DELIVERING OXYGEN TO THE MUSCLE CELLS.

• cardiac output (transporting oxygen to the cells)
• VO2 (oxygen consumption with exercise)
• measures of cardiovascular fitness
• skeletal muscles
• changes in CV physiology with age

The cardiovascular (heart and blood vessels) and pulmonary (lungs) systems work together to deliver the oxygen necessary for efficient (aerobic) energy metabolism to the exercising muscle. Oxygen is extracted from air in the lungs and then transported in the blood to the cells where it is extracted and utilized. The byproduct of energy production, carbon dioxide, is then transported back to the lungs by the circulating blood and leaves the body in expired air.

CARDIAC OUTPUT

The increase in stroke volume results from both a hypertrophy (enlargement) of the left ventricle muscle (athlete's heart) as well as an enhancement of the heart's contractile state, probably mediated by the autonomic nervous system.

THE LUNGS

OXYGEN CONSUMPTION (VO2)

VO2max. is determined:

• lung capacity (getting oxygen from the air we breath into the blood which is passing through the lungs
• and the ability of the muscle cells to extract oxygen from the blood passing through them (the arterio-venous or A-V O2 difference)

At levels of exertion greater than the VO2 max., the energy needs of the cells outstrip the ability of the cardiovascular system to deliver the oxygen requiredfor aerobic metabolism, and oxygen independent or anaerobic energy production begins. Anaerobic metabolism is not only less efficient (less ATP is formed per gram of muscle glycogen metabolized) resulting in more rapid depletion of muscle glycogen stores, but also results in a build up of lactic acid and other metabolites which impair muscle cell performance (even when adequate glycogen stores remain). The build up of excess lactic acid will be ultimately be eliminated when exercise levels decrease to an aerobic level and adequate oxygen is again available to the muscle cell. The build up of these metabolites (and amount of oxygen which will ultimately be needed to eliminate it) during anaerobic metabolism is responsible for oxygen debt (the period of time required to remove them) and recovery phase that follows anaerobic exercise. The lactic acid produced in the cellduring energy production appears to be serve a protective function to prevent the development of excessive acidity, while other acidic compounds are actually responsible for the discomfort and the impairment of muscle cell function.

MEASURES OF CARDIOVASCULAR FITNESS

The following all indicate that an individual's VO2max has been reached:

• VO2 plateau - no further increase in oxygen use per minute even with an increase in work performed
• heart rate within 10 beats of the age predicted maximum heart rate - this is the basis for using your maximum heart rate as a surrogate for your VO2 max when designing a personal training program)
• plasma (blood) lactate levels greater than 7 mmol/liter

• Ranges of VO2maxby age/sex
• Calculating %VO2max based on your % of your MHR (Maximum Heart Rate).

Both systems are working at all times, however at low levels of activity almost all the ATP required by the cells is produced via the aerobic system and only a small amount of ATP being produced by the anaerobic system. The small amount of lactic acid being produced as a byproduct is quickly metabolized by the body.

The amount of lactic acid in the blood is the result of the balance between production and removal (by the heart, liver, kidneys, and non working muscles). Although being produced at only a low rate, lactic acid can be measured in the blood even when you are at rest. As you increase your activity, the percentage (and total amount) of ATP energy produced by the anaerobic system increases - as does the production of lactic acid. At a certain level of activity the amount of lactic acid produced becomes greater can be removed by the body's metabolism and the blood concentration begins to rise. This level of activity is known as the anaerobic or lactate threshold (AT or LT). The more one's level of exercise exceeds their lactate threshold, the greater the lactic acid buildup (concentration in the blood) , and the greater its negative impact on physical performance. The lactic acid itself does not directly create the slight drop in the blood's pH (from 7.4 to about 7.2) that can be measured and is thought to be the cause of fatigue, labored breathing, and reduced power of muscle contractions experienced at higher intensities of exercise. It is thought that the associated proton (H+) accumulation, coinciding with, but not caused by lactate production, leads to the acidosis which forces the athlete to back off.

LT is typically expressed as a percentage of maximum heart rate (which in turn is thought to reflect VO2max). Although LT occurs at 50 to 60% of MHR in untrained individuals, it rises with training and is between 80 to 85% MHR in trained athletes. The majority of the improvement is felt to be from an improved processing (or removal) of lactic acid although there is almost certainly some decrease in production for a set level of activity as well (see article at the end of this section).

Why is the lactate threshold important? Having a higher lactate threshold means an athlete can perform at a higher intensity for a longer time. The higher the threshold, the faster an athlete can bike (or run) before they reach their exercise limit and are forced to slow down from the lactic acid/acidosis build up. Generally, in two people with the same VO2 max, the one with a higher LT will ride faster in continuous-type endurance events.

The margin by which lactate threshold is exceeded is inversely proportional to the time the athlete is able to sustain that pace. For example, let's say you have a LT of 170 beats per minute. You can sprint with a HR of 180 for a short time, or 185 for even a shorter time, before you become uncomfortable and are forced to slow down. On the other hand you could exercise at your LT of 170 BPM for hour(s). Thus LT defines the pace you can maintain for an endurance event. This suggests an easy way to determine your LT - the average heart rate you can maintain over a 30 minute (about 10 mile) time trial will be a good approximation of your LT.

