Chapter 11: High Intensity Intermittent Exercise

what is high intensity intermittent exercise?

periods of exercise that are characterized by fluctuations in exercise intensity over a given time

repeated periods of high-intensity activity (near maximal or supra-maximal) interspersed with exercise of low to moderate intensity or, in some cases, complete inactivity (i.e. rest)

which energy systems are used to regulate metabolism during HIIE?

a mix of anaerobic and aerobic energy systems

during vigorous bouts, primarily anaerobic

during recovery or low to moderate intensity, primarily aerobic → carbohydrate is main substrate for aerobic metabolism at moderate intensity, lipid is main substrate during instances of recovery or low intensity

energy systems function on a continuum!

what does glycogen phosphorylase regulate?

glycogenolysis

upregulated by calcium, magnesium, inorganic phosphate, epinephrine, ADP, AMP

downregulated by hydrogen ions, G-6-P, ATP

what does PFK regulate?

glycolysis

upregulated by glycolysis intermediates, ADP, AMP, ammonia, cAMP

downregulated by ATP, hydrogen ions, magnesium, PCr, citrate, phosphoenolpyruvate

what does pyruvate dehydrogenase (PDH) regulate?

progression of complete carbohydrate oxidation (pyruvate to acetyl-CoA)

upregulated by calcium, magnesium, ADP, hydrogen ions

downregulated by ATP, citrate, inorganic phosphate

what does calcium activate at the beginning of exercise?

phosphorylase kinase (turns glycogen phosphorylase on)

PDH → activation is continued with greater pyruvate and hydrogen ion accumulation

what molecules continue glycogen phosphorylase action allosterically as exercise continues?

inorganic phosphate (from PCr breakdown), AMP, ADP

eventually downregulated by accumulated hydrogen ions

what is the benefit of an increased rest duration during HIIT training?

aids in recovery from longer sprints

reduced markers of metabolic stress and better-preserved time to completion

likely allows for greater PCr restoration

blood flow and oxygen delivery is critical for recovery → better trained athletes will recover more quickly between bouts

what must be considered when creating a fueling strategy?

substrate availability (consumption), exercise intensity, and duration

what is glycemic index (GI)?

a system that ranks foods based on their effect on blood glucose levels

foods toward 1 have a smaller and slower effect, while foods toward 100 have the greatest and quickest effect

higher GI foods spike blood glucose

what is the effect of carb intake during HIIE?

increases glucose availability and insulin levels; reduces glycerol and plasma FFA, prolongs time to exhaustion

what muscle adaptations occur with HIIE?

increased aerobic enzymes, buffering capacity, muscle glycogen content, increased PDH protein content and activity

decreased reliance on carbs at submaximal exercise intensity

improved insulin sensitivity

*conserves time and reduces training volume

what do the aerobic and anaerobic adaptations of HIIE demonstrate?

the importance of intensity, not only total exercise duration

how does HIIT training cause aerobic adaptations with lower total exercise volume?

likely due to repeated stimulation of early signaling mechanisms

engaging in repeated vigorous bouts causes repeated metabolic stress that offsets the energy charge of the cell

activates signaling kinases (responsible for transferring phosphates from ATP to other substrates

repeated activation of kinases eventually changes PGC1 gene expression (upregulates expression of proteins used for mitochondrial biogenesis)

repeated high intensity stimulus is required for adaptations, causes greater cellular stress to induce adaptations

what are the two instances of fatigue?

progressive fatigue across a workout or match

instantaneous fatigue following singular bout of vigorous activity

what are the causes of progressive fatigue across a workout?

central fatigue → neural and psychological reasons (preceding motor neuron)

peripheral fatigue → substrate related fatigue, metabolite accumulation

what is the cause of instantaneous fatigue following a single bout of vigorous activity?

peripheral fatigue → metabolite accumulation

what are the substrate-related mechanisms of fatigue?

reduced glycogen availability → greater effect following speed endurance type HIT, glycogen in type II fibers mostly reduced, impairs calcium release from SR, glycogen supplementation during exercise delays fatigue

PCr depletion → contributes to temporary/instantaneous fatigue during HIT or matches, when PCr is temporarily depleted, intensity must decline (restored during rest in type I fibers first)

what metabolites accumulate to contribute to fatigue?

H+ ions from lactate (reduces pH & calcium release from SR and PFK activity)

potassium accumulation in the extracellular space (from muscle contraction)

calcium (impaired release causes fatigue, high phosphate from PCr hydrolysis produces calcium-phosphate)

free radicals (atoms or molecules with one or more unpaired electrons)

muscles contain antioxidants but exercise creates oxidative stress (overproduction of reactive oxygen species [ROS], antioxidants can’t keep up, ROS attach to molecules, oxidizing them causes loss of function)