Intrinsic and extrinsic biological factors

Human biology plays a fundamental role in CRF response to training that extends beyond heredity and genes. Intrinsic factors such as sex, age, as well as baseline V̇O_2max and HRV each modulate physiological responses from homeostatic perturbations, including changes in V̇O₂. The recent review by Meyer et al4 comprehensively addresses biological factors modulating the variability of V̇O₂ responses. While sex and age have been suggested to account for less than a combined 10%–16% of the V̇O₂ response,23 24 baseline HRV, more precisely high frequency power (high vagal activity), has so far been one of the strongest factors, predicting 27% of the V̇O₂ response (n=39), following 8 weeks of aerobic training24 and 34% (n=16) following 12 weeks of interval training.25 Interestingly, baseline V̇O₂, as a predictor, has demonstrated mitigating results as Bouchard and Rankinen23 showed that it only explained 1% of the V̇O₂ response in their cohort and Hautala et al26 approximately 2%. Subsequent analyses revealed that this phenomenon may be more complex than originally described, and possibly predict more of the V̇O₂ response to training, with negative relationships observed, where individuals with lower fitness and initial values would attain greater V̇O₂ gains.27 28 This would logically explain diminishing returns from training and a V̇O₂ ceiling reached in elite athletes where gaining additional CRF can become nearly impossible and is, in itself, a polarising field of V̇O₂ work. Moreover, training studies conducted over weeks, if not months, require the consideration of external factors that further influence biological responses. While many lifestyle, behavioural and environmental factors may come into play, the impact of physical recovery, nutritional status and stress can largely influence phenotyping effects of training.3 The degree of homeostatic perturbation from a predetermined exercise load regulates the timeline associated with musculoskeletal, neural and metabolic recovery necessary to initiate the next training load. Low intensity and volume exercise can require less than 24 hours, whereas high-intensity and volume exercise may need more than 48 hours to adequately recover. Cardiac autonomic recovery, assessed via HRV, offers an excellent assessment of physical recovery despite, its assumed flaws that it does not reflect all physiological systems impacted by exercise.29 Finally, endocrine, immunological and metabolic dysregulation occur from exercise and cannot be easily assessed during recovery but are known to play a critical role in exercise readiness and response to physiological change.

Methodological factors: training volume

The current ACSM guidelines prescribe a fixed volume of aerobic training consisting of at least ≥150 min per week of moderate intensity or ≥75 min per week of vigorous intensity or a combination of the two (figure 1). Furthermore, for those not meeting the guidelines, it is recommended to gradually work towards meeting the goals commencing with short 5 min activity periods.30 However, individual differences in training status and response exist and therefore appropriate training dose should be considered individually. Sisson et al28 observed a reduced number of V̇O₂ non-responders with increasing weekly training volume, via higher energy expenditure relative to body weight, while maintaining training intensity. Montero and Lundby13 studied healthy young males who did not participate in organised sports and observed a decreasing occurrence of non-responders with increasing weekly training volume from 60 (n=16 with 11 non-responders) to 120 (n=15 with six non-responders) and 180 (n=14 with four non-responders) min per week and no non-responders with 240 or 300 min of weekly endurance training. Furthermore, they observed that adding two extra sessions of 60 min per week eliminated non-responders. Importantly, 29% of those exercising 180 min per week, at both moderate and high intensity, did not improve their CRF. Therefore, the current volume guideline of at least 150 min of moderate aerobic exercise might not be sufficient to increase CRF for a significant number of healthy adults. Montero and Lundby13 further suggested a critical duration of >180 ≤240 min per week to increase positive training responses. However, it is crucial to note that a significant proportion of the participants showed a positive training response at training volumes of 60 or 120 min per week highlighting the fact that sufficient training volume to elicit improvement in CRF remains highly individual. Furthermore, a fixed training volume may lead to decreasing gains in aerobic fitness after a training period of 3–6 months, and a subsequent volume increase may be required to pursue further improvements.31

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Figure 1

Physical activity and exercise guidelines across intensity domains for aerobic and resistance training for individuals aged from 18 to 64 years old (adapted from Gibson, Wagner, Hayward 202082).

