PHYSIOLOGIC RESPONSES DURING INDOOR CYCLING

PHYSIOLOGIC RESPONSES DURING INDOOR CYCLING

Rebecca A Battista, Carl Foster, Jessica Andrew, Glenn Wright, et al.. Journal of Strength and Conditioning Research. Champaign: Jul 2008. Tomo 22, Nº 4; pg. 1236, 6 pgs

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ABSTRACT

Battista, RA, Foster, C, Andrew, J, Wright, G, Lucia, A, and Porcari, JP. Physiologic responses during indoor cycling. J Strength Cond Res 22: 1236-1241, 2008-During the last decade, there has been active interest in indoor cycling (e.g., spinning) as a method of choreographed group exercise. Recent studies have suggested that exercise intensity during indoor cycling may be quite high and may transiently exceed VO^sub 2^max. This study sought to confirm these findings, as the apparent high intensity of indoor cycling has implications for both the efficacy and the risk of indoor cycling as an exercise method. Twenty healthy female students performed an incremental exercise test to define VO^sub 2^max and performed 2 videotaped indoor exercise classes lasting 45 minutes and 35 minutes. VO^sub 2^, heart rate (HR), and rating of perceived exertion (RPE) were measured during the indoor cycling classes, with VO^sub 2^ data integrated in 30-second intervals. The mean %VO^sub 2^max during the indoor cycling classes was modest (74 ± 14%VO^sub 2^max and 66 ± 14%VO^sub 2^max, respectively). However, 52% and 35% of the time during the 45- and 35-minute classes was spent at intensities greater than the ventilatory threshold (VT). The HR response indicated that 35% and 38% of the session time was above the HR associated with VT. In 10 of the 40 exercise sessions, there were segments in which the momentary VO^sub 2^ exceeded VO^sub 2^max observed during incremental testing, and the cumulative time with exercise intensity greater than VO^sub 2^max ranged from 0.5 to 14.0 minutes. It can be concluded that although the intensity of indoor cycling in healthy, physically active women is moderate, there are frequent observations of transient values of VO^sub 2^ exceeding VO^sub 2^max, and a substantial portion of the exercise bouts at intensities greater than VT. As such, the data suggest that indoor cycling must be considered a high-intensity exercise mode of exercise training, which has implications for both efficacy and risk.

KEY WORDS spinning, group exercise, exercise training

INTRODUCTION

Despite the popularity of indoor cycling (e.g., spinning) within the fitness community, limited research has been performed to document the physiological responses during indoor cycling classes (8,19,22). Recently, Caria et al. (8) demonstrated that the intensity during indoor cycling classes ranged from moderate to very heavy, with approximately 25% of the class duration being performed at intensities in excess of the ventilatory threshold (VT). This value is comparable to, if not greater than, the percentage of high-intensity exercise routinely undertaken by competitive endurance athletes during training (4,12-14,33-35) or even during competition in the most demanding endurance events in the world, the 3-week Grand Tours in cycling (25). Remarkably, Caria et al. (8) observed that in 9 of 12 subjects, all of whom were indoor cycling instructors, the highest VO^sub 2^ during a simulated class was transiently greater than the VO^sub 2^max achieved during incremental exercise. This observation is of interest for at least 3 reasons. First, in already well-trained individuals, higher-intensity training is required to improve VO^sub 2^max and performance (10,20,23,37), although the spontaneous training pattern of athletes is characterized by a majority of time at relatively low-intensity training and with only a limited amount of moderately high-intensity training (4,12-14,3335). Indeed, distance runners may have greater improvement with up to 80% of their training hours at low intensity (12,13), although a minimum of approximately 10% of training at intensities above the respiratory compensation threshold (RCT) appears to be required for athletic success. Thus, if the tendency of indoor cycling to require high intensity exercise is confirmed, it can serve as a beneficial method for off-season conditioning in athletes. Second, in middle-aged and older individuals, for whom indoor cycling is a very popular form of group exercise, the tendency toward high exercise intensities may represent a risk factor for catastrophic outcomes during exercise. Exertional myocardial infarction is usually triggered by unaccustomed heavy exercise in previously sedentary individuals (38). In patients with exercise-induced ischemia, the ischemic abnormalities usually do not occur until intensity exceeds that associated with VT (7,26). Third, given that one of the ongoing debates in the literature over the last decade has been concerning the physiological meaning of VO^sub 2^max (1,2,11,18,29-31,36), the observation of VO^sub 2^ values during nonexhaustive work that are greater than Vo2max during incremental exercise are of interest relative to the understanding of this fundamental parameter in exercise physiology. Accordingly, the intent of this study was to make observations ofthe intensity of indoor cycling, with the intent of documenting the exercise intensity during indoor cycling.

