Fatigue Adaptations in Resistance Training PDF

Fatigue ADAPTATIONS IN COACTIVATION AFTER ISOMETRIC RESISTANCE TRAINING Factors influencing fatigue Fatigability: the rate of decline in an objective measure of motor performance (e.g. MVC force, reaction time, movement accuracy). Enoka (2015) Defining central fatigue u What is central fatigu...

Fatigue ADAPTATIONS IN COACTIVATION AFTER ISOMETRIC RESISTANCE TRAINING Factors influencing fatigue Fatigability: the rate of decline in an objective measure of motor performance (e.g. MVC force, reaction time, movement accuracy). Enoka (2015) Defining central fatigue u What is central fatigue? “A progressive exercise-induced reduction in voluntary activation or neural drive to the muscle” Do we maximally activate muscles? u Occasional stories of feats of strength under situations of high stress prompt the question: u Do we use our muscles maximally? u Or do we have a reserve of muscular force that we do not use in normal circumstances? Do we maximally activate muscles? u Electrical stimulation elicits more force than MVC after fatiguing exercise (Reid, 1928) u Evidence that muscle is capable of more than it is producing u Suggests reduced central drive  Stimulation of motor cortex Interpolated twitch Descending motor commands Force Time Electrical stimulation of peripheral nerve axon Interpolated twitch Merton (1954), Voluntary strength and fatigue, J Physiol (Lond).  Interpolated twitch u Interpolated twitch amplitude does not scale linearly with voluntary force. u There is a more rapid drop in IT amplitude than would be expected at low levels of force. Simulated vs. real muscle twitches u This suggests that the reserve of muscle force is depleted rapidly at low force levels but retained at high force levels. u Why might the CNS use this strategy? Evidence for central fatigue u Peripheral nerve stimulation can produce movement even when voluntary activation cannot (Reid, J Physiol (Lond), 1928) u Finger flexions against 3 kg load at 24/s produce task failure u Subsequent median nerve stimulation lifts the load – compensates for central component of fatigue u Full rest for 60 s restores both peripheral and central fatigue components  Defining central fatigue u Early evidence suggested that ‘muscle’ fatigue can be induced by cognitive tasks alone u Extent of loaded (3kg) finger movements reduced following mental exertion – lecturing (Mosso, 1904) Note: I will be unavailable after class due to fatigue Main points u Central fatigue is caused by a reduced capacity for activating alpha motoneurons u Central fatigue can be assessed using the interpolated twitch technique u Reduced central drive to muscles can be caused by either physiological fatigue or the perception of fatigue Changes in motor system function with fatigue Changes in motoneuron function with fatigue u CNS activation of motoneurons changes during fatigue u Motoneuron firing rate diminishes to 50% of maximum during an MVC u Muscle relaxation time increases Impact of fatigue on contractile characteristics u Muscle twitch characteristics change during fatigue u After 60 s MVC, twitch force diminishes, muscle relaxation time increases. u Reduction in “ripple” with 7 Hz tetanus Voluntary over-ride of central fatigue u Telling people to produce a ‘super’ effort reduces the rate of voluntary force decay u Rate of force decay becomes similar to tetanic force decay u Why might this occur? Do we maximally activate muscles? u Startling stimuli can also elicit greater contractions than MVC (Ikai and Steinhaus, 1961) Startle responses are known to be caused by activation of pathways descending from the brainstem. Suggests that non- corticospinal pathways can recruit additional motor neurons. Cortical & subcortical effects of fatigue u Interpolated twitches obtained by stimulating the cortex and brainstem provides information about where CNS drive is altered during fatigue. Gandevia et al., J Physiol (Lond), 1999 Fatigue affects cortical & subcortical pathways differently u In humans, spinal stretch reflex but not long- loop reflex affected by MVC in Abd Pol Brev Long latency stretch reflex Short latency stretch reflex Implications for measuring maximal motor capacity u MVCs should be accompanied by some instruction and practice u Feedback of performance should be given during the MVC u The gain of any real-time visual feedback should be varied so that the subject is not aware of the magnitude of any decline in performance u Standardised verbal encouragement should be given u Subjects must be allowed to reject efforts that they do not regard as “maximal” u Provision of rewards should be considered Main points u Fatigue is associated with changes in the firing characteristics of motor neurons that reduce muscular force and coordination u Non-corticospinal inputs to motoneurons can recruit motor neurons unable to be recruited by the cortex u Fatigue does not have the same effects on cortical and subcortical motor systems Fatigue pathways A neurophysiological view of preparation of voluntary force α-mn and γ-mn’s receive multiple peripheral and central inputs Effects of fatigue on muscle afferents Effects of fatigue on muscle afferents Effects of fatigue on muscle afferents Fatigue alters synergist activity u Changes in non- stimulated muscle (MG) as a consequence of electrical stimulation of LG u Mediated by agonist to synergist reflex pathways Main points u All proprioceptive afferents are affected by fatigue u Type III/IV afferents particularly important for signalling fatigue u Changes in proprioceptive afferents can reduce force capacity in non-fatigued muscles Factors causing fatigue Factors causing fatigue “No single factor has yet been identified in exercise of normal human volunteers as the cardinal ‘exercise-stopper’ ” (Gandevia, 2001) Factors causing fatigue u Some suggestions: u Decreased CNS drive u Decreased facilitation of motoneurons u Muscle acidification (Inc. H+) u Reduced blood flow u Hypoxia (Altitude) u Glycogen depletion u Core temperature u Lack of use (learned non-use) Does the type of load affect fatigue? u Yes! u Position-matching tasks(with inertial load) result in earlier failure than force-matching tasks (same force requirements) u Motor unit recruitment occurs at lower forces in position- matching u Motor units fatigue earlier u Why? Main points u Many factors may contribute to fatigue, but none are known to be the primary cause of exercise cessation u Muscle fibre acidosis does NOT appear to contribute to fatigue u The type of load encounter does influence fatigue, suggesting that cortical involvement may induce more fatigue