Exercise physiology is a science based largely on those systems associated with the consumption, distribution, and use of molecular oxygen. For steady-state, easy to moderately intense, aerobic exercise, the measurement of steady-state oxygen uptake is considered the gold standard for the estimation of energy expenditure, usually reported as a rate function measurement where power output and energy costs are proportional (kJ·min−1) (1). During brief, intense exercise, oxygen uptake may not obtain a steady state and may not rise in proportion with work (5,7); it is reported as a capacity-type measure: with glucose as a “fuel”, 1 L of oxygen consumed = 21.1 kJ.
In addition to the volume of oxygen consumed, the energy costs of muscular strength and power also are derived from non-oxygen-related or anaerobic biochemistry, so the sole measurement of oxygen uptake can underreport the true cost of weight training (11). Such underreporting may be readily found in MET tables, for example, where golfing is listed as increasing metabolic rate to a greater extent than “light” weight lifting, calisthenics, and gymnastics (see, as an example, Appendix D in Nieman ). Clearly, a better knowledge of the caloric costs of physical activity can benefit an overweight society, yet for cardiac, pulmonary, or other physically limited patients, accurate and precise information concerning the metabolic intensity of physical activity (shoveling, load carrying, etc.) is essential to the minimization of health risks (13).
We asked the question, do non-steady-state energy expenditure measurements of a single bout of brief and intense exercise-oxygen uptake, blood lactate, and excess postexercise O2 consumption-collectively estimate the energy costs of weight lifting in a reliable and valid fashion? In the current investigation, triplicate non-steady-state O2 and blood lactate measurements are scrutinized in the estimation of energy expenditure during and after the bench press exercise (using a Smith machine). Our hypothesis is that in addition to exercise O2 uptake, measures of blood lactate and modified excess postexercise O2 consumption will improve the overall estimation of the costs of strength and power.
Experimental Approach to the Problem
The best estimates of energy expenditure are derived from steady-rate oxygen uptake measurements. Because brief, intense exercise does not consist of steady-rate oxygen uptake, we estimated aerobic and anaerobic energy expenditure in aliquot measurements for a given amount of work. Brief, intense exercise also invokes recovery, and this also may significantly increase overall energy expenditure. We attempt here to determine the reliability and validity of aliquot estimates of aerobic and anaerobic exercise and recovery energy expenditure before, during, and after repeated bouts of the bench press.
This investigation was approved by a human subjects institutional review board at the University of Southern Maine. Four men and 4 women subjects were informed of the experimental risks and signed an informed consent document before the investigation. All had histories of weight training (i.e., weight training 3 times per week for at least 3 months). To facilitate a comparison between other studies, and because of population size, the men's and women's data were pooled together to form a heterogonous sample for statistical analysis (n = 8, Table 1).
Subjects reported weekly to the lab for 10 total visits; they were asked to refrain from exercise on the day of testing and to have fasted for at least 4 hours before testing. On the first visit, a 1-repetition maximum (1RM) for the bench press was recorded on a Smith machine consisting of a horizontal bar that slides on vertical tracks where weight can only be lifted in the vertical plane; the exerciser does not have to balance the weight (York Barbell Co., York, Pa). Weight was gradually increased until a single repetition could not be completed. Subjects warmed up with light weights of their own choice. Subjects also chose the weight increase and rest time between attempts. To minimize fluctuations in power output, subjects also practiced lifting and lowering the bar at a cadence, set by metronome, of 1.5 seconds up and 1.5 seconds down. A small flywheel attached to a processor was connected to the moving cable on the Smith machine that recorded the distance the bar traveled. The coefficient of variation (CV) for 10 individual repetitions at a distance of 106.6 cm was 0.25%. For 6 separate sets of varying but explicit distances with 10 reps per set, the CV was 0.74%. Work (J) was recorded as the product of weight lifted and distance.
During the following 9 visits to the lab, subjects were randomly assigned to bench pressing 50% of their 1RMs for 7, 14, or 21 reps; thus, 3 sets of data were recorded for each of the assigned numbers of repetitions. Each day of lifting consisted of the following measurements: 5 minutes, resting, supine, energy expenditure (liter of O2 per minute); resting blood lactate (mmol); exercise O2 uptake (kJ); peak recovery blood lactate (mmol); and excess postexercise O2 consumption (kJ) and work (J).