METHODS

Experimental Approach to the Problem

The focus of the current study was a controlled observation of a simulated indoor cycling class. These observations were designed to test the hypothesis that indoor cycling would produce transiently very high exercise intensities (i.e., >Vo^sub 2^max).

Subjects

The subjects were 20 healthy, physically active, female university students and staff (age, 24.4 ± 5.8 years; height, 168 ± 6 cm; body mass, 64.7 ± 6.8 kg) enrolled in an indoor cycling class. All were experienced with indoor cycling, and all had been participating in indoor cycling at least twice weekly for the preceding 2 months at the time of study. Each subject provided written informed consent, and the protocol was approved by the university's human subjects committee. The subjects were screened by questionnaire prior to class participation (38).

Procedures

Exercise Test Protocol. Each subject performed 3 exercise tests. The first test was an incremental exercise test to fatigue on an electrically braked cycle ergometer (Lode Excalibur, Groningen, The Netherlands) designed to document maximal power output, Vo^sub 2^max, VT, RCT, and maximal heart rate (HRmax). Subjects were free to pedal within the range of 60 to 90 rpm. The initial power output was 25 W and was increased by 25 W every 2 minutes. Gas exchange data were measured by using open circuit spirometry (Applied Electrochemistry, Inc., Pittsburgh, PA) and were integrated over 30-second intervals to measure oxygen uptake (VO^sub 2^), pulmonary ventilation (V^sub E^), and ventilatory equivalents for oxygen (V^sub E^*Vo^sub 2^^sup -1^) and carbon dioxide (V^sub E^*VcO^sup 2^^sup -1^).

VO^sub 2^max was accepted as the highest VO^sub 2^ observed during a continuous 30-second sampling period, with inclusion criteria consistent with conventional guidelines for Vo^sub 2^max (21) (e.g., an inability to maintain pedaling rate despite strong verbal encouragement, relative HR >95% age predicted, or respiratory exchange ratio at peak exercise >1.1). Because of the infrequency of an actual plateau (i.e., <50% ofthe rate of increase in VO^sub 2^ compared to the body ofthe test) of VO^sub 2^ in even high-level athletes (24), the development of a decreased slope ofthe VO^sub 2^-time relationship was not included as an inclusion variable. Nevertheless, 7 of the 20 subjects demonstrated decreases in the slope ofthe VO^sub 2^-versus-power output relationship consistent with a plateau of VO^sub 2^, which is reasonable considering that fewer than 50% of elite athletes can demonstrate a plateau of Vo2 during incremental exercise (24).

The VT was determined by using the criterion of an increase in V^sub E^-VO^sub 2^^sup -1^ with no increase in V^sub E^-VCO^sub 2^^sup -1^, and the RCT was determined by using the criterion of an increase in both V^sub E^-VO^sub 2^^sup -1^ and V^sub E^-VO^sub 2^^sup -1^. The VO^sub 2^ at the VT and RCT was confirmed based on the v-slope method (17). Heart rate was measured by using radiotelemetry (Polar Electro Oy, Kempele, Finland) integrated over 5 seconds.