Methodological factors: training intensity

A key concern remains across aerobic training studies: how intense should endurance training be in order to elicit optimal adaptations. When examining the health and performance effects of training intensity both minimum effective and optimal training intensity should be considered. The minimum training intensity necessary to improve V̇O_2max is dependent on its initial value, where individuals with a V̇O_2max of >40 mL/kg/min require a minimum intensity of 45% of V̇O₂ reserve to increase their V̇O_2max, and those with a lower CRF may obtain improvements at 30%–45% of their V̇O₂ reserve.32 Exercise at a vigorous intensity (≥6 Metabolic Equivalent (METs), typically ≥60% of V̇O_2max) conveys more cardioprotective benefits compared with moderate intensity exercise when total energy expenditure is matched. In their meta-analysis, Scribbans et al33 found no significant difference in improvements in V̇O₂ between moderate (60%–70% of V̇O_2max), heavy (80%–92.5% of V̇O_2max) and severe (100%–250% of V̇O_2max) training. However, a meta-analysis conducted by Milanović et al34 that included 723 participants demonstrated that while both endurance and HIT training improved V̇O_2max compared with no exercise, HIT training elicited larger improvements (5.5±1.2 mL/kg/min) compared with traditional endurance training (4.9±1.4 mL/kg/min). Moreover, the progression of training intensity also requires considerations. While previously untrained subjects improved their V̇O_2max by training at 60% of their heart rate reserve (HRR) 3 days per week for 45 min per session up to 6 months, significant changes in endurance capacity were not observed after the 6-month period.31

When integrating a broader array of indices of cardiometabolic health (body composition, V̇O_2max, blood pressure, lipid profile and glucose metabolism) in overweight/obese untrained individuals, high-intensity interval training (HIIT) was more effective in promoting a greater number of positive adaptations compared with moderate intensity continuous training. Recently however, Burton et al35 observed that intense training (80%–90% of V̇O_2peak) improved V̇O_2peak, but markers of cardiometabolic health (fat metabolism and stress) showed no improvement when daily PA (step count) was reduced. Therefore, a combination of HIT and low/moderate intensity training seems more effective in improving cardiometabolic health compared with HIT alone. Huang et al36 investigated the dose–response relationship of CRF adaptation in sedentary older adults and found the largest V̇O_2max adaptations at a mean intensity of 66%–73% of HRR, while a higher training intensity (75%–80% of HRR) resulted in large declines of V̇O_2max. Therefore, while in some subject populations the benefits of HIT are undeniable, they might result in negative adaptations in other.

HIIT has received a lot of attention as an effective and time-efficient way of increasing cardiovascular and skeletal muscle health and performance.37 In their meta-analysis, Bacon et al38 observed that HIIT resulted in a greater increase in V̇O_2max compared with moderate continuous training. The largest increases were observed when longer intervals of 3–5 min were used. The minimum effective volume of HIT training was also investigated. In inactive and overweight but otherwise healthy men performing one bout of 4 min at 90% HR_max rate three times a week for 10 weeks brought similar improvements to V̇O_2max compared with four bouts of 4 min at 90% HR_max.21 However, increasing the number of interval repetitions was more effective in reducing body fat, total cholesterol, low-density lipoproteins (LDL)-cholesterol and ox-LDL cholesterol. The results suggest that already small doses of HIT are an effective way of training for previously inactive subjects.21 Thus, commencing a training programme with a low dose of HIT and gradually increasing the dose as CRF improves might be an effective training strategy.

In elite athletes, a polarised training approach combining moderate and high intensity training has been shown to be superior to both low-volume HIIT and heavy intensity training.39 Laursen40 argued that HIIT and more prolonged training sessions at a lower intensity might cause similar aerobic adaptations in the skeletal muscle via different molecular pathways, while low-intensity training might also promote autonomic balance and recovery. Therefore, their combination in a training programme might result in highest performance improvements. These findings from the training practices of high-level athletes have not yet been fully investigated in the context of general population health. There is limited but emerging evidence that a polarised approach to training might be more effective also in recreational athletes and in young overweight and obese subjects.41 42 Polarised training targets most training time at exercise intensities below VT1 (ventilatory threshold) and above ventilatory threshold 2 (VT2), with limited training between VT1 and VT2, following an undulating non-linear periodisation structure. Studies reporting endurance performance effect sizes comparing polarised (more training time below VT1 and above VT2 – ES 0.85–2.80) versus threshold training (significant training between VT1 and VT2– ES −0.42–2.16) have consistently shown greater improvements following polarised training.39 41 43 In real-life situations, a combination of training intensities might be more practicable and enjoyable, cardiometabolic health benefits more thorough, the physiological training stimulation more varied and a combination of different training intensities could result in better management of training stress.

