The intra-push velocity profile of the over-ground racing wheelchair sprint start

The aim of this study was to analyse the first six pushes of a sprint start in over-ground racing wheelchair propulsion. One international male wheelchair athlete (age=28 years; body mass=60.6 kg; racing classification=T4) performed maximal over-ground sprint trials, over approximately 10 m, in his own racing wheelchair fitted with a velocometer. Each trial was filmed at 200 Hz using a "Pan and Tilt" system. Eight trials were manually digitised at 100 Hz. Raw co-ordinate data were smoothed and differentiated using a quintic spline routine. Across the period from pushes one to six the duration of each push cycle decreased (0.82+/-0.02-0.45+/-0.01 s) with the mean duration of the propulsive phase decreasing from 0.62+/-0.02 to 0.21+/-0.01 s and the recovery phase increasing from 0.20+/-0.01 to 0.24+/-0.02 s. The push-rim was contacted progressively closer to top dead centre and released progressively closer to bottom dead centre with each push. The data indicate that peak velocity occurred after release. The main findings of this study support the observation that racing wheelchair sprint propulsion is a complex form of locomotion and cannot be described accurately by using just the established definitions of a propulsive and a recovery phase.     

Adaptation to sudden unexpected loading of the low back--the effects of repeated trials

The purpose of this study was to investigate short-term changes in reactions to sudden unexpected loading of the low back. The study utilized a set-up where a horizontal force of 58 N pointing forward suddenly was applied to the upper part of the subject's trunk. EMG activity from the erector spinae muscles and trunk movement data were recorded during 10 trials for 19 subjects. The analysis included EMG reaction time, mean rectified EMG amplitude during the period 50-250 ms after the sudden loading, and time elapsed until stopping of the forward movement of the trunk (stopping time). Reaction time means ranged from 66 to 97 ms (79+/-9 ms), and no difference was found between the trials. Conversely, the mean stopping time for the first trial (468 ms) was significantly higher than for trials 3-10 (359- 371 ms), and the average EMG amplitude during the period 50-250 ms after the sudden loading was lower for the first trial. This study showed that some subjects adapted to sudden unexpected loadings of the low back through a reduction in stopping time and a progression in EMG response during the first few trials. This possible adaptation to repeated trials have been overlooked in previous studies.     

Venous occlusion to the lower limb attenuates vasoconstriction in the nonexercised limb during posthandgrip muscle ischemia.

We investigated the effects of increases in calf volume on cardiovascular responses during handgrip (HG) exercise and post-HG exercise muscle ischemia (PEMI). Seven subjects completed two trials: one control (no occlusion) and one venous occlusion (VO) session. Both trials included a baseline measurement followed by 15 min of rest (REST), 2 min of HG, and 2 min of PEMI. VO was applied at 100 mmHg via cuffs placed around both distal thighs during REST, HG, and PEMI. Mean arterial pressure, heart rate, forearm blood flow (FBF) in the nonexercised arm, and forearm vascular resistance (FVR) in the nonexercised arm (FVR) were measured. During REST and HG, there were no significant differences between trials in all parameters. During PEMI in the control trial, mean arterial pressure and FVR were significantly greater and FBF was significantly lower than baseline values (P < 0.05 for each). In contrast, in the VO trial, FBF and FVR responses were different from control responses. In the VO trial, FBF was significantly greater than in the control trial (4.7 +/- 0.5 vs. 2.5 +/- 0.3 ml x 100 ml(-1) x min(-1), P < 0.05) and FVR was significantly lower (28.0 +/- 4.8 vs. 49.1 +/- 4.6 units, respectively, P < 0.05). These results indicate that increases in vascular resistance in the nonexercised limb induced by activation of the muscle chemoreflex can be attenuated by increases in calf volume.     

