Muscle acidosis during static exercise is associated with calf vasoconstriction

In this study we measured (n = 6) the phosphocreatine-to-inorganic phosphate ratio (PCr/Pi), Pi, and pH with 31P-nuclear magnetic resonance (31P-NMR) in the human forearm during static work at 30% of maximal voluntary contraction (MVC) for 2 min followed immediately by 3 min of circulatory arrest (forearm arterial occlusion). Static exercise, with its central volitional and skeletal muscle metabolic and mechanical afferent components, caused a rise in heart rate (HR, 32%), blood pressure (BP, 29%), and calf vascular resistance (calf R, 30%). During forearm occlusion after static exercise, HR returned to base line, the increase in BP was attenuated by 30%, and calf R remained elevated and unchanged. The percent change in calf R was correlated with forearm cellular pH (R = 0.56, P less than 0.001) but only weakly associated with PCr/Pi (R = 0.33, P less than 0.042). 30% MVC for 1 min followed by arterial occlusion (3 min) reduced PCr/Pi by 65% and pH by 0.16 U (P less than 0.05). Calf R was unchanged. Circulatory arrest alone (20 min) caused no change in either pH or calf R but large changes in PCr/Pi (50% reduction). We conclude that 1) there is an association between forearm cellular acidosis and calf vasconstriction during static forearm exercise and 2) large changes in PCr/Pi without concomitant changes in pH are not associated with changes in calf R.     

Differential control of forearm and calf vascular resistance during one-leg exercise

The purpose of this study was to determine whether blood flow (BF) and vascular resistance (VR) are controlled differently in the nonactive arm and leg during submaximal rhythmic exercise. In eight healthy men we simultaneously measured BF to the forearm and calf (venous occlusion plethysmography) and arterial blood pressure (sphygmomanometry) and calculated whole limb VR before (control) and during 3 min of cycling with the contralateral leg at 38, 56, and 75% of peak one-leg O2 uptake (VO2). During the initial phase of exercise (0-1.5 min) at all work loads, BF increased and VR decreased in the forearm (P less than 0.05), whereas calf BF and VR remained at control levels. Thereafter, BF decreased and VR increased in parallel and progressive fashion in both limbs. At end exercise, forearm BF and VR were not different from control values (P greater than 0.05); however, in the calf, BF tended to be lower (P less than 0.05 at 75% peak VO2 only) and VR was higher (23 +/- 9, 44 +/- 14, and 88 +/- 23% above control at 38, 56, and 75% of peak VO2, respectively, all P less than 0.05). In a second series of studies, forearm and calf skin blood flow (laser-Doppler velocimetry) and arterial pressure were measured during the same levels of exercise in six of the subjects. Compared with control, skin BF was unchanged and VR was increased (P less than 0.05) in the forearm by end exercise at all work loads, whereas calf skin BF increased (P less than 0.05) and VR decreased (P less than 0.05). The present findings indicate that skeletal muscle and skin VR are controlled differently in the nonactive forearm and calf during the initial phase of rhythmic exercise with the contralateral leg. Skeletal muscle vasodilation occurs in the forearm but not in the calf; forearm skin vasoconstricts, whereas calf skin vasodilates. Finally, during exercise a time-dependent vasoconstriction occurs in the skeletal muscle of both limbs.     