Physiological thresholds

Prescribing exercise relative to maximal anchors such as V̇O_2max, maximum power or velocity during an incremental test (W_max, V_max) or HR_max is a common and traditional approach that fails to account for large individual variation in metabolic response to exercise at a similar percentage of the maximum value.44 The use of physiological thresholds such as VT, lactate threshold (LT) or critical power (CP) and the delta concept representing the difference between thresholds and maximum values have been suggested to lower the instance of non-responders to training.4 45 Threshold-based exercise bouts are, by design, individually modulated to reach the physiological thresholds of each individual rather than a homogenous homeostatic perturbation, inducing heterogeneous responses. Physiological thresholds have a considerable range across individuals where, for example, CP can be as low as ~50% in the untrained, to ~95% V̇O_2max in elite athletes.46 The use of VT instead of HRR have previously resulted in improved training responses and fewer non-responders.15 47 Work by Weathermax et al47 demonstrated that those training following an individualised ventilatory thresholds (VT)-based protocol were all considered to respond positively to training (19/19), but 40% of those following an HHR protocol were considered non responders (∆≤4.7% of baseline V̇O_2max) (8/20). Training at 95% LT also induced more homogeneous responses among trained and untrained individuals compared with an intensity of 70% V̇O_2max.45 While individual thresholds seem to be superior to establishing appropriate and effective training intensities for aerobic training, their application at a population level may be challenging, if not completely impractical. Defining individual ventilatory thresholds, or other physiological limits, requires expensive laboratory equipment and skilled labour and is therefore not an option at a population level.

RPE and self-assessed efforts

While popular among athletes and the general population because of accessible technologies, the efficacy of %HR (or % V̇O₂) in guiding exercise intensity has been questioned, and the rating of perceived exertion (RPE) has been shown as a more effective method of targeting exercise intensity corresponding to VT, particularly in less experienced subjects with an RPE of ~13 correlating to VT1.20 The relationship between VT1 and RPE has been investigated in numerous populations, and it is estimated that an RPE of 11–14 will match VT in most individuals, in trained and untrained, independently of body composition.48–50 Two scales have been developed by Borg for RPE. A 6–20 points scale was designed to represent the linear increase of HR with increasing intensity, and another CR10 category ratio scale with a 1–10 system, which was suggested to better represent the non-linear responses of physiological thresholds.51 A review by Hydren and Cohen41 further categorised training zones where intensities at VT1 and below would represent ≤13 and ≤4, intensities between VT1 and VT2 would be 14–16 and 5–6, and ≥VT2 would be ≥17 and ≥7, on the 6–20 and CR10 Borg RPE scales, respectively. These zones and their correspondent RPE score have been suggested to implement polarised training models, which have demonstrated superior outcomes compered with threshold models.41

Beyond athletic training purposes, Tjønna et al21 further suggested the use of RPE for monitoring exercise intensity in public health programmes with a rating of 16 on the Borg scale corresponding to 90% HR_max and would, in most individuals, be above LT and VT1. Importantly, to better prescribe aerobic training to individuals without access to laboratory testing, RPE-based intensity guidelines can be applied. Targeting an RPE of ≤4 (CR10 scale) would be appropriate for moderate, an RPE of 5–6 for vigorous and ≥7 for severe exercise intensity. Supplementing RPE with information on external work (ie, pace in walking/running) and internal physiological response such as heart rate from wearable devices would bring an additional level of sophistication to prescribing and monitoring training.

The application of RPE for exercise and training is easily accessible and effective but nonetheless retains limitations and consideration in its applications, similarly to other methods to assess exercise intensity. Perceived effort (PE) has been defined differently across the literature leading to different ways to how individuals perceive effort per se and to assess the scale validity. Halperin and Emanuel52 have recently addressed this ongoing issue as individuals may understand and perceive the concepts of fatigue, discomfort and heaviness (all key terms used to describe PE yet theoretically independent) differently, directly affecting scale results and selection of exercise intensity. The development of new and population-specific scales, along with variety in each of their versions (aerobic vs resistance exercise, age groups, sports, etc), and how the scale may be defined and presented may also lead to confusion. For adequate implementation of RPE to the general population, the use of the original CR-10 Borg scale would be recommended, along with a simple definition of the scale, and the simple question ‘How effortful is the task?’ should be used.52 The integration of a rating of fatigue scale, which would help remind individuals of the differences in concepts between fatigue and exertion, may help improving RPE validity.53

Another alternative based on intrinsic self-assess effort, termed the ‘isoeffort’ model, has been proposed as an alternative to prescribing intensity instead of physiological or work rate related prescriptions.22 In this model, the total number of intervals, interval duration and rest period duration are prescribed (eg, 4×8 min with 2 min of recovery), and the individual is instructed to ‘solve’ the intensity based on what they can maintain throughout the entire session. Thus, the interval prescription guides towards the desired physiological response.22 The upside of this method is that it requires no physiological testing, but high motivation is needed and is mainly applicable to high intensity training since the aim is to maintain the highest intensity possible.