Effects of wave reflection timing on left ventricular mechanics

The aim of this study was to evaluate how the timing of the pressure pulse produced by peripheral reflection affects the left ventricle (stroke volume, ventricular work, coronary driving pressure). Ten isolated perfused rabbit hearts were attached to rubber tubes of different lengths (0.5, 0.8 and 1 m) connected to a hydraulic resistance. The different lengths produced reflections at different times and the reflected pulse returned to the ventricle in early (at 84 ms), middle (at 134 ms) and late systole (at 168 ms) for the three tubes, respectively. The loading parameters (ventricular filling pressure and hydraulic resistance) were not changed during the procedure. Ventricular and aortic pressure and aortic flow were monitored continuously and recorded; cardiac cycle was fixed at 800 ms. An operator-independent procedure was used to calculate instantaneous and total systolic external work, mean diastolic aorto-ventricular pressure difference and ventricular stroke volume. RESULTS: The mean value of stroke volume for the three different length rubber tubes was 320 +/- 71, 348 +/- 77 and 368 +/- 87 microliters, respectively. The mean value of total external work was 20.3 +/- 8.3, 22.5 +/- 8.8 and 24.2 +/- 9.6 mJ, respectively. The mean aortoventricular pressure difference was 40 +/- 12, 46 +/- 13, 50 +/- 14 mmHg, respectively (1 mmHg = 133 Pa). The differences between the parameters measured in the three conditions were statistically significant (p < 0.05). A reduction of reflection timing, reduces, on a pure mechanical basis, cardiac output and external ventricular work and has a negative effect on coronary driving pressure.     

Muscle glycogenolysis during differing intensities of weight-resistance exercise

Skeletal muscle glycogen metabolism was investigated in eight male subjects during and after six sets of 70% one repetition maximum (1 RM, I-70) and 35% 1 RM (I-35) intensity weight-resistance leg extension exercise. Total force application to the machine lever arm was determined via a strain gauge and computer interfaced system and was equated between trials. Compared with the I-70 trial, the I-35 trial was characterized by almost double the repetitions (13 +/- 1 vs. 6 +/- 0) and half the peak concentric torque for each repetition (12.4 +/- 0.5 vs. 24.2 +/- 1.0 Nm). After the sixth set, muscle glycogen degradation was similar between I-70 and I-35 trials (47.0 +/- 6.6 and 46.6 +/- 6.0 mmol/kg wet wt, respectively), as was muscle lactate accumulation (13.8 +/- 0.7 and 16.7 +/- 4.2 mmol/kg wet wt, respectively). After 2 h of passive recovery without caloric intake, muscle glycogen increased by 22.2 +/- 6.8 and 14.2 +/- 2.5 mmol/kg wet wt in the I-70 and I-35 trials, respectively. Optical absorbance measurement of periodic acid-Schiff-stained muscle sections after the 2 h of recovery revealed larger absorbance increases in fast-twitch than in slow-twitch fibers (0.119 +/- 0.024 and 0.055 +/- 0.024, P = 0.02). Data indicated that when external work was constant, the absolute amount of muscle glycogenolysis was the same regardless of the intensity of resistance exercise. Nevertheless the rate of glycogenolysis during the I-70 trial was approximately double that of the I-35 trial.     

Growth hormone responses to repeated maximal cycle ergometer exercise at different pedaling rates.

The present study examined the growth hormone (GH) response to repeated bouts of maximal sprint cycling and the effect of cycling at different pedaling rates on postexercise serum GH concentrations. Ten male subjects completed two 30-s sprints, separated by 1 h of passive recovery on two occasions, against an applied resistance equal to 7.5% (fast trial) and 10% (slow trial) of their body mass, respectively. Blood samples were obtained at rest, between the two sprints, and for 1 h after the second sprint. Peak and mean pedal revolutions were greater in the fast than the slow trial, but there were no differences in peak or mean power output. Blood lactate and blood pH responses did not differ between trials or sprints. The first sprint in each trial elicited a serum GH response (fast: 40.8 +/- 8.2 mU/l, slow: 20.8 +/- 6.1 mU/l), and serum GH was still elevated 60 min after the first sprint. The second sprint in each trial did not elicit a serum GH response (sprint 1 vs. sprint 2, P < 0.05). There was a trend for serum GH concentrations to be greater in the fast trial (mean GH area under the curve after sprint 1 vs. after sprint 2: 1,697 +/- 367 vs. 933 +/- 306 min x mU(-1) x l(-1); P = 0.05). Repeated sprint cycling results in an attenuation of the GH response.     

Measurement of cardiac muscle relaxation in hypothyroidism.