Sympathetic neural discharge and vascular resistance during exercise in humans

The purpose of this study was to determine the relationship between changes in efferent muscle sympathetic nerve activity (MSNA) to the lower leg and calf vascular resistance (CVR) during isometric exercise in humans. We made intraneural (microneurographic) determinations of MSNA in the right leg (peroneal nerve) while simultaneously measuring calf blood flow to the left leg, arterial pressure, and heart rate in 10 subjects before (control), during, and after (recovery) isometric handgrip exercise performed for 2.5 min at 15, 25, and 35% of maximal voluntary contraction (MVC). Heart rate and arterial pressure increased above control within the initial 30 s of handgrip at all levels, and the magnitudes of the increases at end contraction were proportional to the intensity of the exercise. In general, neither MSNA nor CVR increased significantly above control levels during handgrip at 15% MVC. Similarly, neither variable increased above control during the initial 30 s of handgrip at 25 and 35% MVC; however, during the remainder of the contraction period, progressive, parallel increases were observed in MSNA and CVR (P less than 0.05). The correlation coefficients relating changes in MSNA to changes in CVR for the individual subjects averaged 0.63 +/- 0.07 (SE) (range 0.30-0.91) and 0.94 +/- 0.06 (range 0.80-0.99) for the 25 and 35% MVC levels, respectively. During recovery, both MSNA and CVR returned rapidly toward control levels. These findings demonstrate that muscle sympathetic nerve discharge and vascular resistance in the lower leg are tightly coupled during and after isometric arm exercise in humans. Furthermore, the exercise-induced adjustments in the two variables are both contraction intensity and time dependent.     

Heart rate and blood pressure responses to isometric exercise in young and older men

The aim of this study was to examine the isometric endurance response and the heart rate and blood pressure responses to isometric exercise in two muscle groups in ten young (age 23–29 years) and seven older (age 54–59 years) physically active men with similar estimated forearm and thigh muscle masses. Isometric contractions were held until fatigue using the finger flexor muscles (handgrip) and with the quadriceps muscle (one-legged knee extension) at 20%, 40%, and 60% of the maximal voluntary contraction (MVC). Heart rate and arterial pressure were related to the the individual's contraction times. The isometric endurance response was longer with handgrip than with one-legged knee extension, but no significant difference was observed between the age groups. The isometric endurance response averaged 542 (SEM 57), 153 (SEM 14), and 59 (SEM 5) s for the handgrip, and 276 (SEM 35), 94 (SEM 10) and 48 (SEM 5) s for the knee extension at the three MVC levels, respectively. Heart rate and blood pressure became higher during one-legged knee extension than during handgrip, and with increasing level of contraction. The older subjects had a lower heart rate and a higher blood pressure response than their younger counterparts, and the differences were more apparent at a higher force level. The results would indicate that increasing age is associated with an altered heart rate and blood pressure response to isometric exercise although it does not affect isometric endurance. Accepted: 23 October 1997     

Evidence for sympatholysis at the onset of forearm exercise.

The effect of augmented sympathetic outflow on forearm vascular conductance after single handgrip contractions of graded intensity was examined to determine whether sympatholysis occurs early in exercise (n = 7). While supine, subjects performed contractions that were 1 s in duration and 15, 30, and 60% of maximal voluntary contraction (MVC) in intensity. The contractions were repeated during control and lower body negative pressure (LBNP) (-40 mmHg) sessions. Forearm blood flow (FBF; Doppler ultrasound) and mean arterial pressure were measured continuously for 30 s before and 60 s after the single contractions. Vascular conductance (VC) was calculated. Total postcontraction blood flow increased in an exercise intensity-dependent manner. Compared with control, LBNP caused a reduction in baseline and postexercise FBF (P < 0.05), VC (P < 0.01), as well as total excess flow (P < 0.01). Specifically, during LBNP, baseline FBF and VC were reduced by 29 and 34% of control, respectively (P < 0.05). After the 15% MVC contraction, peak VC during LBNP was reduced by a magnitude similar to that during baseline (i.e., ~30%), but it was only reduced by 15% during both the 30 and 60% MVC trials (P < 0.01). It was concluded that the stimuli for exercise hyperemia during moderate and heavy, but not mild, handgrip exercise intensities, diminish the vasoconstrictor effects of LBNP. Furthermore, these data demonstrate that this sympatholysis occurs early in exercise.     

NADH content in type I and type II human muscle fibres after dynamic exercise.