For a trained athlete, the LT roughly corresponds to Zone 4 (of the 5 heart rate training zones) which is defined as 84-90% of Maximal Heart Rate. This level of cardiovascular activity produces enough lactic acid from anaerobic activity to stimulate improvement in CV and LT status, but not enough to limit the amount of time you can maintain this heart rate. The result is the most efficient heart rate to stimulate maximal improvement in aerobic performance.

Riders with a higher LT are also more efficient in their use of internal glycogen stores. At all levels of activity there are always muscle cells (not entire muscles, but a small number of cells within those muscles) that are relatively deficient in oxygen and thus making ATP inefficiently via anaerobic pathways, producing lactic acid as a result. As the LT is approached, more and more of these inefficient cells are being utilized to power your muscles. Those with an higher LT not only experience less physical deterioration in muscle cell performance for any level of %MHR, but also use less glycogen for ATP production at any level of performance. Thus an improvement in LT will allow the individual to perform at higher intensities for a longer period of time before running out of adequate energy (glycogen) stores.

An individual's LT will improve with training, and cyclists with a higher LT can work at a higher level of energy expenditure for longer periods, defeating opponents of equal (or even greater) physical strength but with lower LTs. That is why interval training, which improves LT, improves endurance performance.

Some of the many factors that affect the rate of lactate accumulation (and thus the intensity of activity at which the LT is reached) include:

• Exercise intensity - as intensity increases, more and more muscle fibers are operating in the anaerobic range and more lactate is produced
• Distribution of muscle fiber type - slow twitch fibers, which are best for endurance activity, produce less lactate for any level of exercise intensity than fast twitch fibers
• Training status - with training, muscle cell changes improve the rate at which lactate is metabolized and as a result, the rate of removal of lactate from the blood stream increases. Thus the balance between production and removal is shifted towards removal.
• Efficiency of effort - distribution of effort amongst across more muscle groups will lead to less lactate production (as they are all working at a lower intensity level than if the same amount of work was being done by fewer muscles groups. A great example is a rower, who must train to effectively use the muscles of the legs, back, and arms, rather than just the legs to maximize the amount of work they can do while minimize lactate production and exceeding their lactate threshold.

• How do I race based on my LT? This will depend on the duration of the event. For shorter, spring type activities which last minutes at most, you do not generally worry about your LT and lactate levels will often be quite high at the finish. In events of longer duration (30 minutes to 1 hour) you will want to aim for a rate at or just slightly above your LT.

• Does LT change based on the type of exercise? Yes, the lactate threshold is specific to the exercise task. So if a cyclist moves to rowing, (s)he will fatigue at a lower heart rate initially as rowing employs different muscles, generally less trained. Since these muscles are less trained, the cyclist's rowing LT will be considerably lower. Thus heart rate cannot be used as a guide in cross training activities.

• Can I improve my LT? Yes, training results in a decrease in lactate production for any given exercise intensity. Thus untrained individuals usually reach their LT at about 60% of VO2 max. With training, LT can increase from 60% to above 70% or even higher. And elite athletes have LTs at or above 80% of VO2 max.

Resting heart rate - your heart rate on awakening in the morning, is a simple but effective indicator of your level of training. It will fall as you train, but then begin to rise again with overtraining.

Heart Rate Variability - A measure of recovery to help "tweak" or modify the intensity of a training program.

Cardiac Stress Testing - for asymptomatic coronary artery disease.

HEART RATE VARIABILITY

Normal values for HRV are an average of 100 msec in the first decade of life and decline by approximately 10 msec per decade lived. At age 30-40, the average is 70 msec; age 60-70, it's 40 msec; and at age 90-100, it's 10 msec.

This article includes an annecdotal report of a fall and then rapid rebound of HRV after COVID-19 and influenza vaccinations.

And an article in bicycling.com suggested it might be useful in a training program. "HRV can be very helpful in training because a higher HRV usually indicates good training readiness.....For example, if you notice your HRV is on the high end of your average, you might increase the intensity or duration of your upcoming workout. And on the flip side, if your number is dipping lower than usual not just one day but consistently, it could indicate that you've been pushing it too hard and need to dial things back. Although logic suggests HRV might play a useful role, closer study failed to support any helpful information. Below are 3 (of many).

• Nine experienced endurance athletes increased their usual amount of training by 100% within 4 weeks.  In the conditions of the present study, HRV during sleep does not seem to be a valid marker of overtraining in male endurance athletes.

• Resting HRV is largely unaffected by overreaching.

• Exhaustive endurance training for 6-9 weeks did not induce changes in intrinsic heart rate and cardiac autonomic modulation in female athletes,

THE SKELETAL MUSCLES

Along with these visible changes in the muscle cells, there are microscopic and metabolic changes at the muscle cell level with training. These include an increase in the size and number of the muscle cell mitochondria, an increase in the activity of various metabolic enzymes in the muscle cells, and an increase in the number of capillaries in the muscle that supply blood to the individual muscle cells. The net result is an increase in the amount of oxygen extracted from the blood in a single pass through the muscle (the arterial - venous oxygen difference).

SUBMAXIMAL EXERCISE

CHANGES IN EXERCISE PHYSIOLOGY WITH AGE