The effect of oral creatine supplementation on the curvature constant parameter of the power-duration curve for cycle ergometry in humans.

For high-intensity cycle ergometer exercise, the tolerable duration (t) is well characterized as a hyperbolic function of power output, P : t = W'/(P-thetaF), where thetaF may be termed the "fatigue threshold." The purpose of this study was to determine the effect of oral creatine (Cr) supplementation on the curvature constant parameter (W') of the power-duration curve. A double-blind research method and a cross-over design were employed for creatine/placebo supplementation. Eight healthy male subjects (aged 18 to 22 years) each performed four or five high-intensity square-wave exercise bouts on an electrically braked cycle ergometer after 5 d of Cr monohydrate (CR: 20 g of Cr with artificial sweetener/d) or placebo (PL: 6 g of glucose/d) supplementation. Each subject performed a single high-intensity exercise trial per day for four or five successive days to determination the P-t hyperbolic relation. After 6 weeks (the washout time of Cr from the muscles), each subject performed the other condition (i.e., PL or CR) and repeated the same experimental procedure. There was no significant difference for thetaF between PL and CR conditions (PL: 214.4 +/- 23.6, CR: 207.0 +/- 19.8 W, mean +/- SD). In contrast, W' was significantly increased by the Cr supplementation (PL: 10.9 +/- 2.7, CR: 13.7 +/- 3.0 kJ; p<0.05). The results indicated that Cr and/or PCr content in muscles seems to be one of the important determinants of the curvature constant parameter (W') of the power-duration hyperbolic curve for cycle ergometry.     

Comparison between maximal power in the power-endurance relationship and maximal instantaneous power

The purpose of this study was to analyze the relevance of introducing the maximal power ( P _m) into a critical-power model. The aims were to compare the P _m with the instantaneous maximal power ( P _max) and to determine how the P _m affected other model parameters: the critical power ( P _c) and a constant amount of work performed over P _c ( W ). Twelve subjects [22.9 (1.6) years, 179 (7) cm, 74.1 (8.9) kg, 49.4 (3.6) ml/min/kg] completed one 15 W/min ramp test to assess their ventilatory threshold (VT), five or six constant-power to exhaustion tests with one to measure the maximal accumulated oxygen deficit (MAOD), and six 5-s all-out friction-loaded tests to measure P _max at 75 rpm, which was the pedaling frequency during tests. The power and time to exhaustion values were fitted to a 2-parameter hyperbolic model (NLin-2), a 3-parameter hyperbolic model (NLin-3) and a 3-parameter exponential model (EXP). The P _m values from NLin-3 [760 (702) W] and EXP [431 (106) W] were not significantly correlated with the P _max at 75 rpm [876 (82) W]. The P _c value estimated from NLin-3 [186 (47) W] was not significantly correlated with the power at VT [225 (32) W], contrary to other models ( P <0.001). The W from NLin-2 [25.7 (5.7) kJ] was greater than the MAOD [14.3 (2.7) kJ, P <0.001] with a significant correlation between them ( R =0.76, P <0.01). For NLin-3, computation of W _{P >P c}, the amount of work done over P _C, yielded results similar to the W value from NLin-2: 27.8 (7.4) kJ, which correlated significantly with the MAOD ( R =0.72, P <0.01). In conclusion, the P _m was not related to the maximal instantaneous power and did not improve the correlations between other model parameters and physiological variables.     

Correlation between single-slice muscle anatomical cross-sectional area and muscle volume in thigh extensors, flexors and adductors of perimenopausal women

Quantitative data on muscle volume (MV) are important for estimating maximal muscle power. The objective of this study was to determine the correlation between anatomical cross-sectional areas (ACSAs) and the MV in thigh muscles (extensors, flexors, adductors, and sartorius) in perimenopausal women, and to identify at which proximal-to-distal level a single-slice ACSA measurement with magnetic resonance imaging (MRI) displays the highest correlation in each of these. Axial MRIs of the thigh were acquired in 41 perimenopausal women aged 50.8 ± 3.2 years. Segmentation of the extensors, flexors, adductors, and the sartorius was performed between the femoral neck (0%) and the distal end of the intermediate vastus (100%). MVs were determined by numerical integration, and the ACSA was calculated from three-dimensional reconstructions at 10% intervals from proximal to distal. The extensors contributed 50%, the flexors 19%, the adductors 28%, and the sartorius 3% of the total thigh MV. Maximal correlations between ACSA and MV were observed at the 20–40% proximal-to-distal level in the extensors (R ² = 0.73), at 30% in the adductors (R ² = 0.82), and at 70% in the flexors (R ² = 0.72) and sartorius (R ² = 0.85), respectively. ACSA at 50% displayed the highest overall correlations (R ² ≥ 0.69) with MV for all muscle groups. Single-slice ACSAs from MRI displayed high correlations with MVs. Although the (proximal to distal) measurement locations with the highest correlation varied between the muscle groups, a single slice at the 50% location achieved the best compromise in terms of correlation between ACSA and MV across extensors, flexors, adductors and the sartorius.     