The effect of dynamic exercise on the NADH content of human type I (slow-twitch) and II (fast-twitch) muscle fibres was investigated. Muscle biopsy samples were obtained from the quadriceps femoris of seven healthy subjects at rest and after bicycle exercise at 40, 75 and 100% of the maximal oxygen uptake [VO2(max.)]. At rest and after exercise at 100% VO2(max.), muscle NADH content was significantly higher (P less than 0.05) in type I than in type II fibres. After exercise at 40% VO2(max.), muscle NADH decreased in type I fibres (P less than 0.01), but was not significantly changed in type II fibres. After exercise at 75 and 100% VO2(max.), muscle NADH increased above the value at rest in both type I and II fibres (P less than 0.05). Muscle lactate was unchanged at 40% VO2(max.), but increased 20- and 60-fold after exercise at 75 and 100% VO2(max.) respectively. The finding that NADH decreased only in type I fibres at 40% VO2(max.) supports the idea that type I is the fibre type predominantly recruited during low-intensity exercise. The increase of NADH in both fibre types after exercise at 75% and 100% VO2(max.) suggests that the availability of oxygen relative to the demand is decreased in both fibre types at high exercise intensities.     

Leg exercise conditioning increases peak forearm blood flow.

To examine whether forearm vascular adaptations could occur after upright-leg exercise training, the reactive hyperemic blood flow after 10 min of forearm circulatory arrest (RHBF10) was studied. RHBF10 was examined in seven subjects before, at 2 wk, and after the completion of 4 wk of bicycle ergometer training. Maximal O2 consumption (VO2max) for leg ergometer work increased 13% (P less than 0.05) over 4 wk. Over that period of time RHBF10 in the forearm increased 50% (P less than 0.05), with a reciprocal drop in minimum vascular resistance. Resting heart rate decreased 15% (P less than 0.05) during the same period. Changes in RHBF10 and VO2max were noted after 2 wk of training. Mean arterial pressure did not change. We conclude that vascular adaptations can occur in the forearm muscle beds, even though the training regimen is designed to condition the lower extremities.     

Finger and forearm vasodilatatory changes after local cold acclimation

To determine the vascular changes induced by local cold acclimation, post-ischaemia and exercise vasodilatation were studied in the finger and the forearm of five subjects cold-acclimated locally and five non-acclimated subjects. Peak blood flow was measured by venous occlusion plethysmography after 5 min of arterial occlusion (PBFisc), after 5 min of sustained handgrip at 10% maximal voluntary contraction (PBFexe), and after 5 min of both treatments simultaneously (PBFisc + exe). Each test was performed at room temperature (25 degrees C, SE 1 C) (non-cooled condition) and after 5 min of 5 degrees C cold water immersion (cooled condition). After the cold acclimation period, the decrease in skin temperature was more limited in the cold-acclimated compared to the non-acclimated (P less than 0.01). The PBFisc was significantly reduced in the cooled condition only in the cold-acclimated subjects (finger: 8.4 ml.100 ml-1.min-1, SE 1.1, P less than 0.01; forearm: 5.8 ml.100 ml-1.min-1, SE 1.5, P less than 0.01) compared to the non-cooled condition. Forearm PBFexe was significantly decreased in the cooled condition only in the cold-acclimated subjects (non-cooled: 7.4 ml.100 ml-1.min-1, SE 1.2; cooled: 3.9 ml.100 ml-1.min-1, SE 2.6, P less than 0.05) indicating that muscle blood flow was also reduced.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of one-legged exercise on the strength, power and endurance of the contralateral leg. A randomized, controlled study using isometric and concentric isokinetic training.

