An individualization of exercise prescription is implemented mainly in terms of intensity but not for duration. To survey the need for an individualized exercise duration prescription, we investigated acute physiologic responses during constant‐load exercise of maximal duration ( t max ) and determined so‐called duration thresholds. Differences between absolute (min) and relative terms (% t max ) of exercise duration were analyzed. Healthy young and trained male and female participants ( n = 11) performed an incremental exercise test and one t max constant‐load exercise test at a target intensity of 10% of maximal power output below the second lactate turn point (LTP 2 ). Blood lactate, heart rate, and spirometric data were measured during all tests. t max was markedly different across subjects (69.6 ± 14.8 min; range: 40–90 min). However, distinct duration phases separated by duration thresholds (DTh) were found in most measured variables. These duration thresholds (except DTh1) were significantly related to t max (DTh2: r 2 = 0.90, p < 0.0001; DTh3: r 2 = 0.98, p < 0.0001) and showed substantial interindividual differences if expressed in absolute terms (DTh2: 24.8 ± 6.0 min; DTh3: 47.4 ± 10.6 min) but not in relative terms (DTh2: 35.4 ± 2.7% t max ; DTh3: 67.9 ± 2.4% t max ). Our data showed that (1) maximal duration was individually different despite the same relative intensity, (2) duration thresholds that were related to t max could be determined in most measured variables, and (3) duration thresholds were comparable between subjects if expressed in relative terms. We therefore conclude that duration needs to be concerned as an independent variable of exercise prescription.

Duration is a rarely investigated marker of exercise prescription. The aim of this study was to test the feasibility of the methodological approach, assessing effects of different duration constant-load exercise (CLE) on physiological responses (internal load) and recovery kinetics. Seven subjects performed an incremental exercise (IE) test, one maximal duration CLE at 77.6 ± 4.8% V˙O2max, and CLE’s at 20%, 40%, and 70% of maximum duration. Heart rate (HR), blood lactate (La), and glucose (Glu) concentrations were measured. Before, 4, 24, and 48 h after CLE’s, submaximal IE tests were performed. HR variability (HRV) was assessed in orthostatic tests (OT). Rating of perceived exertion (RPE) was obtained during all tests. CLE’s were performed at 182 ± 27 W. HRpeak, Lapeak, V˙Epeak, and RPEpeak were significantly higher in CLE’s with longer duration. No significant differences were found between CLE’s for recovery kinetics for HR, La, and Glu in the submaximal IE and for HRV or OT. Despite no significant differences, recovery kinetics were found as expected, indicating the feasibility of the applied methods. Maximum tests and recovery tests closer to CLE’s termination are suggested to better display recovery kinetics. These findings are a first step to prescription of exercise by both intensity and duration on an individual basis.