The gas exchange threshold (GET) and respiratory compensation point (RCP) have been used to demarcate the exercise intensity domains (moderate, heavy, or severe) [1
]. It has been suggested that the GET demarcates the moderate from heavy domains [1
], while the RCP demarcates the heavy from severe domains [2
]. During constant power output (PO), cycle ergometry within the moderate domain, oxygen consumption (
), heart rate (HR), and blood lactate concentration ((La−
) reach steady state within 3 min [1
]. In the heavy domain, however, (La−
appearance exceeds its rate of removal for the first 10–20 min then reaches a steady state [1
]. Thus, each domain is characterized by unique patterns of physiological responses.
A number of studies have examined the physiological and perceptual responses during continuous, constant PO cycle ergometry [1
] within both moderate and heavy exercise intensity domains. During fatiguing constant PO exercise there are predictable time-dependent patterns of responses for
, HR, rating of perceived exertion (RPE), breathing frequency (
), and minute ventilation (
) that are, in part, dependent upon the exercise intensity associated with the task. There are, however, different patterns of responses for various physiological variables when continuous exercise is maintained at a constant physiological or perceptual parameter instead of PO [6
]. For example, Stoudemire et al.
] reported increases in (La−
and HR, but decreases in
and velocity during treadmill running at RPE levels within the moderate and heavy domains. In addition, Lajoie et al.
] reported increases in
, HR, and RPE during 60 min of cycling exercise while maintaining maximal lactate steady-state, which is related to the anaerobic threshold. More recently, Cochrane et al.
] reported dissociations among RPE,
, and respiratory exchange ratio (RER) during cycle ergometry at a constant RPE, 20% below the GET (within the moderate domain). To explain these patterns of fatigue-related responses, Cochrane et al.
] made some hypotheses regarding the applicability of fatigue models [11
]. It was hypothesized [6
] that small nerve afferents associated with respiratory muscles responsible for mediating
were the likely mediators of the perception of effort during constant RPE cycle ergometry within the moderate domain. No previous study, however, has investigated whether the dissociations among physiological and perceptual variables are dependent upon the exercise intensity of the task, as seen during constant PO exercise. Intensity dependent dissociations may further explain mediators of perception during both moderate and heavy intensity exercise.
Although specific metabolic, cardiovascular, and respiratory responses have been documented during constant PO exercise, less is known regarding pacing strategies associated with these responses and how responses may differ during exercise at a constant perception of effort. It has been theorized [16
] that there is some level of brain involvement in the perception and manifestation of fatigue, with some theories suggesting the brain is responsible for conscious and unconscious integration of feedback from the working muscles and external stimuli (environment and time) [15
], those that suggest the brain performs regulation without peripheral feedback [13
], and others advocating a greater influence of unconscious “reflexes” on responses during exercise [11
]. However, there is still debate regarding which, if any, of these theories best explains pacing and the onset of fatigue during dynamic exercise. There may be a dual contribution from automatic “reflex” actions as well as large metabolic disturbances to the conscious integration of the perception of effort that helps form an individual’s pacing strategy. The mechanisms underlying this conscious awareness of physiological responses are not fully understood. Thus, by controlling the sense of effort (RPE), dissociations among physiological and perceptual responses may point to potential mediators of both conscious and unconscious interpretations of effort and the awareness that determines pace.
It is hypothesized that the variable(s) that track RPE are associated with similar underlying mediators and/or mechanisms, and those that are experimentally uncoupled from perception (i.e.
, they do not follow the same pattern of responses as RPE), may not contribute to the perception of effort during moderate and/or heavy intensity cycle ergometry exercise. Based on the findings of previous studies [6
], it is also hypothesized that variables related to respiration may be the most potent mediators of perception during prolonged cycling exercise. If specific variables can be identified which may mediate perception, a framework can be provided to athletes and coaches to improve pacing strategies, training programs, and athlete performance during prolonged, aerobic exercise. No previous study, has investigated the patterns of responses for these metabolic, cardiovascular, and respiratory variables during prolonged (>30 min) cycle ergometry maintained at a constant perceptual intensity within both the moderate and heavy domains. Like constant PO cycle ergometry, there may be an effect of intensity on the patterns of responses for identified variables, and therefore, there may be mediators of perception that are dependent upon the intensity at which exercise is performed. In addition, it is unclear how controlling perception affects pacing and the metabolic cost of exercise (
) across an extended period of time (>30 min). Therefore, the purposes of this study were to (1) examine the metabolic (
and RER), cardiovascular (HR), respiratory (
), and work intensity (PO) responses during continuous, constant RPE cycle ergometry within the moderate and heavy domains; and (2) to determine the level of agreement and sustainability of two perceptually grounded exercise intensities (RPEGET
) to corresponding
values (GET and 15% above GET) from an incremental and continuous cycling test.
