterior) decrease following training i.e. it is easier to activate the muscle – long term potentiation of the tissue Motor Unit Conduction Velocity Vila-Cha, et al. (2010). Motor unit behaviour during submaximal contractions following six weeks of either endurance or strength training. Journal of Applied Physiology. Corticospinal Excitability Potentiation The increase in strength of nerve impulses along pathways that have been used previously, either short-term or long-term With Training: Potentiation appears to have acute and chronic implications Coordination  Coordination All of the above have been suggested as alterations to neural function resulting in increased force Coordination of movement and muscle activity is likely the greatest contributor to increased force from a neural perspective i.e. you learned!!! Co-activation of Antagonists Carolan & Cafarelli (1992). Adaptations in coactivation after isometric resistance training. The Journal of Applied Physiology. 73: 911-917. Method: Assessed activation of quad and hamstring during isometric knee extension pre and post 8 wks of training Results: Decreased hamstring coactivation 20% - Wk 1 Decreased hamstring coactivation by a further 13% - Wk 2 Control group did not see a change Limitation Most research has been done isometrically in fully stable situations Does the same response occur in open chain situations? Is there utility to having coactivation of antagonists? Muscular Adaptations: Beyond hypertrophy Force can change based on morphology of the muscle Morphological Change – i.e. ↑pennation angle Wells et al (2014). Vastus lateralis exhibits non-homogenous adaptations to resistance training DOI: 10.1002/mus.24222 Morphological Change – i.e. ↑CSA Doblow & Gorgey (2016). Effects of use and disuse on non-paralyzed and paralized skeletal muscles DOI: 10.14336/AD.2015.0826 Morphological Change – i.e. altered fiber type ratio Fry et al. (2003). Muscle fiber characteristics of competitive power lifters DOI: 10.1519/1533-4287(2003)0172.0.co;2 Remember the characteristics of the muscle fibers Methenitis et al. (2016  What does this mean to adaptation of tissue? Marathoner Sprinter Athletes who produce high power have a different distribution of fiber types Myosin heavy chain distribution of isolated single muscle fibers from the vastus lateralis of a world championship sprinter Trappe et al. (2015) Does load influence the response? These data suggest that well trained Olympic weightlifters have a low proportion of IIx, but very high proportion of IIa Serrano et al. (2019) Take Home Message Hypertrophy is an important adaptation to allow for increased muscle force output; however, neural adaptations allow for more economical force output and muscle fiber type transitions to more type II fibers and selective hypertrophy of these fibers are needed to maximize strength and power