The isovolumetric relaxation time of the left ventricle (IRT) in 20 hypothyroid patients (133 +/- (SE of mean) 4 ms) was significantly longer than that in 23 normal subjects (95 +/- 3 ms). During a trial of thyroxine replacement the IRT in 12 hypothyroid patients fell from 143 +/- 4 ms to 107 +/- 4 ms. The IRT seems to be a useful index of end-organ function in hypothyroidism.     

Spike-triggered average torque and muscle fiber conduction velocity of low-threshold motor units following submaximal endurance contractions.

The motor unit twitch torque is modified by sustained contraction, but the association to changes in muscle fiber electrophysiological properties is not fully known. Thus twitch torque, muscle fiber conduction velocity, and action potential properties of single motor units were assessed in 11 subjects following an isometric submaximal contraction of the tibialis anterior muscle until endurance. The volunteers activated a target motor unit at the minimum discharge rate in eight 3-min-long contractions, three before and five after an isometric contraction at 40% of the maximal torque, sustained until endurance. Multichannel surface electromyogram signals and joint torque were averaged with the target motor unit potential as trigger. Discharge rate (mean +/- SE, 6.6 +/- 0.2 pulses/s) and interpulse interval variability (33.3 +/- 7.0%) were not different in the eight contractions. Peak twitch torque and recruitment threshold increased significantly (93 +/- 29 and 12 +/- 5%, P <0.05) in the contraction immediately after the endurance task with respect to the preendurance values (0.94 +/- 0.26 mN.m and 3.7 +/- 0.5% of the maximal torque), whereas time to peak of the twitch torque did not change (74.4 +/- 10.1 ms). Muscle fiber conduction velocity decreased and action potential duration increased in the contraction after the endurance (6.3 +/- 1.8 and 9.8 +/- 1.8%, respectively, P <0.05; preendurance values, 3.9 +/- 0.2 m/s and 11.1 +/- 0.8 ms), whereas the surface potential peak-to-peak amplitude did not change (27.1 +/- 3.1 microV). There was no significant correlation between the relative changes in muscle fiber conduction velocity or surface potential duration and in peak twitch torque (R2= 0.04 and 0.10, respectively). In conclusion, modifications in peak twitch torque of low-threshold motor units with sustained contraction are mainly determined by mechanisms not related to changes in action potential shape and in its propagation velocity.     

The effect of warm-up on high-intensity, intermittent running using nonmotorized treadmill ergometry

The aim of this study was to investigate the effect of previous warming on high-intensity intermittent running using nonmotorized treadmill ergometry. Ten male soccer players completed a repeated sprint test (10 x 6-second sprints with 34-second recovery) on a nonmotorized treadmill preceded by an active warm-up (10 minutes of running: 70% VO2max; mean core temperature (Tc) 37.8 +/- 0.2 degrees C), a passive warm-up (hot water submersion: 40.1 +/- 0.2 degrees C until Tc reached that of the active warm-up; 10 minutes +/- 23 seconds), or no warm-up (control). All warm-up conditions were followed by a 10-minute static recovery period with no stretching permitted. After the 10-minute rest period, Tc was higher before exercise in the passive trial (38.0 +/- 0.2 degrees C) compared to the active (37.7 +/- 0.4 degrees C) and control trials (37.2 +/- 0.2 degrees C; p < 0.05). There were no differences in pre-exercise oxygen consumption and blood lactate concentration; however, heart rate was greater in the active trial (p < 0.05). The peak mean 1-second maximum speed (MxSP) and group mean MxSP were not different in the active and passive trials (7.28 +/- 0.12 and 7.16 +/- 0.10 m x s(-1), respectively, and 7.07 +/- 0.33 and 7.02 +/- 0.24 m x s(-1), respectively; p > 0.05), although both were greater than the control. The percentage of decrement in performance fatigue was similar between all conditions (active, 3.4 +/- 1.3%; passive, 4.0 +/- 2.0%; and control, 3.7 +/- 2.4%). We conclude that there is no difference in high-intensity intermittent running performance when preceded by an active or passive warm-up when matched for post-warm-up Tc. However, repeated sprinting ability is significantly improved after both active and passive warm-ups compared to no warm-up.     

Effect of exercise duration on postprandial hypertriglyceridemia in men with metabolic syndrome.

