The Determinants of Energy Cost during Exercise; Anaerobic and Lactate Threshold. A lab report

Introduction:

Determination of energy cost during exercise only on the basis of maximal oxygen consumption (VO_2max) is not perfect, Wilmore, Costill and Kenney, (2008). According to studies of Astrand and Rodahl, (1977) and Wilmore, (1977), well-trained athletes especially endurance athletes like long distance runners, cyclists may keep their performance in a steadily improving level even after reaching the plateau stage of VO_2max.These studies recommend that there are some other determinants as well which need to be considered to describe endurance performance other than VO_2max.

One such determinant is an anaerobic threshold (AT), defined as the point of abrupt increase in ventilatory equivalent caused by non-metabolic carbon dioxide production owing to lactate buffering, Mc Ardle, Katch, F., & Katch, V., (2000). Since the anaerobic threshold is the point of body’s shift to anaerobic metabolism or lactate formation, AT is closely related with Lactate Threshold (LT) of a body. LT is the point of beginning of lactate accumulation above basal lactate level with linear increment in exercise intensity as the result of more production of lactate than removal by the body. AT may also be defined as the energy cost or oxygen uptake just below the disproportionate increase in minute ventilation and carbon dioxide production. Mc. Ardle, et.al. (2000), proposed AT as a non-invasive measure of the onset of anaerobiosis. Lactate accumulation is the major cause of a muscle fatigue. So, it is always beneficial to maintain a higher lactate threshold in proportion to the percentage of maximum oxygen consumption. AT is usually expressed in relation to minute ventilation whilst LT is usually expressed as the percentage of maximal oxygen uptake (% VO_2max) at which it occurs.

This study was designed to estimate the AT and LT and how they vary with the linear increment in exercise intensity at regular 3 minutes interval of time. We hypothesized that: a) with the increase in exercise intensity, blood lactate level increases showing the accumulation of lactate and hence the onset of anaerobic metabolism, b) an abrupt disproportionate linear increase in minute ventilation is always associated with increase in blood lactate level showing the onset of anaerobic metabolism and hence determines the anaerobic threshold,  c) both AT and LT are obtained with the higher exercise intensity and differ from person to person & d) ventilatory equivalent for oxygen is increased with the beginning of exercise from rest at low work intensity, becomes linear with increase in intensity and just below the point of anaerobic threshold there will be abrupt increase  in V_E & e) respiratory exchange ratio increases with increase in work intensity.

We investigated the AT and LT with a smooth resistance increment cycling exercise test at the regular 3 minutes of intervals.

Methods and Materials

Subjects

Two subjects (both female students) who were physically fit for stationary cycling volunteered to participate in the study. The experimental subject 1and subject 2 weighed 55 kg and 53 kg respectively. Both the subjects were informed about the study procedures, protocol, safety measures, purpose, duration, benefits and the risk factors of the test.

Overview

Both the subjects cycled for 12 minutes at the speed of 70 rpm. The subjects were regularly encouraged to maintain the speed throughout the test with the regular increment in resistance. We measured blood lactate, the total amount of oxygen consumption, carbon dioxide production and the total volume of air expired.

Protocol

The cycling started with a warm up phase of 3 minutes with the resistance of 60 W. The workload or resistance of the cycle was increased by 30 watts at the end of each 3 minutes and reached up to the resistance of 120 W completing the 5 stages of the test.  Before the start of the test, a blood sample was taken from both the subjects by pricking a finger to measure the blood lactate at rest. Blood lactate was then measured by taking the sample within the final 30 seconds of each 3-minute workload. Expired gas sample was collected within the final 60 seconds of each 3 minutes resistance increment in the cycle to analyze the percentage of oxygen in expired air, carbon dioxide produced, and a total volume of air expired.

Results

Overall, we found that blood lactate increased with increase in the resistance of the cycle and the duration of the test, (Table 1). The lactate level increased by 0.8 and 1.5 mM in subject 1 and 2 at the end of cycling with 60W and 90W respectively. However, at first nine minutes of the test, there were no significant change in lactate level and was below 4.0mM to both the subjects. There was marked increase in lactate level at the end of 120W and 150W, (6.3 and 11.2 respectively) in subject 1 whereas two marked increased points (4.2 and 7.7 mM) are found at 120W and 180 W cycling in subject 2. Maximum oxygen uptake and minute ventilation increased in proportion to the increase in lactate concentration level and a load of exercise.

Conclusion

The determination of anaerobic threshold and lactate threshold depend on the exponential increase in blood lactate concentration as a result of a person overcoming a certain level of exercise intensity or oxygen consumption. Also, the breaking point or the abrupt increment point in pulmonary ventilation versus oxygen uptake has been taken into consideration to determine the anaerobic threshold. However, it is difficult to establish a well-defined point to determine the anaerobic and lactate threshold as the rate and nature of metabolic reactions differ within an individual according to the type and nature of the muscles and muscle fibers used during work, the type of fuel substrate being oxidized, and the oxidative capacity, capillary density and enzyme pattern of the muscles being used.

Read full article here