Changes in maximal aerobic power,aerobic capacity,and muscle enzyme activities at two stages of the annual training cycle in ski-runners

Summary Changes of cardiorespiratory capacity, of the activity of seven enzymes involved in energy metabolism and of laboratory endurance were investigated in a group of nine male ski-runners before and after exhausting training and a competing period during the winter.Despite the decrease in laboratory endurance and total work oxygen consumption between the investigations, O₂ max, O₂-pulse max and O₂ debt did not change; and O₂-pulse per kg b.w. showed a significant increase.In biopsy samples of the vastus lateralis muscle, the activity of enzymes of carbohydrate metabolism, both anaerobic and total (triose phosphate dehydrogenase — TPDH, lactate dehydrogenase — LDH, hexokinase — HK), and of total aerobic metabolism (citrate synthetase — CS, malate dehydrogenase — MDH), was decreased during this period by 27 to 59% (mean values for different enzymes). The mean activity of cytoplasmic glycerol phosphate dehydrogenase (GPDH) and of hydroxyacyl — CoA dehydrogenase (HOADH) did not change, although the activity of the latter enzyme was decreased in the muscle of those ski-runners who were trained predominantly for speed, and it was increased in those trained mainly for endurance.The changes in activity of the muscle enzymes associated with glycolysis (TPDH and LDH) and of MDH, connected with metabolism and hydrogen transport between cytoplasmic and aerobic mitochondrial compartments, correlate inversely with those of aerobic capacity (total work O₂ consumption).     

The use of magnetic resonance images to investigate the influence of recruitment on the relationship between torque and cross-sectional area in human muscle

The purpose of the present study was to investigate the effect of recruitment on the relationship between peak torque and physiological cross-sectional area (PCSA) in human muscle. A group of 11 healthy men participated in this study. Isokinetic knee extension torques at seven (0, 30, 60, 120, 180, 240, and 300° · s⁻¹) velocities were determined. Magnetic resonance imaging (MRI) was performed to calculate PCSA of right quadriceps femoris (QF) muscle. Exercise-induced contrast shifts in spin-spin relaxation time (T2)-weighted MRI were taken at rest and immediately after repetitive knee-extension exercise and T2 of QF were calculated. The MRI pixels with T2 values more than 1 SD greater than the means at rest were considered to represent QF muscle that had contracted. The area of activated PCSA within the total in QF was expressed as percentage activated PCSA and used as an index of muscle recruitment. The PCSA correlated with peak torque at 0° · s⁻¹ (r=0.615, P < 0.05); in contrast, activated PCSA correlated with peak torque at 120° · s⁻¹ (r=0.603, P < 0.05) and 180° · s⁻¹ (r=0.606, P < 0.05). Additionally, there was a significant difference in correlation coefficients between the activated PCSA-peak torque relationship and the PCSA-torque relationship (P < 0.05). These results suggested that muscle recruitment affects the PCSA-torque relationship. Accepted: 11 August 2000     

Relationship between internal calcium and outward current in mammalian ventricular muscle; a mechanism for the control of the action potential duration?

In sheep and calf ventricular bundles, increasing the internal calcium by increasing the frequency of voltage-clamping to plateau range potentials increased the time-independent outward current. This effect was more marked with higher [Ca]o, and was reduced if the Ca current blockers Verapamil or D 600 were used. 2. If the internal Ca was increased by the addition of cyanide and reduction of external sodium the outward current was also increased. The frequency-dependent increase in outward current also occurred in this Na-poor (12 mM) solution. 3. Tension measurement on the ventricular bundles showed that a Na-free solution with cyanide did not cause a contracture. On changing from Tyrode to a Na-free solution containing cyanide, and on changing back to Tyrode there was a potentiation of the twitch. 4. In Na-poor solution with cyanide, although no contracture was found, ECa was less positive, suggesting that under these circumstances Ca accumulates at the inner side of the membrane, but not around the myofibrils. 5. The prolongation of the action potential in Cl-free solution is frequency-dependent. A greater prolongation is seen at lower frequencies suggesting that Cl current is relatively more important for repolarization at lower frequencies of stimulation. 6. It is suggested that calcium at the inner side of the membrane sets the level of the background outward current. A feed-back mechanism on this basis is proposed for the control of the action potential duration. Various factors that could influence this basic mechanism are discussed.