We examined the effect of exercise on postprandial hypertriglyceridemia (PHTG) and insulin resistance in individuals with metabolic syndrome. Subjects were 10 hypertriglyceridemic men with insulin resistance [age = 35.0 +/- 1.8 yr, body weight = 90.7 +/- 3.3 kg, fasting triglyceride (TG) = 2.6 +/- 0.4 mmol/l, peak oxygen consumption ((.)Vo(2peak)) = 36.0 +/- 1.3 ml(-1).kg(-1).min(-1), and homeostatic model assessment of insulin resistance (HOMA-IR)= 3.1 +/- 0.3]. Each participant performed a control trial (Ctr; no exercise) and three exercise trials at 60% of their (.)Vo(2peak) for 30 min (30 min-Ex), 45 min (45 min-Ex) and 60 min (60 min-Ex). All subjects had a fat meal in each trial. In the exercise trials, the subject jogged on a treadmill for a designated duration of 12 h before ingestion of a fat meal. Blood samples were taken at 0 h (before the meal) and at 2, 4, 6, and 8 h after the meal. The plasma TG, area score under TG concentration curve over an 8-h period (TG AUC) after the meal, and HOMA-IR were analyzed. The TG AUC scores in both the 45 min-Ex and 60 min-Ex were 31 and 33% lower, respectively, than Ctr (P < 0.02). There were no significant differences in TG AUC scores between the 30 min-Ex and the Ctr (P > 0.05). There were no trial differences in the fasting plasma glucose concentration (P > 0.05). HOMA-IR values in the 30 min-Ex, 45 min-Ex, and 60 min-Ex trials were lower than the Ctr (P < 0.03), but no significant differences were found in HOMA-IR among the exercise trials. The results suggest that for physically inactive individuals with metabolic syndrome, exercising at moderate intensity for 45 min effectively attenuates PHTG while exercise for 30 min is sufficient to improve insulin action.     

Effect of carbohydrate ingestion and ambient temperature on muscle fatigue development in endurance-trained male cyclists.

The aim of the present study was to determine the effect of carbohydrate (CHO; sucrose) ingestion and environmental heat on the development of fatigue and the distribution of power output during a 16.1-km cycling time trial. Ten male cyclists (Vo(2max) = 61.7 +/- 5.0 ml.kg(-1).min(-1), mean +/- SD) performed four 90-min constant-pace cycling trials at 80% of second ventilatory threshold (220 +/- 12 W). Trials were conducted in temperate (18.1 +/- 0.4 degrees C) or hot (32.2 +/- 0.7 degrees C) conditions during which subjects ingested either CHO (0.96 g.kg(-1).h(-1)) or placebo (PLA) gels. All trials were followed by a 16.1-km time trial. Before and immediately after exercise, percent muscle activation was determined using superimposed electrical stimulation. Power output, integrated electromyography (iEMG) of vastus lateralis, rectal temperature, and skin temperature were recorded throughout the trial. Percent muscle activation significantly declined during the CHO and PLA trials in hot (6.0 and 6.9%, respectively) but not temperate conditions (1.9 and 2.2%, respectively). The decline in power output during the first 6 km was significantly greater during exercise in the heat. iEMG correlated significantly with power output during the CHO trials in hot and temperate conditions (r = 0.93 and 0.73; P < 0.05) but not during either PLA trial. In conclusion, cyclists tended to self-select an aggressive pacing strategy (initial high intensity) in the heat.     

新生大鼠离体脊髓薄片侧角中间外侧核细胞的电生理特性

在新生大鼠离体脊髓薄片的中间外侧核作细胞内记录,研究细胞膜的静态与动态电生理特性。细胞的静息电位(RP)变动于-46—-70mV,膜的输入阻抗为108.3±67.9MΩ(X±SD,下同),时间常数9.9±5.6ms,膜电容138.6±124.2pF。用去极化电流进行细胞内刺激时,大部份细胞(85.4％)能产生高频率连续发放,其余细胞(15.6％)仅产生初始单个发放。胞内直接刺激引起的动作电位(AP)幅度为63.4±9.0mV,时程2.4±0.6ms,阈电位水平在RP基础上去极18.7±6.2mV。大部份细胞的锋电位后存在明显的超极化后电位,其幅度为5.1±2.7mV、持续90±31.8ms。刺激背根可在记录细胞引起EPSP或顺向AP,少数细胞尚出现IPSP。而刺激腹根则可引起逆向AP。     

Conduction velocity of low-threshold motor units during ischemic contractions performed with surface EMG feedback.

