title: Influence of duty cycle on the power-duration relationship: observations and potential mechanisms.
abstract: The highest sustainable rate of aerobic metabolism [critical power (CP)] and the finite amount of work that can be performed above CP (W' [curvature constant]) were determined under two muscle contraction duty cycles. Eight men completed at least three constant-power handgrip tests to exhaustion to determine CP and W' for 50% and 20% duty cycles, while brachial artery blood flow (Q̇BA) and deoxygenated-[hemoglobin + myoglobin] (deoxy-[Hb+Mb]) were measured. CP was lower for the 50% duty cycle (3.9 ± 0.9 W) than the 20% duty cycle (5.1 ± 0.8 W; p < 0.001), while W' was not significantly different (50% duty cycle: 452 ± 141 J vs. 20% duty cycle: 432 ± 130 J; p > 0.05). At the same power output, Q̇BA and deoxy-[Hb + Mb] achieved higher end-exercise values for the 20% duty cycle (9.87 ± 1.73 ml·s(-1); 51.7 ± 4.7 μM) than the 50% duty cycle (7.37 ± 1.76 ml·s(-1), p < 0.001; 44.3 ± 2.4 μM, p < 0.03). These findings indicate that blood flow influences CP, but not W'.
authors.affiliation: Department of Kinesiology, Kansas State University, Manhattan, KS, USA; Department of Anatomy and Physiology, Kansas State University, Manhattan, KS, USA. Electronic address: rbrox@ksu.edu.
authors.forename: R M
authors.initials: RM
authors.lastname: Broxterman
authors.affiliation: Department of Kinesiology, Kansas State University, Manhattan, KS, USA; Department of Anatomy and Physiology, Kansas State University, Manhattan, KS, USA.
authors.forename: C J
authors.initials: CJ
authors.lastname: Ade
authors.affiliation: Department of Kinesiology, Kansas State University, Manhattan, KS, USA.
authors.forename: S L
authors.initials: SL
authors.lastname: Wilcox
authors.affiliation: Department of Kinesiology, Kansas State University, Manhattan, KS, USA.
authors.forename: S J
authors.initials: SJ
authors.lastname: Schlup
authors.affiliation: Department of Kinesiology, Kansas State University, Manhattan, KS, USA.
authors.forename: J C
authors.initials: JC
authors.lastname: Craig
authors.affiliation: Department of Kinesiology, Kansas State University, Manhattan, KS, USA.
authors.forename: T J
authors.initials: TJ
authors.lastname: Barstow
mesh_terms: D000328:Adult; D004576:Electromyography; D016552:Ergometry; D019054:Evoked Potentials, Motor / Q000502:physiology; D005080:Exercise Test; D017079:Exercise Tolerance / Q000502:physiology*; D018737:Hand Strength / Q000502:physiology*; D064368:Healthy Volunteers; D006454:Hemoglobins / Q000378:metabolism; D006801:Humans; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D010101:Oxygen Consumption; D010108:Oxyhemoglobins / Q000378:metabolism; D012039:Regional Blood Flow; D019265:Spectroscopy, Near-Infrared; D013997:Time Factors; D018608:Ultrasonography, Doppler; D055815:Young Adult
chemical_list: D006454:Hemoglobins; D010108:Oxyhemoglobins; C100003:deoxyhemoglobin
keywords: Critical power; exercise tolerance; muscle blood flow; muscle contraction