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Aim This study characterised the decoupling of internal-to-external workload in marathon running and investigated whether decoupling magnitude and onset could improve predictions of marathon performance. Methods The decoupling of internal-to-external workload was calculated in 82,303 marathon runners (13,125 female). Internal workload was determined as a percentage of maximum heart rate, and external workload as speed relative to estimated critical speed (CS). Decoupling magnitude (i.e., decoupling in the 35–40 km segment relative to the 5–10 km segment) was classified as low ( Results The overall internal-to-external workload decoupling experienced was 1.16 ± 0.22, first detected 25.2 ± 9.9 km into marathon running. The low decoupling group (34.5% of runners) completed the marathon at a faster relative speed (88 ± 6% CS), had better marathon performance (217.3 ± 33.1 min), and first experienced decoupling later in the marathon (33.4 ± 9.0 km) compared to those in the moderate (32.7% of runners, 86 ± 6% CS, 224.9 ± 31.7 min, and 22.6 ± 7.7 km), and high decoupling groups (32.8% runners, 82 ± 7% CS, 238.5 ± 30.7 min, and 19.1 ± 6.8 km; all p p p Conclusion Durability characteristics, assessed as internal-to-external workload ratio, show considerable inter-individual variability, and both its magnitude and onset are associated with marathon performance.

New findings What is the topic of this review? We review biological and methodological factors associated with the variable changes in cardiorespiratory fitness in response to endurance training. What advances does it highlight? Several biological and methodological factors exist that each contribute, to a given extent, to response variability. Notably, prescribing exercise intensity relative to physiological thresholds reportedly increases cardiorespiratory fitness response rates compared to when prescribed relative to maximum physiological values. As threshold-based approaches elicit more homogenous acute physiological responses among individuals, when repeated over time, these uniform responses may manifest into more homogenous chronic adaptations thereby reducing response variability. Abstract Changes in cardiorespiratory fitness (CRF) in response to endurance training (ET) exhibit large variations, possibly due to a multitude of biological and methodological factors. It is acknowledged that ∼20% of individuals may not achieve meaningful increases in CRF in response to ET. Genetics, the most potent biological contributor, has been shown to explain ∼50% of response variability, whilst age, sex, and baseline CRF appear to explain a smaller proportion. Methodological factors represent the characteristics of the ET itself including the type, volume, and intensity of exercise, as well as the method used to prescribe and control exercise intensity. Notably, methodological factors are modifiable and, upon manipulation, alter response rates to ET, eliciting increases in CRF regardless of an individual's biological predisposition. Particularly, prescribing exercise intensity relative to a physiological threshold (e.g. ventilatory threshold) is shown to increase CRF response rates compared to when intensity is anchored relative to a maximum physiological value (e.g. maximum heart rate). It is, however, uncertain whether the increased response rates are primarily attributable to reduced response variability, greater mean changes in CRF, or both. Future research is warranted to elucidate whether more homogenous chronic adaptations manifest over time among individuals, as a result of exposure to more homogenous exercise stimuli elicited by threshold-based practices. This article is protected by copyright. All rights reserved.

What is the topic of this review? Biological and methodological factors associated with the variable changes in cardiorespiratory fitness in response to endurance training. What advances does it highlight? Several biological and methodological factors exist that each contribute, to a given extent, to response variability. Notably, prescribing exercise intensity relative to physiological thresholds reportedly increases cardiorespiratory fitness response rates compared to when prescribed relative to maximum physiological values. As threshold-based approaches elicit more homogeneous acute physiological responses among individuals, when repeated over time, these uniform responses may manifest as more homogeneous chronic adaptations thereby reducing response variability.Changes in cardiorespiratory fitness (CRF) in response to endurance training (ET) exhibit large variations, possibly due to a multitude of biological and methodological factors. It is acknowledged that ∼20% of individuals may not achieve meaningful increases in CRF in response to ET. Genetics, the most potent biological contributor, has been shown to explain ∼50% of response variability, whilst age, sex and baseline CRF appear to explain a smaller proportion. Methodological factors represent the characteristics of the ET itself, including the type, volume and intensity of exercise, as well as the method used to prescribe and control exercise intensity. Notably, methodological factors are modifiable and, upon manipulation, alter response rates to ET, eliciting increases in CRF regardless of an individual's biological predisposition. Particularly, prescribing exercise intensity relative to a physiological threshold (e.g., ventilatory threshold) is shown to increase CRF response rates compared to when intensity is anchored relative to a maximum physiological value (e.g., maximum heart rate). It is, however, uncertain whether the increased response rates are primarily attributable to reduced response variability, greater mean changes in CRF or both. Future research is warranted to elucidate whether more homogeneous chronic adaptations manifest over time among individuals, as a result of exposure to more homogeneous exercise stimuli elicited by threshold-based practices.