The purpose of this investigation was to study the effect of one-legged exercise on the strength, power and endurance of the contralateral leg. The performance of the knee extensor and flexor muscle of 20 healthy young adults (10 men and 10 women) was first tested by Cybex II+ and 340 dynamometers. Then 10 subjects were chosen at random to train using one leg three times a week for 7 weeks whilst the other 10 served as controls. During the 8th week, the tests were repeated. Both quadriceps and hamstring muscles of the trained subjects showed a cross-transfer effect from the trained limb to the untrained side. This concerned the strength and power, as well as endurance characteristics of these muscles. The average change in peak torque of the quadriceps muscle was +19% (P less than 0.001) in the trained limb, +11% (P less than 0.01) in the untrained limb and 0% in the control limbs. In hamstring muscles the changes were +14% (P less than 0.01), +5% and -1%, respectively. Concerning muscle endurance (work performed during the last 5 contractions in the 25-repetition test) the corresponding changes were +15% (P less than 0.01), +7% (P less than 0.01), and -1% in quadriceps muscle, and +17% (P less than 0.05), +7%, and -3% in hamstring muscles. The average strength benefit in the untrained limb was +36% (hamstring muscles) and +58% (quadriceps muscle) of that achieved in the trained limb. Untrained hamstring muscle showed better benefits in the endurance parameters than in strength or power parameters, while in the quadriceps muscle this effect was reversed. A positive relationship was observed between the changes (greater improvement in the trained limb resulted in greater improvement in the untrained limb) (hamstring muscles: r = 0.83, P less than 0.001, quadriceps muscle: r = 0.53, P less than 0.001). In endurance parameters, this relationship was almost linear while in the strength and power parameters the results were more in favour of a curvilinear relationship with limited benefit.     

Forearm blood flow responses to fatiguing isometric contractions in women and men

Previous studies suggest that women experience less vascular occlusion than men when generating the same relative contractile force. This study examined forearm blood flow (FBF) in women and men during isometric handgrip exercise requiring the same relative force. Thirty-eight subjects [20 women and 18 men, 22.8 +/- 0.6 yrs old (means +/- SE)] performed low- and moderate-force handgrip exercise on two occasions. Subjects performed five maximum voluntary contractions (MVC) before exercise to determine 20% and 50% MVC target forces. Time to task failure (TTF) was determined when the subject could not maintain force within 5% of the target force. Mean blood velocity was measured in the brachial artery with the use of Doppler ultrasonography. Arterial diameter was measured at rest and used to calculate absolute FBF (FBFa; ml/min) and relative FBF (FBFr; ml.min(-1).100 ml(-1)). Women generated less (P < 0.05) absolute maximal force (208 +/- 10 N) than men (357 +/- 17 N). The TTF was longer (P < 0.05) at 20% MVC for women (349 +/- 32 s) than for men (230 +/- 23 s), but no difference between the sexes was observed at 50% MVC (women: 69 +/- 5 s; men: 71 +/- 8 s). FBFa and FBFr increased (P < 0.05) from rest to TTF in both women and men during 20% and 50% MVC trials. FBFr was greater in women than in men at > or =30% TTF during 50% MVC. At exercise durations > or =60% of TTF, FBFa was lower (P < 0.05) in women than in men during handgrip at 20% MVC. Despite the longer exercise duration for women at the lower contraction intensity, FBFr was similar between the sexes, suggesting that muscle perfusion is matched to the exercising muscle mass independent of sex.     

Mechanical effects of muscle contraction do not blunt sympathetic vasoconstriction in humans

Sympathetic vasoconstrictor responses are blunted in the vascular beds of contracting muscle (functional sympatholysis), but the mechanism(s) have been difficult to elucidate. We tested the hypothesis that the mechanical effects of muscle contraction blunt sympathetic vasoconstriction in human muscle. We measured forearm blood flow (Doppler ultrasound) and calculated the reductions in forearm vascular conductance (FVC) in response to reflex increases in sympathetic activity evoked via lower body negative pressure (LBNP). In protocol 1, eight young adults were studied under control resting conditions and during simulated muscle contractions using rhythmic forearm cuff inflations (20 inflations/min) with cuff pressures of 50 and 100 mmHg with the arm below heart level (BH), as well as 100 mmHg with the arm at heart level (HL). Forearm vasoconstrictor responses (%DeltaFVC) during LBNP were -26 +/- 2% during control conditions and were not blunted by simulated contractions (range = -31 +/- 3% to -43 +/- 6%). In protocol 2, eight subjects were studied under control conditions and during rhythmic handgrip exercise (20 contractions/min) using workloads of 15% maximum voluntary contraction (MVC) at HL and BH (similar metabolic demand, greater mechanical muscle pump effect for the latter) and 5% MVC BH alone and in combination with superimposed forearm compressions of 100 mmHg (similar metabolic demand, greater mechanical component of contractions for the latter). The forearm vasoconstrictor responses during LBNP were blunted during 15% MVC exercise with the arm at HL (-1 +/- 3%) and BH (-2 +/- 3%) compared with control (-25 +/- 3%; both P < 0.005) but were intact during both 5% MVC alone (-24 +/- 4%) and with superimposed compressions (-23 +/- 4%). We conclude that mechanical effects of contraction per se do not cause functional sympatholysis in the human forearm and that this phenomenon appears to be coupled with the metabolic demand of contracting skeletal muscle.     