Oxygen uptake was recorded using a standard metabolic cart (MMS-2400, ParvoMedics, Sandy, Utah). The metabolic cart was calibrated a minimum of two times immediately before all testing, using room air and calibration gas (16% O2, 4% CO2) and a 3-L syringe. Resting oxygen uptake was recorded in 20-second measurement periods and was averaged for a 5-minute period with each subject lying supine with his or her back on the bench (feet on the floor). At the end of the 5-minute rest, each subject began lifting at the required cadence while O2 uptake continued to be measured throughout the exercise period. Gas exchange measurement periods were based on the completion of 7 reps in approximately 20 seconds, 14 reps in approximately 40 seconds, and 21 reps in approximately 60 seconds. Resting oxygen uptake was computed using the respiratory exchange ratio (RER). Aerobic exercise energy expenditure was recorded as 1 L of O2 = 21.1 kJ. After the lifts were completed and the weight was racked, each subject had his or her feet elevated on a stool; the subject lay completely supine, and excess postexercise oxygen consumption was recorded until 2 subsequent measurements fell below 5.0 ml·kg−1·min−1 (a standing, resting O2 uptake). Excess postexercise oxygen consumption was converted to energy expenditure as 1 L of O2 = 19.6 kJ to dismiss the glycolytic component from the oxygen uptake measure (10-13). Each subject's resting O2 uptake was subtracted during the course of the exercise and excess postexercise oxygen consumption energy expenditure measurements.
All blood lactate measurements were recorded in quadruplicate using blood lactate strips and 4 handheld lactate analyzers (a standard calibration strip was inserted into each machine before testing; Lactate Pro, Arkray, Inc.). For data analysis, the high and low lactate measures were dropped, and the 2 remaining measures were averaged. Resting blood lactate was collected with subjects lying supine before the resting O2 uptake measurement. Blood lactate also was recorded in the supine recovery from exercise at 2, 4, and possibly 6 minutes postexercise. Peak blood lactate was taken as the highest blood lactate concentration recorded (this usually occurred at 2 minutes postlift). Anaerobic exercise energy expenditure was calculated as the difference between resting and peak lactate values multiplied by body weight (kg), then by 3.0 ml of O2 (4). This oxygen equivalence measure was converted to joules as 1 L of O2 = 21.1 kJ. Total energy expenditure was recorded as the sum of aerobic and anaerobic exercise energy expenditures and excess postexercise O2 consumption.
Statistical significance was set at p ≤ 0.05. Triplicate measurements were tested for reliability using repeated-measures analysis of variance (ANOVA). Variability was determined using the CV (SD ÷ mean). Differences among triplicate measures of CV were determined using ANOVA. Correlation was performed using the Pearson product moment correlation. Linear regression also was performed.
Subject characteristics are listed in Table 1. Repeated-measures ANOVA revealed that during the course of triplicate trials, no significant differences were found in any of the 7 variables within 7, 14, or 21 reps of the bench press: 7 reps resting O2, p = 0.81; 14 reps resting O2, p = 0.79; 21 reps resting O2, p = 0.89; 7 reps resting lactate, p = 0.68; 14 reps resting lactate, p = 0.73; 21 reps resting lactate, p = 0.87; 7 reps anaerobic exercise energy expenditure, p = 0.74; 14 reps anaerobic exercise energy expenditure, p = 0.80; 21 reps anaerobic exercise energy expenditure, p = 0.86; 7 reps aerobic exercise energy expenditure, p = 0.48; 14 reps aerobic exercise energy expenditure, p = 0.26; 21 reps aerobic exercise energy expenditure, p = 0.45; 7 reps excess postexercise oxygen consumption, p = 0.89; 14 reps excess postexercise oxygen consumption, p = 0.88; 21 reps excess postexercise oxygen consumption, p = 0.70; 7 reps total energy expenditure, p = 0.93; 14 reps total energy expenditure, p = 0.97; 21 reps total energy expenditure, p = 0. 86; 7 reps work, p = 0.98, 14 reps work, p = 0.91; 21 reps work, p = 0.99.