The aim of this study was to analyze the effect of ischemia on low-threshold motor unit conduction velocity. Nine subjects were trained to isolate the activity of a single motor unit (target motor unit) in the abductor pollicis brevis muscle with feedback on surface EMG signals recorded with a 16-electrode linear array. After training, the subjects activated the target motor unit at approximately 8 pulses per second (pps) for five 3-min-long contractions. During the third and fourth contractions, a cuff inflated at 180 mmHg around the forearm induced ischemia of the hand. The exerted force (mean +/- SE, 4.6 +/- 2.1% of the maximal voluntary contraction force), discharge rate (8.6 +/- 0.4 pps), interpulse interval variability (34.8 +/- 2.5%), and peak-to-peak amplitude of the target motor unit action potentials (176.6 +/- 18.2 microV) were not different among the five contractions. Conduction velocity, mean power spectral frequency, and action potential duration were the same in the beginning of the five contractions (2.8 +/- 0.2 m/s, 195.2 +/- 10.5 Hz, and 5.4 +/- 0.3 ms, respectively) and changed over the 3 min of sustained activation only during the fourth contraction. Conduction velocity and mean power spectral frequency decreased (10.05 +/- 1.8% and 8.50 +/- 2.18% during the 3 min, respectively) and action potential duration increased (8.2 +/- 4.6% in the 3 min) during the fourth contraction. In conclusion, 1) subjects were able to isolate the activity of a single motor unit with surface EMG visual feedback in ischemic conditions maintained for 16 min, and 2) the activation-induced decrease in single motor unit conduction velocity was significantly larger with ischemia than with normal circulation, probably due to the alteration of mechanisms of ion exchange across the fiber membrane.     

Ultrafine and fine particulate matter inhalation decreases exercise performance in healthy subjects

The purpose of this study was to investigate effects of PM1 (particulate matter with aerodynamic diameter 0.02-2 microm) inhalation on exercise performance in healthy subjects. Inhalation of internal combustion-derived PM is associated with adverse effects to the pulmonary and muscle microcirculation. No data are available concerning air pollution and exercise performance. Fifteen healthy college-aged males performed 4 maximal effort 6-min cycle ergometer trials while breathing low or high PM1 to achieve maximal work accumulation (kJ). Low PM1 inhalation trials 1 and 2 were separated by 3 days; then after a 7 day washout, trials 3 and 4 (separated by 3 days) were done while breathing high PM1 generated from a gasoline engine; CO was kept below 10 ppm. Lung function was done after trial 1 to verify nonasthmatic status. Lung function was normal before and after low PM1 exercise. PM1 number counts were not different between high PM1 trials (336,730 +/- 149,206 and 396,200 +/- 82,564 for trial 3 and 4, respectively) and were different from low PM1 trial number counts (2,260 +/- 500) (P < 0.0001). Mean heart rate was not different between trials (189 +/- 6.0, 188 +/- 7.6, 188 +/- 7.6, 187 +/- 7.4, for low and high PM1 trials; respectively). Work accumulated was not different between low PM1 trials (96.1 +/- 9.38 versus 96.6 +/- 10.83 kJ) and the first high PM1 trial (trial 3, 96.8 +/- 10.65 kJ). Work accumulated in the second high PM1 trial 4, 91.3 +/- 10.04 kJ) was less than in low PM1 trials 1 and 2, and high PM1 trial 3 (P = 0.004, P = 0.003, P = 0.0008; respectively). Acute inhalation of high (PM1) typical of many urban environments could impair exercise performance.     