Evidence for impaired skeletal muscle contraction-induced rapid vasodilation in aging humans

We tested the hypothesis that aging is associated with an impaired contraction-induced rapid vasodilation in healthy adults. We reasoned that employing single contractions of a small muscle mass would allow us to isolate the local rapid vasodilatory responses independent of systemic hemodynamic and sympathetic neural influences on forearm hemodynamics. We measured forearm blood flow (Doppler ultrasound) and arterial blood pressure (Finapres) on a beat-by-beat basis and calculated the changes in forearm vascular conductance (DeltaFVC) in response to forearm contractions in 18 young (24 +/- 1 yr) and 13 older (62 +/- 2 yr) healthy subjects. Single, 1-s dynamic forearm contractions were performed with the experimental arm slightly above heart level at 5, 10, 20, and 40% of the subjects' maximal voluntary contraction (MVC) in random order. In general, muscle contractions evoked a rapid increase in FVC that reached a peak within approximately four to five cardiac cycles postcontraction in both age groups. At 5% MVC, there were no significant age-related differences in contraction-induced forearm vasodilation. However, the peak vasodilatory responses were impaired approximately 40-45% in older adults at 10, 20, and 40% MVC, as were the total vasodilatory responses (area under curve approximately 40-50%; all P < 0.05). Additionally, the immediate vasodilation (first cardiac cycle postcontraction) for the 20% and 40% MVC trials was also impaired approximately 50% with age (P < 0.05). There were no significant age-group differences in MVC or forearm fat-free mass, and these variables were not correlated with local vasodilation within a given exercise intensity. Under the experimental conditions employed, the blunted responses with age reflect impaired local contraction-induced rapid vasodilation.     

Effect of arterial oxygenation on quadriceps fatigability during isolated muscle exercise

The effect of various levels of oxygenation on quadriceps muscle fatigability during isolated muscle exercise was assessed in six male subjects. Twitch force (Q(tw)) was assessed using supramaximal magnetic femoral nerve stimulation. In experiment 1, maximal voluntary contraction (MVC) and Q(tw) of resting quadriceps muscle were measured in normoxia [inspired O(2) fraction (Fi(O(2))) = 0.21, percent arterial O(2) saturation (Sp(O(2))) = 98.4%, estimated arterial O(2) content (Ca(O(2))) = 20.8 ml/dl], acute hypoxia (Fi(O(2)) = 0.11, Sp(O(2)) = 74.6%, Ca(O(2)) = 15.7 ml/dl), and acute hyperoxia (Fi(O(2)) = 1.0, Sp(O(2)) = 100%, Ca(O(2)) = 22.6 ml/dl). No significant differences were found for MVC and Q(tw) among the three Fi(O(2)) levels. In experiment 2, the subjects performed three sets of nine, intermittent, isometric, unilateral, submaximal quadriceps contractions (62% MVC followed by 1 MVC in each set) while breathing each Fi(O(2)). Q(tw) was assessed before and after exercise, and myoelectrical activity of the vastus lateralis was obtained during exercise. The percent reduction of twitch force (potentiated Q(tw)) in hypoxia (-27.0%) was significantly (P < 0.05) greater than in normoxia (-21.4%) and hyperoxia (-19.9%), as were the changes in intratwitch measures of contractile properties. The increase in integrated electromyogram over the course of the nine contractions in hypoxia (15.4%) was higher (P < 0.05) than in normoxia (7.2%) or hyperoxia (6.7%). These results demonstrate that quadriceps muscle fatigability during isolated muscle exercise is exacerbated in acute hypoxia, and these effects are independent of the relative exercise intensity.     