and PO values that corresponded to RPEGET
(RPE = 14 ± 0.5) and RPEGET+15%
(RPE = 16 ± 0.5) intensities were 30.3 ± 1.5 (167 ± 10 W) and 34.9 ± 1.7 mL·kg−1
(196 ± 12 W), respectively. The results of the polynomial regression analyses (Figure 2
a–f) indicated that during the 60 min rides at RPEGET
, there were significant, negative, linear relationships for mean, normalized
= 0.90) and (r2
= 0.96) vs.
time; negative, quadratic relationships for mean, normalized HR (R2
= 0.90), RER (R2
= 0.96), and PO (R2
= 0.98) vs.
time; and no significant change for mean, normalized
= 0.04) vs.
time relationship. In addition, the results of the polynomial regression analyses during the 60 min rides at RPEGET+15%
, indicated that there was a significant, negative, linear relationship for mean, normalized HR (r2
= 0.72) vs.
time; negative, quadratic relationships for mean, normalized
= 0.82), RER (R2
= 0.98), and PO (R2
= 0.98) vs.
time, and no significant change for the mean, normalized
= 0.001) vs.
The results of the RM ANOVA indicated that there was a significant main effect for O2 during rides at RPEGET (p<0.05 = 0.40). Follow-up pairwise comparisons revealed O2 at 7–19 min was significantly (p<0.01) greater than O2 at min 35–47 and at min 49–50. In addition, O2 at 21–33 min was significantly (p<0.01) greater than that at min 35–47 and min 49–60. However, O2 corresponding to GET from the incremental test to exhaustion was not significantly different from measured O2 across the 60 min ride at RPEGET. There was also a significant main effect for O2 during rides at RPEGET+15% (p < 0.05; = 0.67). Follow-up pairwise comparisons revealed that O2 at min 7–19 was significantly greater than O2 at all other time points (21–33 min, 35–47 min, and 49–60 min) during the 60 min ride at RPEGET+15%. Furthermore, there was no difference between O2 at 35–47 min and 49–60 min, but O2 during the last 25 min of the 60 min ride was significantly different (p < 0.01) from that collected during 7–19 min and 21–33 min. However, O2 corresponding to 15% above GET from the incremental test to exhaustion was not significantly different from O2 at 7–19 min, but differed from all other time points (21–33 min, 35–47 min, and 49–60 min).
The results of the paired samples t
-tests indicated that the
associated with the perceptual intensities, as calculated from the incremental test to exhaustion (GET and GET + 15%), were significantly different (p
< 0.01; d
= 0.55). In addition, comparison of the
associated with the initial stage of each constant RPE ride indicated that subjects maintained a higher
< 0.01; d
= 0.67) during the first 20 min of rides at RPEGET+15%
than those at RPEGET
. There was no significant difference between RPEGET
at min 21–33 (p
< 0.01; d
= 0.10), 35–47 min (p
< 0.01; d
= 0.03), or min 49–60 (p
< 0.01; d
= 0.14) (Figure 3
In the present study, RPEGET
were 70.1% ± 1.6% and 80.8% ± 1.9% of
, respectively (Table 1
). The GET typically occurs at between 70% and 80% of
in endurance-trained individuals [24
], but at between 50% and 60% of
for those who are untrained [25
]. The RCP, however, typically occurs at about 50% of the difference between
and GET (i.e.
, 85%–90% of
) in endurance-trained individuals [1
]. Thus, in the present study, the RPEGET
corresponded to relative intensities similar to those of previous studies [1
] and represented the demarcation of the moderate and heavy exercise intensity domain (RPEGET
), and an intensity within the heavy domain (RPEGET+15%
During exercise at a constant perception of effort, there are dissociations among the patterns of responses for physiological and perceptual variables typical of constant PO exercise [6
]. For example, Stoudemire et al.
] reported that during treadmill running at a constant RPE corresponding with 2.5 mmol (La−
and 4.0 mmol (La−
, there were dissociations among RPE, (La−
, and velocity. To maintain RPE, velocity was reduced which resulted in a decreased metabolic cost and
across time [8
]. More recently, Cochrane et al.