The relationship between consistency of propulsive cycles and maximum angular velocity during wheelchair racing

The purposes of this study were to examine the consistency of wheelchair athletes' upper-limb kinematics in consecutive propulsive cycles and to investigate the relationship between the maximum angular velocities of the upper arm and forearm and the consistency of the upper-limb kinematical pattern. Eleven elite international wheelchair racers propelled their own chairs on a roller while performing maximum speeds during wheelchair propulsion. A Qualisys motion analysis system was used to film the wheelchair propulsive cycles. Six reflective markers placed on the right shoulder, elbow, wrist joints, metacarpal, wheel axis, and wheel were automatically digitized. The deviations in cycle time, upper-arm and forearm angles, and angular velocities among these propulsive cycles were analyzed. The results demonstrated that in the consecutive cycles of wheelchair propulsion the increased maximum angular velocity may lead to increased variability in the upper-limb angular kinematics. It is speculated that this increased variability may be important for the distribution of load on different upper-extremity muscles to avoid the fatigue during wheelchair racing.     

Combined ingestion of protein and free leucine with carbohydrate increases postexercise muscle protein synthesis in vivo in male subjects

The present study was designed to determine postexercise muscle protein synthesis and whole body protein balance following the combined ingestion of carbohydrate with or without protein and/or free leucine. Eight male subjects were randomly assigned to three trials in which they consumed drinks containing either carbohydrate (CHO), carbohydrate and protein (CHO+PRO), or carbohydrate, protein, and free leucine (CHO+PRO+Leu) following 45 min of resistance exercise. A primed, continuous infusion of L-[ring-13C6]phenylalanine was applied, with blood samples and muscle biopsies collected to assess fractional synthetic rate (FSR) in the vastus lateralis muscle as well as whole body protein turnover during 6 h of postexercise recovery. Plasma insulin response was higher in the CHO+PRO+Leu compared with the CHO and CHO+PRO trials (+240 +/- 19% and +77 +/- 11%, respectively, P < 0.05). Whole body protein breakdown rates were lower, and whole body protein synthesis rates were higher, in the CHO+PRO and CHO+PRO+Leu trials compared with the CHO trial (P < 0.05). Addition of leucine in the CHO+PRO+Leu trial resulted in a lower protein oxidation rate compared with the CHO+PRO trial. Protein balance was negative during recovery in the CHO trial but positive in the CHO+PRO and CHO+PRO+Leu trials. In the CHO+PRO+Leu trial, whole body net protein balance was significantly greater compared with values observed in the CHO+PRO and CHO trials (P < 0.05). Mixed muscle FSR, measured over a 6-h period of postexercise recovery, was significantly greater in the CHO+PRO+Leu trial compared with the CHO trial (0.095 +/- 0.006 vs. 0.061 +/- 0.008%/h, respectively, P < 0.05), with intermediate values observed in the CHO+PRO trial (0.0820 +/- 0.0104%/h). We conclude that coingestion of protein and leucine stimulates muscle protein synthesis and optimizes whole body protein balance compared with the intake of carbohydrate only.     

Physiological responses of exercised-fatigued individuals exposed to wet-cold conditions.

Thirteen healthy and fit men [age = 27 +/- 8 (SD) yr, height = 177 +/- 5 cm, mass = 75 +/- 7 kg, body fat = 14 +/- 5%, maximal O2 consumption = 51 +/- 4 ml. kg-1. min-1] participated in an experiment designed to test their thermoregulatory response to a challenging cold exposure after 5 h of demanding mixed exercise during which only water was consumed. Subjects expended 7,314 +/- 741 kJ on cycling, rowing, and treadmill-walking machines, performed 8,403 +/- 1,401 kg. m of mechanical work during resistance exercises, and completed 120 inclined sit-ups. Subjects then assumed a seated position in a 10 degrees C air environment while wearing shorts, T-shirt, rain hat, and neoprene gloves and boots. After 30 min the subjects were showered continuously with cold water ( approximately 920 ml/min at 10 degrees C) on their backs accompanied by a 6 km/h wind for up to 4 h. Blood samples were taken from the nondominant arm every 30 min during the exposure and assayed for energy metabolites, hormones, indexes of hydration, and neurotransmitters. Counterbalanced control trials without prior exercise were also conducted. Blood insulin was higher during the control trial, whereas values of glycerol, nonesterified fatty acids, beta-hydroxybutyrate, lactate, cortisol, free triiodothyronine, and thyroxine were lower. Three subjects lasted the maximum duration of 4.5 h for control and fatigue trials, with final rectal temperatures of 36.43 +/- 0.21 and 36.08 +/- 0.49 degrees C, respectively. Overall, the duration of 172 +/- 68 (SD) min for the fatigue trial was not significantly different from that of the control trial (197 +/- 72 min) and, therefore, was not affected by the preexposure exercise. Although duration was positively correlated to body fatness and shivering intensity, the latter was not correlated to any physical characteristic or the fitness level of the individual.     