Finally, the talk test is also an inexpensive, practical and reliable tool to assess exercise intensity, particularly VT1, in healthy and diseased populations.54 55 Described briefly, this test requires exercising individuals to recite a 10–15 s text at the end of an exercise stage of an incremental protocol, following which they are asked the following question: ‘can you talk comfortably?’. They can answer yes, not sure and no, where the answer yes implies the need to increase exercise intensity and not sure or no leads to the end of the test. The reasoning of this test is grounded in the competitive requirements of ventilation for the gas exchange and metabolism of exercise versus speech, as the first inflection point of ventilation that begins to rise exponentially, VT1, represents the point where speech becomes uncomfortable and difficult due to the increased ventilatory demand from exercise.54–57 While simple, this test has shown meaningful validity and reliability when compared with physiological assessed VT1 in elite athletes,56 57 health adults58 59 and even cardiac patients.60 61 Although some considerations are needed when using this test, as most studies have examined the talk test in relation to VT1 only. Its relationship to other thresholds (VT2, lactate), as well as across different exercise testing protocols, also remain unclear.

Mechanisms linking self-assessed to physiological thresholds

The mechanisms describing how perception of effort is so closely associated with VT1, and other physiological variables and thresholds, remain unclear, and whether exertion is primarily modulated via central, peripheral or local pathways requires further investigations. Several concepts have nonetheless been brought forward to explain exertion including metabolically sensitive afferent muscle fibres relaying information to the central nervous system from homeostatic state of muscles under exercising conditions.62 63 The reduction on central motor drive, and subsequently physical performance, seems to also be regulated by exogenous factors,64 psychological preparedness65 and environmental stressors,66 which may further be governed by a sensory tolerance limit, as suggested by Amann and colleagues.63 Although both Borg scales have been heavily used for rate of perceived exertion, the 10-point scale was constructed to reflect the non-linearity of physiological responses during graded exercise tests.19 To that effect, physiologically relevant responses occurring during graded exercise testing, most particularly those occurring at ventilatory, lactate or gas thresholds, would theoretically mirror those of the scale. As such, the oxygen-conforming response, a term coined from the work of Hochachka,67 supports the association between oxygen availability/tissue oxygenation level with local muscle metabolism and contractile functions.68 Interestingly, Drouin et al69 recently demonstrated that perception of effort follows changes in muscle oxygenation during isometric handgrip exercises. It has been established that muscle oxygenation (as determined from changes in O₂Hb, HHb, and %O₂sat) is closely related to the non-linear ventilatory responses of incremental exercise70 as a result in intracellular fluctuations of mitochondrial oxygen uptake and ATP/Pi ratio. Moreover, changes in muscle oxygenation state seem to be sensitive to both ventilatory thresholds (VT1 and VT2) as well as sexes and training status.70 These insights on the regulatory pathways of RPE and physiological thresholds may not present a complete spectrum of factors; nonetheless, evidence suggests that the oxygen-conformer response and the association between muscle oxygenation and ventilatory thresholds may be, at least in part, responsible.

A new framework for exercise prescription to the general population

Using RPE to determine exercise intensities for exercise programming have demonstrated a close relationship to VT1 and VT2 and could be just as effective to improve CRF in non-responders and negative responders. This approach is free and widely accessible and should constitute the basis of the exercise prescription guidelines for the general adult population. This method is personalised and reflects the individual’s thresholds, essential to improve CRF. While the foundation of current exercise guidelines includes the recommendation of 150 min/week of moderate PA and is the most accessible way to reach the wider population, this approach remains inheritably flawed and does not result in improved CRF in the general population. Application of more rigorous methods, such as laboratory-based thresholds, for establishing training intensities could be considered based on individual and personalised needs but remains inaccessible for most individuals. To facilitate the understanding of this concept, we developed a hierarchical framework (figure 2) that demonstrates that when both accessibility and effectiveness are considered, prescribing exercise to the general population should favour the use of RPE or other self-assessed thresholds methods, followed by laboratory thresholds or the current PA guidelines, and finally, relative intensities via HR or VO₂. This novel framework can be used by clinicians, as well as health and fitness experts, to improve CRF of patients, clients and the general population, in an accessible and effective fashion.

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Figure 2

Hierarchical theoretical framework presenting the level of accessibility and effectiveness of different methods to assess exercise intensity. CP, critical power; LT, lactate threshold; RPE rate of perceived exertion; VT, ventilatory threshold.