] reported dissociations among RPE,
, HR, E
, RER, and PO during continuous cycle ergometry at a constant RPE in the moderate domain. During the constant RPE rides,
, HR, E
, and RER tracked the time-dependent decreases in PO [6
]. In the present study, there were similar dissociations among RPE vs.
, HR, E
, RER, and PO during continuous cycle ergometry at RPEGET
. For example,
, HR, RER, E
, and PO decreased across time during both 60 min rides. Thus, the dissociations among RPE vs.
, HR, E
, RER, and PO in the present study were similar to those of previous studies [6
] and further supported the hypothesis that metabolic, cardiovascular, and intensity related variables do not solely mediate the perception of effort during cycle ergometry at a constant RPE within the moderate or heavy exercise intensity domains.
The pattern of responses for both the mean, normalized HR and E
differed between the rides at RPEGET
. At RPEGET
, the mean, normalized HR and E
responses exhibited quadratic and linear decreases, respectively, while at RPEGET+15%
the mean, normalized HR and E
exhibited linear and quadratic decreases across the 60 min rides. Although the patterns for HR decline over time were different between the two rides, the mean, normalized HRs across the 60 min rides were similar (85.7% vs.
). In addition, HR did not change proportionately with reductions in PO during the constant RPE rides. For example, at RPEGET+15%
, greater reductions in PO were required at the initiation and throughout the 60 min rides, which likely explained the lack of a plateau and linear decline for HR during the more intense rides (Figure 2
d). The participants, therefore, spent more time at their initial %HRpeak
during the moderate intensity rides than during rides in the heavy domain. It has been reported [26
] that during sustained, moderate intensity exercise (50%–75%
), HR “drifts” upward as the intensity of exercise is maintained. This drift in HR has been attributed to increases in cutaneous blood flow due to increases in core temperature [26
]. When riding at a constant RPE within the moderate exercise intensity domain, HR initially drifted up across time, but to a lesser degree than would be expected during constant PO cycle ergometry, due to reductions in PO, which resulted in an overall quadratic decrease across time. In the heavy domain (RPEGET+15%
), however, HR exhibited a linear decrease across time, which was attributed to a more rapid initial reduction in PO. The patterns of response for HR in the present study differed from those of Stoudemire et al.
] who reported a continuous rise in HR throughout two, 30 min runs at a constant RPE. Although the mechanisms responsible for the rise in HR over time during the constant RPE runs were not discussed [8
], the dissociations for RPE and HR were consistent with the findings of the present study. That is, the uncoupling of HR and RPE indicates that HR did not mediate the perception of effort during constant RPE exercise within the moderate and heavy exercise intensity domains [8
In the present study, the mean, normalized E
exhibited a linear decrease across time during the constant RPE rides within the moderate intensity domain. During the rides within the heavy domain (RPEGET+15%
), however, the mean, normalized E
exhibited a quadratic decrease across time. Previous studies [4
] have reported different time-dependent patterns of responses for E
that are, in part, dependent upon the intensity of exercise as well as the parameter being held constant. For example, Dempsey [4
] reported that during moderate intensity exercise at a constant PO, E
typically increases as a function of tidal volume, whereas increases at higher intensities are mediated by increases in
]. When cycling at a constant E
within the moderate domain (50%–60%
), however, tidal volume has been shown to decrease, while
, core temperature, and RPE responses increased across time [28
]. In the current study, both 60 min rides at RPEGET
resulted in decreases for metabolic (
and RER) and work intensity (PO) related variables, which indicated a decrease in the metabolic cost of the exercise. Furthermore, RPE and
remained constant across time during the rides in the moderate and heavy exercise intensity domains. Thus, changes for the patterns of responses for E
were not consistent with those of
or the perception of effort, but they tracked decreases in metabolic cost during the 60 min rides at a constant RPE.
During both moderate and heavy intensity rides, only
tracked RPE across time. The mean, normalized
response did not change across time during the rides at RPEGET
. These findings were consistent with those of Cochrane et al.
