RELATIONSHIP BETWEEN HEART RATE AND LACTIC ACID
Howat, R. C., & Robson, M. W. (June, 1992). Heartache or heartbreak. The Swimming Times, 35-37.
The accuracy and precision of a swimmer's estimate of heart rate was determined.
The energy metabolism needed for any workload, and not the workload itself, is the deciding factor for heart rate adjustment as well as for the threshold for prolonged work. The smaller the muscle mass used to accomplish a given workload, the steeper the rise in heart rate and the sooner the work will be interrupted by fatigue.
If the time required for oxygen intake, ventilation, and heart rate to return to resting values is measured after work of a steady-state nature, all variables will reach resting values at the same time. After exhausting work, the reaction is different. Heart rate remains elevated anywhere from five to ten times longer than the other factors. This is caused by the need for glycogen reserves to be restored. To accomplish that muscle metabolism must remain elevated because ATP is required to synthesize glycogen from glucose. This activity is aerobic in nature. Elevated heart rates after exercise support the mechanisms to replenish supplies of glycogen.
"The measurement of the heart rate pattern after exercise is useful in determining whether, and by how much, the endurance [aerobic] performance capacities have been overloaded." (p. 36)
Women must transport the same amount of oxygen to the working muscles with an average of 1.5 gm less hemoglobin per 100 ml of blood. This is equivalent to a -2% capacity. In women, the average heart volume is smaller, so the lesser volume has to be compensated by a higher heart rate.
Swimmers were instructed to perform aerobic swims at threshold. They were asked to count their heart rates after a particular swim. One minute after completion of the exercise a whole blood lactic acid analysis was conducted at poolside. Swimmers were divided in seniors and age groupers.
"Some extremely disturbing aspects of normal training became apparent. Irrespective of the expected wide variations of swimmers' estimates of their heart rates, the percentage whose bodies are gaining any benefit from training is at the most 33 per cent. This is similar to, and with age group swimmers greater than, the degree to which they are inadvertently overtraining. A similar proportion of time can be spent under-training." (p. 37)
"Whole blood lactate levels of around 3 mM equates to the anaerobic threshold for females (3.5 mM for males). . . . it is critical that, at threshold, pulse rate determined training is at least 10 beats per minute less than in males, if the same training effect is desired. Failure to recognize this important fact could result in major degrees of overtraining, especially in age group swimmers." (p. 37)
Using heart rates, such as those typically published to suggest a range within which a particular type of training effect will be achieved, to estimate work will roughly undertrain 33%, stimulate 33%, and overtrain 33% of swimmers in a squad for the purpose of achieving the same training effect.
When swimmers were instructed to train keeping their heart rates within a normal range for aerobic work prescription, the responses within the squad were anything but what was intended. A uniform training intensity based on usual heart rate estimates for aerobic stimulation does not result in a uniform training response, in fact, all manners of adaptation result within a squad. The following table shows that the variety of responses are similar for age-group and senior swimmers.
Seniors Percentage 8.3 33.3 33.3 25 Training nil no increase increase decrease effect aerobically aerobically Age Percentage 4.1 25 33.3 37.5 Training nil no increase increase decrease effect aerobically aerobically Lactate levels (mM) <1.5 1.5 - 2.4 2.5 - 3.4 >3.5
Implications. Using absolute heart rates as an estimate of training intensity across a squad will train, undertrain, and overtrain swimmers in roughly similar proportions, that is, only one in three will respond as intended. If different rates are not used for women, they will be grossly overtrained because a given heart rate needs to be less than males to have an "equivalent" intensity. Using heart rate "norms" to prescribe work intensities in swimming is more erroneous than accurate for achieving particular types of physiological response. When interpreting heart rates, lactate levels, and effort ratings the responses of females should be interpreted against different standards to those of males.
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