Coefficients of variation are listed in Table 2. Resting O2 uptake across repetitions had a CV ranging from 8.6% for 7 reps, 7.6% for 14 reps, and 7.2% for 21 reps; no significant differences were found in CVs among repetitions (p = 0.67). Resting blood lactate values across repetitions had a CV ranging from 20.3% for 7 reps, 24.3% for 14 reps, and 26.7% for 21 reps; no significant differences were found in CVs among repetitions (p = 0.69). Anaerobic exercise energy expenditure across repetitions had a CV ranging from 47.9% for 7 reps, 29.1% for 14 reps, and 14.2% for 21 reps; a significant difference was found between 7 and 21 repetitions (p = 0.002). Aerobic exercise energy expenditure across repetitions had a CV ranging from 47.4% for 7 reps, 28.3% for 14 reps, and 18.4% for 21 reps; no significant differences were found in CVs among repetitions, but a trend was evident (p = 0.06). Excess postexercise oxygen consumption had a CV ranging from 33% for 7 reps, 26.6% for 14 reps, and 29.2% for 21 reps; no significant differences were found in CVs among repetitions (p = 0.73). Total energy expenditure had a CV ranging from 21% for 7 reps, 15.4% for 14 reps, and 15.1% for 21 reps; no significant differences were found in CVs among repetitions (p = 0.73).
Correlations are presented in Table 3 among work, work per kilogram of body weight, and the change in work among trials with anaerobic exercise energy expenditure, aerobic exercise energy expenditure, exercise energy expenditure, and total energy expenditure. Significance is revealed for each (p < 0.05).
The linear relationship between steady-state power output (W) and steady-state O2 uptake (L·min−1) is generally thought to provide validity to the use of O2 uptake in the estimation of energy expenditure. Although true for easy to moderately intense exercise, O2 uptake during intense exercise may not change proportionately with work rate (5,7). Our intent was to determine whether non-steady-state measures could provide a valid estimate of exercise and recovery energy expenditure. Capacity-type measurements (kJ) as opposed to rate function measurements (kJ·min−1) were used in the examination of energy expenditure and work for single bouts of the bench press (at 50% 1RM).
Seven variables were examined in terms of reliability (triplicate measures of each; averages are shown in Table 2): resting O2 uptake (ml·min−1), resting blood lactate (mmol), anaerobic exercise energy expenditure (kJ), aerobic exercise energy expenditure (kJ), excess postexercise oxygen consumption (kJ), total energy expenditure (aerobic exercise energy expenditure + anaerobic exercise energy expenditure + excess postexercise O2 consumption) (kJ), and work (J). Repeated-measures ANOVA revealed that, during the course of triplicate trials, no significant differences were found in any of the 7 variables within 7, 14, or 21 reps of the bench press. Reliability is evident.
We also examined and compared variability among different parameters using the CV (Table 2). We measured resting O2 uptake as a per-minute function, but it is not a true resting metabolic rate. A detailed analysis of resting metabolic rate methodology suggested that a CV of less than 10% during a 5-minute measurement period would imply an accurate enough estimation of resting metabolic rate (3). Thus, our resting O2 uptake levels (CV range, 7.2-8.6%) may be considered reasonable. We electronically recorded work with a device whose CV did not exceed 0.75%, and so most of the variability seen with work (CV range, 4.5-5.4%) seems to be associated with the distance the subject moved the bar for each repetition. Resting lactate values all had similar CVs, ranging from 20.3 to 26.7% across repetitions. Although extensive, this is expected because “normal” resting blood lactate values may range from 1.0 to 2.0 mmol, a 100% difference. For some parameters, then, a CV greater than 10% may be considered acceptable. But, such approval is certainly not universal.
In addition to anxiety, eating, activity, food intake, and so forth, metabolic measurement variability also is associated with the length of the measurement period. Myers et al. (7) have demonstrated that as the O2 uptake measurement period decreases, the SD increases. For our estimates of anaerobic and aerobic exercise energy expenditure, this is likely in that the 7-rep, 20-second CV was greater than 47% for both, decreasing to a 14.2% anaerobic CV and an 18.4% aerobic CV during the course of a 21-rep, 60-second measure (Table 2). However, a significant difference in the CV only was found for anaerobic exercise energy expenditure between 7 and 21 reps. Excess postexercise O2 consumption had an extensive CV that was not different across repetitions (ranging from 26.6 to 33%; p = 0.73). The CVs for total energy expenditure were, for 7 reps, 21%; for 14 reps, 15.4%; and, for 21 reps, 15.1% (p = 0.61; total energy expenditure = aerobic and anaerobic exercise energy expenditure and excess postexercise oxygen consumption). When CVs were averaged within repetitions, ANOVA indicated that the CV for total energy expenditure (17.1%) was significantly lower compared with excess postexercise O2 consumption (31.6%) and aerobic exercise energy expenditure (31.3%; p < 0.01) but not compared with anaerobic exercise energy expenditure (30.4%). Collectively, the CV data raise the interesting possibility that the lowest variation resides for longer measurement periods and/or when all estimations of aerobic and anaerobic energy expenditure for a single bout of weight lifting and recovery are considered.
