Invited Session "The Power-Duration Relationship: Physiological Determinants and Implications for Performance Assessment and Exercise Prescription sponsored by adidas"
Metabolic Determinants of the Critical Power
Vanhatalo, A.
University of Exeter
The power-duration relationship characterises exercise tolerance within the severe exercise intensity domain. The power-asymptote
of the relationship (CP) indicates the boundary between the heavy intensity domain, within which a physiological steady-state is
attainable, and the severe domain, within which exercise is non-steady state and the pulmonary oxygen uptake (VO2) continues to
rise until the VO2max is attained. Whether steady-state is attainable or not has important implications for exercise tolerance and
therefore the establishment of CP is of paramount importance for diagnostic and research purposes. Non-invasive assessment of
skeletal muscle bioenergetics during small muscle mass exercise has indicated that the CP represents a critical threshold for
intramuscular metabolic control (Jones et al. 2008). This interpretation has recently been corroborated, and extended to whole body
exercise, by novel investigations utilising muscle biopsy technique. Collectively, the non-invasive and invasive assessments of muscle
bioenergetics concur in showing that exhaustive exercise above the CP (i.e., in the severe domain) results in the attainment of a
consistent, perhaps limiting, muscle metabolic milieu (as indicated by markers such as pH and concentrations of phosphocreatine
and inorganic phosphate) irrespective of the work rate chosen. The limited exercise tolerance above the CP is quantified as the
curvature constant of the power-duration relationship (W′). As severe-intensity exercise proceeds, the progressive loss of
muscle homeostasis signals a stimulation of ‘excess’ VO2 (i.e., the so-called VO2 slow component), and thereby a loss of efficiency,
which propels the VO2 on a trajectory towards VO2max. Recent evidence suggests that the power-duration relationship is inherently
linked with the VO2 kinetics (Murgatroyd et al. 2011; Vanhatalo et al. 2011). Specifically, interventions which accelerate the rate of
increase in VO2 at exercise onset and reduce the size of the slow component tend to increase the CP and reduce the size of the
W′. These changes are reflected as a rightward shift in the power-duration curve and improved exercise tolerance. The
power-duration relationship may therefore be considered a manifestation of the predictable progression of muscle metabolic
perturbations during severe intensity exercise which ultimately drives VO2 to its maximum and limits exercise tolerance.
References
Murgatroyd SR, Ferguson C, Ward SA, Whipp BJ, Rossiter HB. (2011). J Appl Physiol. 110, 1598-606.
Jones AM, Vanhatalo A, Burnley M, Morton RH, Poole DC. (2010). Med Sci Sports Exerc. 42, 1876-90.
Vanhatalo A, Poole DC, DiMenna FJ, Bailey SJ, Jones AM. (2011). Am J Physiol Regul Integr Comp Physiol. 300, R700-7.