Indoor Cycling Classes. Each subject subsequently performed 2 simulated indoor cycling classes in the laboratory. One class, performed by each subject, was 45 minutes in duration and was choreographed based on conventional principles (i.e., warm-up, systematic interval exercise, and cool-down) widely employed in the indoor cycling community. The second exercise class performed by each subject was 35 minutes in duration but had 4 different variations in choreography, all designed according to general principles used in the indoor cycling community. Each exercise class was performed while watching a videotape. Thus, each subject performed 2 classes resulting in 40 exercise training bouts in the data set. There was no exercise prescription (e.g., target HR) other than to follow the lead provided by the instructor on the videotape. Heart rate data were interfaced directly to the gas analysis system, and the subject did not have direct access to his or her momentary HR via a HR wristwatch.

The simulated classes were performed on a Schwinn indoor cycle (Schwinn, Chicago, IL), which is widely available commercially and is widely used in the indoor cycling community. It was not instrumented to allow measurement of power output. During the simulated classes, respiratory metabolism was measured by using open-circuit spirometry, as in the incremental test, with data integrated over 30 seconds. Heart rate was measured by using radiotelemetry integrated over 5 seconds. The RPE was measured at 10 and 20 minutes and at the end of exercise by using the Category Ratio RPE scale (i.e., 0-10) (6). Because wearing the respiratory valve for prolonged periods often causes subjects to become uncomfortable, they were allowed to remove the breathing valve briefly (i.e., for approximately 20 seconds) at predefined times (i.e., during lower-intensity recovery segments of the choreography) during the exercise bout During this break, they consumed water ad libitum. For the purpose of calculating exercise intensity, the VO^sub 2^ during the periods when the breathing valve was removed was calculated based on the HR response and the HR-VO^sub 2^ relationship for each subject.

Statistical Analyses

The primary statistical comparison was descriptive and categorical, as the interest was to determine the average and highest VO^sub 2^ during the simulated classes and the percentage of time during which either VO^sub 2^ or HR was less than VT, between VT and RCT, or greater than RCT. A comparison of the mean values for the Vo2 observed during the simulated exercise class and during maximal incremental exercise was performed with a repeated-measures analysis of variance. A p ≤ 0.05 was accepted as statistically significant.

RESULTS

Maximal exercise responses during the incremental exercise tests are presented in Table 1. All subjects satisfied criteria for achieving VO^sub 2^max, without depending on demonstrating a plateau in the VO^sub 2^-power output relationship (i.e., fatigue, HR >95% age predicted, or RER >1.1) (21). However, as noted above, 7 ofthe 20 subjects demonstrated a plateau in VO^sub 2^ during the incremental exercise test.

The serial pattern of VO^sub 2^ during the 2 exercise classes is presented in Figure 1, with the data normalized to VO^sub 2^max. While the average intensity was quite moderate (74 ± 14% VO^sub 2^max in class 1 and 66 ± 15% Vo2max in class 2), there was considerable variation in the momentary exercise intensity, which is typical of the general choreographic plan common to indoor cycling, and the intensity during the majority of both classes was in the range of 75% to 80% VO^sub 2^max. The percentage of the exercise bout time with the VO^sub 2^ less than VT, between VT and RCT, between RCT and VO^sub 2^max, and greater than VO^sub 2^max was 48 ± 29%, 36 ± 22%, 11 ± 19%, and 5 ± 3% in class 1 and 65 ± 25%, 24 ± 19%, 7 ± 13%, and 4 ± 3% in class 2, respectively. The pattern of HR responses during the indoor cycling classes paralleled the changes in VO^sub 2^ (Figure 2). When the data were analyzed relative to the proportional intensity relative to HR, the exercise bout time with the HR less tiian VT, between VT and RCT, greater than RCT, and greater than HRmax was 65 ± 25%, 24 ± 19%, 9 ± 13%, and 2 ± 2% in class 1 and 62 ± 31%, 25 ± 21%, 7 ± 13%, and 2 ± 2% in class 2 (Figure 3). Even during the first class, in which all of the subjects were responding to precisely the same instructor cues, the range of exercise intensities performed was widely variable. In 10 of the total of 40 exercise classes studied, there were periods when the momentary VO^sub 2^, integrated over 30 seconds, exceeded the VO^sub 2^max. The highest VO^sub 2^ (i.e., 30-second mean for all 20 subjects combining both classes) during the indoor cycling sessions versus VO^sub 2^max was significantly less (2382 ± 384 mL*min^sup -1^ versus 2570 ± 341 mL-min"1) than VO^sub 2^max during the incremental exercise test (Figure 4). During the combined total indoor cycling classes, the average time that the VO^sub 2^ was above VO^sub 2^max was 1.3 ± 3.2 minutes. However, in the 10 individual sessions in which VO^sub 2^ actually exceeded VO^sub 2^max, the average time that the VO^sub 2^ was above VO^sub 2^max was 5.4 ± 4.5 minutes (range, 0.5-14.0 minutes).