How should validity be tested under brief, non-steady-state conditions? Correlation analyses between work and absolute energy costs indicated somewhat moderate (aerobic exercise energy expenditure, r = 0.58) to very good relationships (anaerobic exercise energy expenditure, r = 0.95; anaerobic + aerobic exercise energy expenditure, r = 0.93; total energy expenditure, r = 0.97) (Table 3). The same was true when the data were adjusted for body weight. These data suggest an estimate of anaerobic exercise energy expenditure as having a better relationship with work than that of aerobic exercise energy expenditure. It is possible, however, that breath holding took place while lifting, confounding the aerobic energy expenditure-work relationship. In fact, total energy expenditure had the best relationship with work (Figure 1). Figure 2 displays the aerobic and anaerobic exercise energy expenditures for 7, 14, and 21 reps; linearity seems evident for both (although the anaerobic energy expenditure component increases to a greater extent as repetition number increases).
Correlation and regression of absolute data, however, do not reveal cause and effect and, thus, cannot ensure validity. To this end, we also investigated the change in work with changes in energy expenditure to determine whether these changes were proportionate. We found somewhat moderate (Δ aerobic exercise energy expenditure, r = 0.54) to good relationships (Δ anaerobic exercise energy expenditure, r = 0.88; Δ anaerobic + Δ aerobic exercise energy expenditure, r = 0.89; Δ total energy expenditure, r = 0.88) with work (Table 3). Change in blood lactate per kilogram of body weight and work are plotted in Figure 3 (r = 0.83, p = 0.014). But, this methodology also has flaws in that fatigue can create additional nonproportional energy expenditure increases with work-so-called “extra energy expenditure”-perhaps as muscle recruitment patterns change during the course of the exercise (with the growing presence of fatigue resulting in the recruitment of more muscle) (5). Additional or extra energy expenditure is typically depicted using O2 uptake and not anaerobic energy expenditure. Using similar terminology, we suggest the possibility of a “slow anaerobic component” to extra energy expenditure during weight lifting (as compared with a “slow O2 component”). It is tempting to speculate from the data in Table 3 and Figure 1 that a predictable relationship is apparent between weight lifting work and its corresponding aerobic and anaerobic energy expenditures during and after the bench press exercise when collected as capacity-type measurements (kJ). Although linearity was found between work and energy expenditure, it is of interest that our methodology (capacity measurements) does not rely on linear assumptions of O2 uptake with work. This may be of benefit in modeling the energy expenditure of more intense work (12).
Robergs et al. (9) have generated a linear extrapolation of weight lifting energy expenditure from easy to moderate workloads based on 5 bouts of 5-minute “steady-state” values (as liters of oxygen per minute). Using this methodology, an extrapolation should portray a linear progression of energy expenditure regardless of whether it was anaerobic, aerobic, or a combination of both. We used the same 1.5-second up and 1.5-second down cadence as they did. By converting their liter-per-minute O2 values into 20-second (7 reps), 40-second (14 reps), and 60-second (21 reps) values, a comparison with the present investigation could be made. Respectively, when our bench press data (using the Smith machine) for total energy expenditure were compared with the bench press data of Robergs et al. (9) (using free weights), we found the following: for 7 reps, 11.5 kJ ± 6.6 vs. 11.4 kJ ± 4.6 (p = 0.88); for 14 reps, 22.2 kJ ± 12.5 vs. 22.8 ± 9.1 (p = 0.44); and for 21 reps, 30.4 kJ ± 16.3 vs. 33.6 kJ ± 14.1 (p = 0.57). It needs to be kept in mind, however, that both studies used easy to moderate workloads.