Autonomic mediation of the pressor responses to isometric exercise in humans

The purpose of this study was to determine the respective contributions of tachycardia and increases in sympathetic nerve activity (SNA) in mediating the pressor responses to fatiguing vs. nonfatiguing levels of isometric handgrip exercise (IHE) in humans. We performed direct (microneurographic) measurements of muscle SNA from the right peroneal nerve in the leg and recorded arterial pressure (AP) and heart rate (HR) in eight healthy subjects before (control), during, and after 2.5 min of IHE at 15, 25, or 35% of maximal voluntary contraction (MVC). At 15% MVC, AP increased during the initial 1.5 min of IHE (7 mmHg, P less than 0.05) and remained at this level; at 25 and 35% MVC, AP increased throughout IHE (22 and 34 mmHg vs. control, respectively, P less than 0.05). HR increased during the initial 1.5 min of IHE at all three levels (5, 12, and 19 beats/min, respectively, P less than 0.05) but did not increase further over the last minute. At 15% MVC, muscle SNA did not increase above control; during 25 and 35% MVC, muscle SNA did not increase during the 1st min of IHE but increased progressively thereafter (109 and 205% vs. control, respectively, P less than 0.05). The magnitudes of the average increases in AP and muscle SNA over the last minute of IHE were directly related (r = 0.99, P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)     

Inflection points of cardiovascular responses and oxygenation are correlated in the distal but not the proximal portions of muscle during incremental exercise.

To test whether there is a regional difference in the exercise pressor reflex within a given muscle, we investigated the relationship between the inflection points of cardiovascular responses and muscle oxygenation during exercise. Seven subjects performed incremental exercise, which consisted of incremental 30-s static knee extensions, each separated by 30 s of recovery. The workload started at 5% maximal voluntary contraction (MVC) and increased by 5% MVC for each increment until exhaustion. Changes (Delta) in the concentrations (denoted by brackets) of oxygenated Hb (O2Hb) and deoxygenated Hb (HHb) were monitored in proximal and distal portions of the vastus lateralis by near-infrared spectroscopy. The inflection points of mean arterial pressure (MAP), calf vascular resistance (CVR), and muscle deoxygenation index (Delta[O2Hb-HHb]) were calculated as the intersection point of two regression equations obtained at lower and higher workloads. The inflection point of Delta[O2Hb-HHb] differed significantly between proximal and distal portions (28.5 +/- 3.0 vs. 39.5 +/- 3.0%MVC, P < 0.05). Linear regression analysis showed significant correlations between the inflection point of Delta[O2Hb-HHb] in the distal portion and MAP (r = 0.89; P < 0.01) and CVR (r = 0.89; P < 0.05), but no significant relationship between the inflection point in the proximal portion and MAP or CVR. These data show that the inflection point of muscle deoxygenation differs between proximal and distal portions within the vastus lateralis during incremental exercise and suggest that the distal portion of the vastus lateralis contributes more to the pressor response than does the proximal portion.     

Abnormal high-energy phosphate metabolism in human muscle phosphofructokinase deficiency.

