This study aimed to examine lactate dynamics during a fixed-load exercise at the work rate of the GET, as determined during Inc-Ex. This was done because the GET level of exercise is often suggested as a convenient initial work rate for cardiac rehabilitation [1, 2], and it is known that with the often-employed percentage of maximal VO2 approach, the same percentage prescription results in a very inhomogeneous metabolic profile, including blood lactate, in different individuals . The GET level exercise is expected to produce a more homogenous response. One study , the primary purpose of which was to assess the arterialized and venous lactate concentration difference during constant-load exercise, investigated lactate levels at the lactate threshold in young male participants. Although LT was determined by lactate measurement (not by GET) and respiratory variables were not reported, the lactate time trend was similar to our results.
The mean BLa increase at the GET was approximately 0.5 mmol/L above the resting level, which is consistent with the results of our and others’ previous reports [3, 4, 5]. Therefore, we can say that this GET level of exercise induces only minimal lactate elevation during Inc-Ex. However, as the continuation of this GET level exercise as a CL-Ex produced a clear increase in the lactate level (1.38 ± 1.21 mmol/L above the resting level), the GET level exercise at this stage cannot be called an exercise that produces only the minimal level of lactate. Further increases in the GET level during CL-Ex may be the result of 2 factors: 1) lactate is formed in the exercising muscle and takes time to reach the bloodstream to be detected [18, 19]; 2) the preferential energy source varies with exercise duration . During Inc-Ex, the workload is continually increased, which may favor glycogen breakdown and induce lactate formation in the working muscle; this is then released with a time delay.
In spite of GET-level exercise being accompanied by distinct elevations in lactate, overall mean changes in lactate, VO2, and RER revealed the attainment of a steady state, although each parameter reached a steady state with different delays during CL-Ex. Previous studies have mostly analyzed changes in blood lactate during steady state exercise using arterialized blood collected from healthy young people [17, 21]. In addition, some previous studies used mixed-venous blood, one of which reported no increase in blood lactate at 60 ± 3% VO2 max . The problems with venous sampling are well known [23, 24].
In our study, CL-Ex at the GET level was individually tolerable for all elderly individuals included in this study. All participants showed significant elevation in BLa during the GET level constant load exercise. The concept of increased lactate levels during exercise has evolved in recent years . Evidence suggests that the increase in BLa is not due to hypoxia, particularly when exercise is not severe, but is instead likely due to increased glycolysis under aerobic conditions . Muscle biopsy studies show decreased muscle glycogen levels, even if the exercise intensity is not severe [26, 27, 28]. The steady state reached by mean VO2, RER, and BLa strongly suggests that our GET level exercise was essentially aerobic in nature. Furthermore, elevated BLa itself may serve as a metabolic signal to stimulate more efficient aerobic energy production . Therefore, an increase in BLa may be a necessary component of optimal exercise training. In this sense, GET-level exercise training can be a good starting point for cardiac rehabilitation.
An explanation is required for the CL-Ex protocol we employed in this study: approximately 3 min of Inc-Ex followed by CL-Ex, instead of step-wise introduction of CL-Ex, as is generally performed [30, 31]. We conducted the study in this manner for the following reason: if we introduce a GET-level work load as a step function, during the first short period, the subject may incur a sudden, undue energy demand. This could generate lactate in the muscle, which may appear in the blood with a delay and interfere with the interpretation of the subsequent BLa during CL-Ex. However, by employing an Inc-Ex protocol, as we routinely do, halting Inc-Ex as soon as we detect the GET (with minimal lactate increase), and transitioning into CL-Ex, we can observe how the naturally occurring GET behaves in CL-Ex.
The first limitation of the study concerns our FL exercise protocol. In this study, we defined a GET-level exercise as an exercise intensity (W) at which the GET in mL of VO2 appeared. It is known that for a step-wise initiation of CL-exercise, there is a delay in the increase of VO2, which is known as the mean response time. This delay in the response to VO2 increases with exercise intensity [32, 33]. Therefore, when the work rate at the GET VO2 is taken as the GET work rate, it may result in overestimation of the GET work rate because the work rate is, in a sense, ahead of VO2. However, there is no currently widely used or standard way to correct for this, although a quantitative way to correct for this has been reported and seems promising . Therefore, our GET-level FL-exercise protocol, which is not corrected for this, may well be overestimating the individual “true” GET level CL-Ex. Despite this, the average respiratory and metabolic responses reached a steady state.
Other limitations of this study include the fact that the size of the sample comprising participants with cardiovascular diseases was not sufficient. Particularly, no difference was observed between the healthy controls and the patient population. Further studies are required to investigate GET-level CL-exercise in cardiac patients with reduced exercise capacity.
Second, only the elderly population was studied. Although lactate dynamics in a younger population are unlikely to be very different at the GET, they may also need to be studied.
Third, the GET is visually determined, and individual GET values determined by different investigators can vary significantly, although the mean values may not show a significant difference . Therefore, there is a possibility that a similar investigation, if conducted by other investigators, may not produce similar results. It would be very interesting to see other studies of a similar nature performed.
In summary, the BLa level during constant-load exercise at GET intensity showed a greater increase (by approximately 1 mmol/L) than it did at the GET during incremental exercise. Nevertheless, BLa reached a steady state, together with VO2 and RER (below 1.0), suggesting that the exercise was primarily aerobic. This GET-level constant-load exercise protocol allowed all elderly participants to complete the 25-min exercise with fairly light to somewhat hard mean perceived exertion. Furthermore, the results suggest that increased blood lactate might serve as a stimulus for furthering aerobic energy metabolism.