No abstract

The physiological equivalents of the curvature constant (W') of the high-intensity power-duration (P-t(LIM)) relationship are poorly understood, although they are presumed to reach maxima/minima at exhaustion. In an attempt to improve our understanding of the determinants of W', we therefore aimed to determine its recovery kinetics following exhaustive exercise (which depletes W') concomitantly with those of O(2) uptake (V(O(2)), a proxy for the kinetics of phosphocreatine replenishment) and blood lactate concentration ([L(-)]). Six men performed cycle-ergometer exercise to t(LIM): a ramp and four constant-load tests, at different work rates, for estimation of lactate threshold, W', critical power (CP), and maximum V(O(2)). Three further exhausting tests were performed at different work rates, each preceded by an exhausting "conditioning" bout, with intervening recoveries of 2, 6, and 15 min. Neither prior exhaustion nor recovery duration altered V(O(2)) or [L(-)] at t(LIM). Postconditioning, the P-t(LIM) relationship remained well characterized by a hyperbola, with CP unchanged. However, W' recovered to 37 +/- 5, 65 +/- 6, and 86 +/- 4% of control following 2, 6, and 15 min of intervening recovery, respectively. The W' recovery was curvilinear [interpolated half time (t(1/2)) = 234 +/- 32 s] and appreciably slower than V(O(2)) recovery (t(1/2) = 74 +/- 2 s) but faster than [L(-)] recovery (t(1/2) = 1,366 +/- 799 s). This suggests that W' determines supra-CP exercise tolerance, its restitution kinetics are not a unique function of phosphocreatine concentration or arterial [L(-)], and it is unlikely to simply reflect a finite energy store that becomes depleted at t(LIM).

Ferguson C, Whipp BJ, Cathcart AJ, Rossiter HB, Turner AP, Ward SA. Effects of prior very-heavy intensity exercise on indices of aerobic function and high-intensity exercise tolerance. J Appl Physiol 103: [812][813][814][815][816][817][818][819][820][821][822] 2007. First published May 31, 2007; doi:10.1152/japplphysiol.01410.2006.-A recent bout of high-intensity exercise can alter the balance of aerobic and anaerobic energy provision during subsequent exercise above the lactate threshold ( L). However, it remains uncertain whether such "priming" influences the tolerable duration of subsequent exercise through changes in the parameters of aerobic function [e.g., L, maximum oxygen uptake (V O2max)] and/or the hyperbolic power-duration (P-t) relationship [critical power (CP) and the curvature constant (WЈ)]. We therefore studied six men performing cycle ergometry to the limit of tolerance; gas exchange was measured breath-by-breath and arterialized capillary blood [lactate] was measured at designated intervals. On different days, each subject completed 1) an incremental test (15 W/min) for estimation of L and measurement of the functional gain (⌬V O2/ ⌬WR) and V O2peak and 2) four constant-load tests at different work rates (WR) for estimation of CP, WЈ, and V O2max. All tests were subsequently repeated with a preceding 6-min supra-CP priming bout and an intervening 2-min 20-W recovery. The hyperbolicity of the P-t relationship was retained postpriming, with no significant difference in CP (241 Ϯ 39 vs. 242 Ϯ 36 W, post-vs. prepriming), V O2max (3.97 Ϯ 0.34 vs. 3.93 Ϯ 0.38 l/min), ⌬V O2/⌬WR (10.7 Ϯ 0.3 vs. 11.1 Ϯ 0.4 ml ⅐ min Ϫ1 ⅐ W Ϫ1 ), or the fundamental V O2 time constant (25.6 Ϯ 3.5 vs. 28.3 Ϯ 5.4 s). WЈ (10.61 Ϯ 2.07 vs. 16.13 Ϯ 2.33 kJ) and the tolerable duration of supra-CP exercise (Ϫ33 Ϯ 11%) were each significantly reduced, despite a less-prominent V O2 slow component. These results suggest that, following supra-CP priming, there is either a reduced depletable energy resource or a residual fatiguemetabolite level that leads to the tolerable limit before this resource is fully depleted. power-duration relationship; WЈ; oxygen uptake kinetics; slow component; lactate threshold THE TOLERABLE DURATION of muscular exercise, such as cycling or running, is dependent on both the limit (i.e., V O 2max ) and the contour (i.e., V O 2 kinetics) of the oxygen uptake (V O 2 ) response profile. The phase 2 (or fundamental) V O 2 kinetics during constant-load cycle ergometer exercise have been shown to reflect those of muscle O 2 consumption [following a short, transit delay, i.e., phase 1 (3, 20)], with V O 2 attaining a steady state with first-order exponential kinetics. Moderateintensity work rates [i.e., below the lactate threshold ( L )] are highly sustainable, there being no sustained arterial lactate concentration ([L Ϫ ]a) increase (reviewed in Refs. 24, 55). For heavy-intensity work rates that lie between L and critical power [CP; i.e., the power asymptote of the power-duration (P-t) relationship], the emerg...

Abstract-Aim:The aim of this study was to investigate feedback control strategies for integration of electric motor assist and functional electrical stimulation (FES) for paraplegic cycling, with particular focus on development of a testbed for exercise testing in FES cycling, in which both cycling cadence and workrate are simultaneously well controlled and contemporary physiological measures of exercise performance derived. A second aim was to investigate the possible benefits of the approach for mobile, recreational cycling.Methods: A recumbent tricycle with an auxiliary electric motor is used, which is adapted for paraplegic users, and instrumented for stimulation control. We propose a novel integrated control strategy which simultaneously provides feedback control of leg power output (via automatic adjustment of stimulation intensity) and cycling cadence (via electric motor control). Both loops are designed using system identification and analytical (model-based) feedback design methods. Ventilatory and pulmonary gas exchange response profiles are derived using a portable system for real-time breath-by-breath acquisition.Results: We provide indicative results from one paraplegic subject in which a series of feedback-control tests illustrate accurate control of cycling cadence, leg power control, and external disturbance rejection. We also provide physiological response profiles from a submaximal exercise step test and a maximal incremental exercise test, as facilitated by the control strategy. Conclusion: The integrated control strategy is effective in facilitating exercise testing under conditions of well-controlled cadence and power output. Our control approach significantly extends the achievable workrate range and enhances exercise-test sensitivity for FES cycling, thus allowing a more stringent characterization of physiological response profiles and estimation of key parameters of aerobic function. We further conclude that the control approach can significantly improve the overall performance of mobile recreational cycling.Index Terms-Cardiopulmonary exercise testing, control systems, functional electrical stimulation (FES), lower limb cycling, spinal-cord injury (SCI) rehabilitation.

VO2 kinetics in intermittent exercise over a range of duty-cycle durations tended to associate with blood [lactate] profiles, similarly to previous demonstrations for sustained constant-load exercise.