The impact of different pacing strategies on five-kilometer running time trial performance

The purpose of this study was to determine the optimal 1.63-km (1-mile) pacing strategy for 5-km running performance in moderately trained women distance runners. Eleven women distance runners (20.7 +/- 0.8 years, 163.8 +/- 2.0 cm, 57.0 +/- 2.2 kg, 51.7 +/- 1.0 ml.kg(-1).min(-1), 18.9 +/- 0.8% fat, 78.1 +/- 1.4% VO(2)max at lactate threshold) performed 2 preliminary 5-km time trials on a treadmill to establish baseline 5-km times. The average 1.63-km split pace of the fastest preliminary trial was manipulated for the first 1.63 km of the experimental trials and run either equal to (EVEN), 3% faster than (3%), or 6% faster than (6%) the current baseline average 1.63-km pace for each subject. Ventilation (V(E)), oxygen consumption VO(2)max )), respiratory exchange ratio, and heart rate were measured continuously. Overall 5-km times were not different (p > 0.05) for the EVEN, 3% and 6% trials finishing in 21:11 (minutes/seconds) +/- 29 seconds, 20:52 +/- 36 seconds and 20:39 +/- 29 seconds, respectively. The fastest time for 8 subjects resulted from the 6% trial and the other 3 subjects' fastest times resulted from the 3% trial. The overall exercise intensity (%VO(2)max , %VO(2)max above lactate threshold, V(E), and respiratory exchange ratio) of the first 1.63-km split was not different between the 3 and 6% trials, despite the 6% trial being 13 seconds faster than the 3% trial. Based on these findings, initial 1.63-km starting paces of a 5-km race can be 3 to 6% greater than current average race pace without negatively impacting performance. In order to optimize 5-km performance, runners should start the initial 1.63 km of a 5-km race at paces 3-6% greater than their current average race pace.     

Impact of repeated increases in shear stress via reactive hyperemia and handgrip exercise: no evidence of systematic changes in brachial artery FMD

Reactive hyperemia (RH) creates an uncontrolled, transient increase in brachial artery (BA) shear stress (SS) for flow-mediated dilation (FMD) assessment. In contrast, handgrip exercise (HGEX) can create similar, sustained SS increases over repeated trials. The purpose of this study was to examine the impact of repeated SS elevation via RH or HGEX and the relationship between RH and HGEX %FMD. BA diameter and blood velocity were assessed with echo and Doppler ultrasound in 20 healthy subjects. Visit A consisted of four 6-min HGEX trials (HGEX trials 1-4) at the intensity required to achieve a shear rate (SR = mean blood velocity/BA diameter; an estimate of SS) of 65 s(-1). Visit B consisted of four RH trials (RH trials 1-4). The RH SR area under the curve (AUC) was higher in trial 1 versus trial 3 and trial 4 (P = 0.019 and 0.047). The HGEX mean SR was similar across trials (mean SR = 66.1 ± 5.8 s(-1), P = 0.152). There were no differences in %FMD across trials or tests (RH trial 1: 6.9 ± 3.5%, trial 2: 6.9 ± 2.3%, trial 3: 7.1 ± 3.5%, and trial 4: 7.0 ± 2.8%; HGEX trial 1: 7.3 ± 3.6%, trial 2: 7.0 ± 3.6%, trial 3: 6.5 ± 3.5%, and trial 4: 6.8 ± 2.9%, P = 0.913). No relationship between subject's RH %FMD and HGEX %FMD was detected (r(2) = 0.12, P = 0.137). However, with response normalization, a relationship emerged (RH %FMD/SR AUC vs. HGEX %FMD/mean SR, r(2) = 0.44, P = 0.002). In conclusion, with repeat trials, there were no systematic changes in RH or HGEX %FMD. The relationship between normalized RH and HGEX %FMD suggests that endothelial responses to different SS profiles provide related information regarding endothelial function.