When you perform any activity, your body uses oxygen in order to provide energy. This oxygen use can be direct, as in aerobic activities, or your body can use oxygen indirectly to replenish fuel supplies depleted in anaerobic activities. Your body will adapt to any activity as long as it is done regularly. The adaptations brought about by anaerobic activities such as strength training are somewhat poorly understood.
Anaerobic Effects on the Body
The majority of scientific studies that concern the adaptation of the body's respiratory and cardiac system to exercise have concentrated on aerobic exercise, or exercise that directly utilizes oxygen. The main indicator of aerobic capacity is VO2Max, or the greatest amount of oxygen consumed during physical activity. Strength training and other anaerobic activities have been shown to have no effect upon VO2Max, but have shown to improve overall performance.
Acute Responses to Strength Training
When you contract a given muscle under a significant amount of resistance, as you would when strength training, these contractions can serve to drive oxygen-depleted blood through your veins back to your heart and lungs. Increasing the amount of blood returning from your veins causes your heart to increase its output to the lungs to oxygenate more blood, and then out to the rest of your body to provide your working muscles with oxygen. This is known as the Frank-Starling Mechanism.
Left Ventricular Hypertrophy
The left ventricle of your heart receives freshly oxygenated blood from your lungs, in preparation for distributing it to the rest of your body. The tissues of the left ventricular walls can thicken, which is known as ventricular hypertrophy. Ventricular hypertrophy is generally associated with high blood pressure, but it can also occur as the result of athletic training. When it is a response to training, ventricular hypertrophy increases your heart's stroke volume, or the amount of blood it can provide per beat. This is one of the few documented benefits of strength training to your cardiorespiratory system.
Strength Training and Improved Lactate Threshold
The lactate threshold is the point at which your working muscle produce more lactate than your circulatory system can effectively clear. This is the level near which your body ceases to rely on oxygen as a direct fuel source. Increasing this threshold level can actually allow you to work at a higher intensity or for a longer duration regardless of your actually VO2Max. Strength training increases the lactate threshold without a corresponding change in your VO2Max.
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