Bonen, A., & Kemp, N. H. (1977). Physiological, metabolic and practical considerations for training swimmers. Research Papers in Physical Education, 3(3), 10-15.

As soon as swimming begins, the volume of air that is inspired increases from about 5-7 L/min at rest up to 100-150 L/min at maximum effort. At the same time the rate of blood flow from the heart to the muscles is increased from about 4-5 L/min to about 25-30 L/min. This increase in blood flow results from an increased heart rate (e.g., 70-190 bpm) and increased volume of blood pumped per beat (70-150 ml). As well, the blood flow is distributed more efficiently.

During swimming the local muscle temperature rises and the acidity around the cells increases. Under those conditions O2 can be extracted more rapidly and completely than at rest. This results in an increase in capacity to use O2 by as much as 1000-1500% when going from rest to maximum swimming.

The cardiac stroke volume of the trained swimmer is greater both at rest and during swimming. The volume increase is extensive in that it offsets a decrease in maximum heart rate as a result of training. These adaptations result in a greater blood flow, and thus, oxygen supply, to the muscles. Trained swimmers are capable of extracting more oxygen from the blood than untrained swimmers.

Training increases the number of units within the muscle cells (mitochondria) where oxygen is used to produce ATP. The enzymes associated with O2 metabolism are also increased. The muscle cell also metabolizes proportionally more fats than when the swimmer is untrained. That use results in a slower rate of glycogen use (it is "spared") and thus, lactic acid production is reduced.

The anaerobic capacity of swimmers can be increased by as much as 20%. However, the levels of lactic acid in children are lower than in adults because of growth and developmental differences.

Physiological capacities achieve a ceiling level (Astrand, P. O., & Rodahl, K. (1970). Textbook of work physiology. McGraw-Hill: Toronto). In many world class mature swimmers the only room for improvement comes from further refinements in technique. Skill development is the secret for development in mature swimmers. In that regard, the specificity of training is paramount.

As a general rule for training swimmers, the shorter the distance the fewer the number of repeats. Research has repeatedly shown that the rate of physiological adaptation occurs primarily in response to the intensity of effort rather than the total amount of effort. Therefore, the principle concept for swim training is effective yardage.

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