] who reported a close agreement between RPE and
, but dissociations for both RPE and vs.
and RER), cardiovascular (HR), and ventilatory (E
) responses during constant RPE cycle ergometry within the moderate intensity domain. It was hypothesized [6
] that reflex actions associated with the exercise pressor reflex model of fatigue [12
] best explained the similarity for RPE and
during the constant RPE rides. According to the exercise pressor reflex model [12
], afferent signals from small group III (mechanical changes) and IV afferents (metabolic changes) from working thigh and respiratory muscles mediate the ventilatory, cardiovascular, and RPE responses during moderate and heavy intensity exercise [2
]. It has also been suggested [11
] that in addition to mediating cardioventilatory responses, group III and IV afferents may have a “critical role” in the regulation of the perception of effort during dynamic exercise. For example, a lower perception of effort was reported during exercise with an intact somatosensory feedback system, which incorporated both group III and IV afferents, than during exercise with blocked spinal receptors [11
]. In the present study, it is unlikely that there was a continuous build-up of metabolic byproducts during either of the constant RPE rides, because of the decreases in metabolic cost
and RER) and work intensity (PO). For the rides beginning in the heavy exercise intensity domain, there may have been an increase in (La−
typical of heavy intensity exercise during the first 20 min of the rides, however, all participants ended the 60-min RPEGET+15%
rides below the GET (65.7% ± 2.2%
) in the moderate domain. These findings did not support an increase in metabolic byproducts ((La−
) or inadequate supplies of ATP [1
]. In addition, there was likely a reduction in required central command due to the decreases in PO, which was tracked by reductions in HR and E
across the 60 min rides. Previous studies [11
] have suggested that group IV afferents may be linked to changes in central command and the cardiovascular system. It has been reported [11
] that an increased rate of discharge from group IV afferents tracked increases in E
, HR, and central command during continuous, constant PO cycle ergometry. In the current study, there were reductions in HR, E
, and, potentially, a reduction in central command during the 60 min constant RPE rides, and therefore, group IV afferents may not have the same patterns of discharge during exercise at a constant RPE as those reported during constant PO exercise [11
The results of the present study also have implications for the regulation of exercise intensity during prolonged cycling exercise. For example, there was no significant difference between the
throughout the 60 min constant RPE rides at RPEGET
). There were differences, however, between the
at 15% above the GET, as determined from the incremental test, and the
during the last 40 min of the 60 min constant RPE ride at RPEGET+15%
). These findings indicated that the use of a perceptual intensity (RPEGET
) may be used for training at the gas exchange threshold, in place of more expensive monitoring methods such as (La−
, heart rate, and oxygen consumption, during cycle ergometry exercise. However, cycling at a higher perceptual intensity (GET + 15%), such as that associated with the heavy exercise intensity domain, did not result in the same starting intensity (
) after 20 min, and therefore, prescribing exercise associated with a higher RPE may only be useful for the first 20 min of cycling exercise. Future studies should compare the effects of multiple training visits at a percentage of
(GET and GET + 15%) and a perceptual intensity (RPE) on markers of aerobic performance.
The patterns of responses for physiological variables during exercise at a constant perceptual rating (RPE) within the moderate and heavy exercise intensity domains differ from those typically observed during constant power output exercise [1
]. Given that it is not uncommon for cyclists to perform rides at varying workloads across time, these findings may provide applications for the prescription and tracking of performance. The finding that the
corresponding with the GET, a commonly used threshold for tempo training exercises, was maintained by controlling perception during a 60 min cycling test also speaks to the ability of cyclists to maintain work at the demarcation point of the moderate and heavy exercise intensity domains. There is, however, a need to extend the findings of the present study on the effect of training at a constant RPE (RPEGET
a more traditional prescription (%HRpeak
) on outcome variables associated with aerobic performance (onset of blood lactate, occurrence of GET, substrate utilization (RER), and
). Our study demonstrates that there may be a relationship between how one perceives exercise and the underlying mechanisms associated with breathing patterns and that the ability to regulate, or pace, cycling exercise at a constant RPE may be tied to the exercise intensity corresponding to the work bout.
In summary, the patterns of response for
, which did not change across time but tracked RPE, were similar during both moderate and heavy intensity cycle ergometry exercise. In addition, the
associated with an RPE grounded threshold corresponding to the GET appeared to be sustainable for up to 60 min of cycling exercise, while that above the GET could only be maintained for the first 20 min of exercise. The findings of the current study, as well as those of previous studies [6
] supported the hypothesis that small group III afferents associated with mechanical distortions within the working thigh and respiratory muscles may influence both perceptual and respiratory responses associated with the frequency of breathing that were independent of changes in metabolic cost, cardiovascular, and ventilation responses. In addition, exercising at the RPE corresponding to the GET may be useful as a pacing or training tool.