Our rationale for portraying aerobic, anaerobic, and recovery energy expenditure was to partition out the contributions of each (12,13). As with a previous investigation of aerobic and anaerobic energy expenditures during weight lifting (11), the inclusion of anaerobic exercise energy expenditure also significantly increased energy expenditure compared with aerobic exercise energy expenditure for all lifts in the present investigation. Our data also include a brief excess postexercise oxygen consumption measure, where the anaerobic glycolytic component has been dismissed (1 L of O2 = 19.6 kJ) (10-13) (Figure 4). We assume that the “high-energy” adenosine triphosphate (ATP) and creatine phosphate (CP) stores are represented within a measure of excess postexercise O2 consumption, although it is unclear how much (12); perhaps, in the brief collection period, most of the recovery O2 uptake was oriented toward restoration of the ATP/CP stores. When compared with the steady-state data of Robergs et al. (9), a brief capacity measure of excess postexercise O2 consumption (containing ATP/PC energy expenditure) along with glycolytic energy expenditure and exercise O2 uptake may indeed portray a valid representation of the energy costs of easy to moderate weight lifting (Table 2).
Our study is not without limitations. Using endurance-related descriptions of lactate appearance and disappearance for aerobic exercise, many exercise physiologists a priori disavow blood lactate measurements as an indication of anaerobic glycolytic ATP turnover. Yet, for intense running, cycling, and swimming, when blood lactate peaks in recovery as opposed to within the exercise period, blood lactate levels have, in fact, adequately portrayed glycolytic energy expenditure (4,6). The conversion of 3 ml O2 per Δ blood lactate may not be similar for weight lifting compared with running, cycling, and swimming, because this value seems to be based on rate function workloads (power in watts) involving pulsatile blood flow through contracting, then relaxing, muscles. The concentric and eccentric phases of resistance training begin to occlude forearm blood flow at about 30% 1RM, with complete occlusion at 70% 1RM (2), certainly affecting both oxygen uptake from blood to muscle and lactate release from muscle to blood (our subjects lifted at 50% 1RM). Regardless of exercise type, however, passive recovery consists of unimpeded blood flow. We also estimated both anaerobic and aerobic energy expenditure as 1 L of O2 = 21.1 kJ, yet each format of metabolism may not have identical efficiency, so this conversion may not be similar for both (12). Our subject size (n = 8) was small, but our intent was triplicate trials for each of the 3 lifts (total data points = 24). Future investigations may also want to separate men from women subjects for comparison because relative exercise energy expenditure contributions were different (Table 4). This study was completed at 50% 1RM, and therefore it does not portray intense fatiguing exercise. Future studies need to investigate the aerobic and anaerobic energy expenditure of repetitions to exhaustion from 50 to 100% 1RM to determine whether proportionality is present between work and exercise O2 uptake, blood lactate, and excess postexercise O2 consumption measurements.
In conclusion, linear increases were found between aerobic exercise energy expenditure, anaerobic exercise energy expenditure, and total energy expenditure with work for a single set of the bench press at 50% 1RM. Anaerobic exercise energy expenditure seems to increase to a greater extent compared with O2 uptake as repetitions increase. Reliability in this small, heterogonous population was indicated for all energy expenditure estimates, but variability could be extensive. Our results agree with those of others, without the need for multiple steady-state measurements or for the assumption of proportional increases in energy expenditure with work. Using capacity estimates, we suggest that non-steady-state estimates of aerobic and anaerobic exercise energy expenditures, along with a modified estimate of excess postexercise O2 consumption, provide an acceptable portrayal of the energy costs of a single bout of weight lifting.
Traditionally, exercise physiologists have quantified energy costs by measuring the volume of oxygen consumed during exercise or activity. Because anaerobic energy expenditure cannot be quantified with a measure of oxygen uptake, it seems possible that oxygen-only measurements of intense weight training have portrayed a misleading estimate of the true energy expenditure of weight training. With this knowledge, the energy costs of the bench press are portrayed here as a composite of oxygen consumed during, lactate produced as a result of, and oxygen consumed in the recovery from, the bench press exercise. Our findings indicate that the amount of “calories burned” during weight lifting are in accordance with the highest estimates found in the current literature.
We thank Tawny Babbitt, Brian Petersen, and Jeremy Doerfler for help with data collection.
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