The exercise classes were perceived to be quite strenuous, with RPE greater than 5 (i.e., hard) at all measured time points (Table 2). Despite the high intensity of exercise, spontaneous comments from the subjects suggested that even with the video recording to guide the exercise pattern, the effort was less than typically experienced in a live class with the instructor and other class members present. Thus, the observed responses plausibly represent a conservative estimate of the exercise intensity during typical indoor cycling classes.

DISCUSSION

The main finding from this study was that although the average intensity of indoor cycling classes was comparatively moderate (i.e., 65-75% VO^sub 2^max), there was an appreciable percentage of the exercise bout when the intensity was greater tiian VT, based on both VO^sub 2^ (approximately 35%) and HR (35-50%) criteria, and the highest VO^sub 2^ observed during the indoor cycling classes was frequently (10 of 40 exercise bouts) greater than VO^sub 2^max observed during incremental cycle exercise, which itself satisfied accepted criteria for achieving VO^sub 2^max (21). Five of the 10 exercise sessions, in which greater than VO^sub 2^max values were observed, occurred in subjects who demonstrated a plateau during the incremental test. The responses of the subjects were quite variable, particularly with the constant cueing provided by the videotape. Since there was no exercise prescription (e.g., target HR) per se, the variability of response potentially represents spontaneous down-regulation of exercise intensity despite cueing that might suggest increasing power output. This would be consistent with the concept that exercise intensity may be intrinsically regulated in a way designed to prevent overexertion injuries (29).

Although the subjects in this study were not adiletes systematically training every day, the percentage of the exercise bout with exercise intensities greater than VT was much higher than routinely observed in athletes during spontaneous training (4,12-14,33-35), aldiough it is within the range of individual high intensity training bouts in athletes (35). On this basis, if indoor cycling were used as an everyday training activity, it is possible that the overall intensity would be too high and possibly contribute to developing nonfunctional overreaching (27,28). However, as an episodic training activity, the comparatively high intensity may be associated with an effective training response. In particular, recent results by Helgerud et al. (20), Laursen et al. (23), and Septo et al. (37) have suggested that in order for already trained adiletes to improve VO^sub 2^max, training intensities approximating the intensity of VO^sub 2^max are required.

