Response to Incremental Exercise

Steady state exercise involves exercising at a low intensity with a constant workload, allowing the body to maintain homeostasis.
The body matches ATP resynthesis to its utilization rate.

Incremental exercise involves exercising against an increasing workload.
- This can be in a stepwise fashion or a ramp-like fashion with fancier equipment.

The assessment involved an incremental exercise test with increasing weight over time.

Actin and myosin need to bind, requiring ATP.
The body attempts to meet demands through oxygen delivery.
With oxygen, glucose metabolism yields 36-39 ATP.
Without oxygen, only 2 ATP are produced per glucose molecule.
There's a trade-off between energy systems.
- Some are fast but have limited capacity.
- The oxidative system is slower but provides ATP for a long time.
During prolonged exercise, the aerobic system is dominant.
The anaerobic threshold supplements aerobic metabolism.

ATP resynthesis must match the increasing work rate.
ATP stores are low, so resynthesis rates must match hydrolysis rates for sustained exercise.

Oxygen consumption matches work rate but with a slight delay.
Once oxygen uptake increases, it does so at a constant rate.
There is an increase of approximately 10 ml of oxygen per watt increase in healthy individuals.

$VO_2$ max is when oxygen consumption plateaus despite further increases in work rate.
Only about 50% of the population plateaus during tests.
$VO_2$ peak is the highest oxygen consumption achieved during the test.

Cardiac output increases proportionally to oxygen uptake but tails off near maximal exertion.
Cardiac output is a limiting factor to $VO_2$ max in healthy individuals.
The Fick equation explains the relationship between $VO_2$ , cardiac output, and arterial-venous oxygen difference.
$VO_2 = Cardiac Output \times (Arterial \; Oxygen \; Content - Venous \; Oxygen \; Content)$
The limiting factor is the delivery of oxygen to working muscles.

Cardiac output = heart rate × stroke volume.
Heart rate increases proportionally to exercise intensity.
Maximum heart rate is estimated by $220 - age$ .
Stroke volume increases just under twofold from rest to exercise.
Stroke volume increases in a curvy linear fashion and is related to blood flow returning to the heart.
Active low-intensity exercise post maximal exercise maintains stroke volume and prevents blood pressure plummeting.

Left ventricular mass increases with exercise training, tracking changes in $VO_2$ max.
Increasing the size of the left ventricle leads to greater stroke volume.
Basett and Howley (1999) comprehensively reviewed physiological limitations to VO2 max.
- They note blood flow to the lower body is reduced during whole-body exercise compared to isolated leg extension.

Beta blockers reduce both heart rate and $VO_2$ max, indicating cardiac output is limited.
Prolonged bed rest reduces both cardiac function cardiac output and $VO_2$ max.
Delivery of oxygen to working muscles limits $VO_2$ max.

The anaerobic or lactic threshold is a better predictor of race performance than $VO_2$ max.
It is the point at which anaerobic metabolism supplements aerobic metabolism.
During an incremental exercise test, aerobic ATP resynthesis cannot meet the rate needed, requiring anaerobic metabolism.
Glycolytic muscle fibers lack mitochondria, necessitating ATP production without oxygen.

Breaking down glucose without oxygen produces lactate and hydrogen ions ( $H^+$ ).
Bicarbonate (HCO3-) in the blood mops up hydrogen ions, ultimately forming water and carbon dioxide.
As exercise intensity increases, anaerobic metabolism contributes more, producing extra CO2.
The carbon dioxide profile changes with exercise intensity; the contribution gets greater and greater.

At rest, carbon dioxide production is typically about 80% of oxygen consumption.
Early in exercise, an equal proportion of carbon dioxide to oxygen consumption is observed.
At the anaerobic threshold, the rate of carbon dioxide increases and production rises faster than oxygen consumption because of that buffering action in hydrogen ions from anaerobic metabolism.
This inflection point marks the anaerobic threshold.
At high intensities, there is an accumulation that we call acidosis because hydrogen ions are acid; receptors that detect that encourage increase in breathing even faster and faster, and that causes us to blow off a load of carbon dioxide.
Beyond the anaerobic threshold, VCO2 increases at a faster rate than VO2 because of this buffering.

Useful for understanding physiological processes and indicates the production of hydrogen ions.
Knowing anaerobic threshold measures what's going on in the muscles by measuring what they are breathing out at the mouth.
Predicts exercise performance better than VO2max because working above the threshold produces fatigue-related metabolites.
Trained endurance athletes can shift the rate of their anaerobic threshold up to a higher percentage of their VO2 max.

The anaerobic threshold relates more to the arterial-venous oxygen difference side of the Fick equation.
Adaptations in this difference shift the threshold.

In most untrained individuals, the anaerobic threshold falls around 40-60% of VO2 max.
Exercising above the anaerobic threshold poses extra physiological challenges and can lead to fatigue.
Critical power or speed represents another threshold beyond which a steady state can never be reached.
The relationship between ATP resynthesis and work rate is valid during constant workload exercise below the anaerobic threshold.