The relatively high percentage of VO^sub 2^ values greater than VO^sub 2^max was a remarkable finding. It is in agreement with the earlier findings of Caria et al. (8) and others (19,22), who found a high overall intensity during indoor cycling with 9 of 12 subjects (i.e., indoor cycling instructors) achieving intensities greater than VO^sub 2^max during simulated indoor cycling classes. However, the current results demonstrate a notably longer duration greater than VO^sub 2^max than reported by Caria et al. (8). This finding could, of course, be explained by an underestimation of VO^sub 2^max during incremental testing. However, in view ofthe stringent criteria for accepting peak exercise results as VO^sub 2^max and the substantial number of subjects who demonstrated a plateau of VO^sub 2^ during the incremental test, the authors feel the frequency of greater than VO^sub 2^max values observed during the classes cannot be attributed to a systematic underestimation of VO^sub 2^max during incremental testing. These data are interpreted as supporting the concept, at least during cycling exercise, that the Vo2max achieved during incremental exercise to fatigue is not a uniquely high value for VO^sub 2^. In that regard, these data may be viewed as supportive of the arguments put forth by Noakes (29,30), challenging the traditional concept of VO^sub 2^max. In that regard, they are contradictory to a series of findings using a double exercise protocol (11,31,36), including a recent report from the authors, laboratory (18). Obviously, this issue deserves and will receive further study.

The current data are consistent with the authors, previous observations of higher than incremental VO^sub 2^max values during cycle time trials in both athletes (15) and well-trained nonathletes (16). However, since running time trials have not been shown to produce higher than incremental VO^sub 2^max responses (9), it may be argued that local muscle fatigue during cycling limits exercise prior to achieving a limitation of central oxygen transport capacity. A wide variety of studies have shown that VO^sub 2^max is systematically lower during cycle ergometry than during running. Thus, cycling may be a somewhat limited model for testing the conceptual underpinnings of VO^sub 2^max, in that for the majority of individuals, cycling exercise is more likely to be limited by local muscular factors, potentially the amount of muscle engaged in exercise, than by the ability of the central circulation to offer oxygen to the exercising musculature.

Regardless of this limitation, the current results reinforce other studies that have demonstrated that VO^sub 2^ values greater than the VO^sub 2^max achieved during incremental exercise can be observed during both submaximal (8) and maximal (3,5,32) exercise. The difference between the current results and those of Rozenek et al. (32) and Billat et al. (3,5) is that the training sessions designed by these investigators were intended to induce VO^sub 2^max. The indoor cycling exercise bouts studied in this study and the study by Caria et al. (8) were anticipated to be submaximal. Thus, the frequent observations of VO^sub 2^ values greater than VO^sub 2^max was unanticipated. This may have unintended consequences in that the risk of serious health consequences (e.g., myocardial infarction) during exercise training is linked to unaccustomed heavy exercise (38), particularly in beginning exercisers. This may be a meaningful concern given that indoor cycling classes are often targeted toward middle-aged fitness participants, a population in whom there may be a significant incidence of subclinical cardiovascular disease and in whom the adequacy of preliminary medical screening may be suboptimal (38). Accordingly, in cases in which less athletic individuals are performing indoor cycling classes, it may be especially prudent to consider both the choreography of the exercise session and the adequacy of pre-exercise screening.

PRACTICAL APPLICATIONS

From the standpoint of using indoor cycling classes to contribute to the off-season conditioning of athletes, it is reasonably well-established that higher-intensity training is necessary to provoke adaptations to the cardiorespiratory system. Thus, in addition to the low impact nature of this mode of exercise, it may be that this would be an effective method of nonspecific conditioning that would be very effective on a result-per-time basis. The current data were collected from university-aged female nonathletes. As such, the generalizability ofthe response is limited. Future studies in competitive athletes would be productive. On the other hand, there is good evidence that unaccustomed high-intensity exercise may contribute to the triggering of acute myocardial infarction in individuals with underlying cardiovascular disease. Given that indoor cycling is widely used in the fitness industry and targeted at middle-aged individuals, those wishing to lead indoor cycling classes should make sure that they have conducted appropriate pre-exercise screening.

ACKNOWLEDGMENTS

This study was funded by a grant from the Office of University Graduate Studies at the University of Wisconsin-La Crosse. There was no extramural funding. None of the authors has conflicts of interest or relationships that require disclosure. The results do not constitute endorsement of any product by the authors or by the National Strength and Conditioning Association.