We studied the pattern of high-energy phosphate metabolism in five patients with phosphofructokinase deficiency (PFKD) and five healthy subjects (HS) during graded rhythmic handgrip performed for 5 min at 17, 33, 50, and 100% of maximal voluntary contraction (MVC). The range of MVC was similar in both groups. Force production was recorded, and intracellular concentrations of phosphorus compounds and pH were measured in the flexor digitorum profundus of the active forearm. At exercise intensities greater than or equal to 50% MVC, changes in concentrations of high-energy phosphate metabolites were abnormal in PFKD. During maximal effort, [ADP], calculated from the creatine kinase reaction, was 64.3 +/- 13.5 (SE) mumol/kg in PFKD vs. 25.7 +/- 4.0 in HS (P less than 0.05). Ammonia (NH3), a product of AMP deamination and an index of muscle [AMP], increased approximately twofold more in venous effluent during maximal forearm exercise in PFKD than in HS (P less than 0.05). Phosphocreatine concentration was 9.4 +/- 1.3 (SE) mmol/kg in HS and 13.0 +/- 1.7 in PFKD (P less than 0.05). Inorganic phosphate concentration was 15.8 +/- 1.4 mmol/kg in HS and 7.4 +/- 0.5 in PFKD (P less than 0.05). During strenuous exercise, PFKD patients exhibit an impairment in the rephosphorylation of ADP related to a subnormal oxidative capacity, an absence of glycolysis, and an attenuated breakdown of phosphocreatine.     

Recovery of the human biceps electromyogram after heavy eccentric, concentric or isometric exercise.

Five men performed submaximal isometric, concentric or eccentric contractions until exhaustion with the left arm elbow flexors at respectively 50%, 40% and 40% of the prefatigued maximal voluntary contraction force (MVC). Subsequently, and at regular intervals, the surface electromyogram (EMG) during 30-s isometric test contractions at 40% of the prefatigued MVC and the muscle performance parameters (MVC and the endurance time of an isometric endurance test at 40% prefatigued MVC) were recorded. Large differences in the surface EMG response were found after isometric or concentric exercise on the one hand and eccentric exercise on the other. Eccentric exercise evoked in two of the three EMG parameters [the EMG amplitude (root mean square) and the rate of shift of the EMG mean power frequency (MPF)] the greatest (P less than 0.001) and longest lasting (up to 7 days) response. The EMG response after isometric or concentric exercise was smaller and of shorter duration (1-2 days). The third EMG parameter, the initial MPF, had already returned to its prefatigued value at the time of the first measurement, 0.75 h after exercise. The responses of EMG amplitude and of rate of MPF shift were similar to the responses observed in the muscle performance parameters (MVC and the endurance time). Complaints of muscle soreness were most frequent and severe after the eccentric contractions. Thus, eccentric exercise evoked the greatest and longest lasting response both in the surface EMG signal and in the muscle performance parameters.     

Age and regional specificity of peak limb vascular conductance in men.

Because of methodological variation in previous studies, age-associated changes in peak limb vascular conductance (VC(peak); a functional index of arterial structure) and its determinants remain poorly defined. The objectives of this study were to describe and compare age-associated changes in peak forearm and calf conductance across a broad age range and to identify physiological characteristics that are predictive of variation in limb-specific VC(peak). Peak conductance (plethysmographic flow/brachial mean arterial pressure) of the forearm (forearm VC(peak)) and calf (calf VC(peak)) after 10 min of arterial occlusion was measured twice in 68 healthy, normally active men aged 20-79 yr. Aerobic capacity (cycle peak oxygen consumption), arterial health (ankle-brachial index, pulse wave velocity), and limb-specific measures of muscle mass (dual-energy X-ray absorptiometry) and isometric strength (grip, plantar flexion) were also assessed. The relative decline in forearm VC(peak) with age (-6.6% per decade; P < 0.001) was greater than the decline in calf VC(peak) (-3.4% per decade; P = 0.004). Limb VC(peak) per kilogram of muscle declined with age in the forearm (-3.8% per decade; P = 0.004) but not in the calf (P = 0.35). Age, Vo(2 peak), and regional muscle mass were significant predictors of peak conductance in both limbs; however, these predictors explained considerably less variance in the calf than in the forearm. These results suggest that healthy aging is associated with a linear decline in limb vasodilator capacity in men, but the magnitude of this effect is reduced in the calf relative to the forearm. This could reflect regional differences in habitual muscle use with aging in normally active men.     

Neuromuscular fatigue during a long-duration cycling exercise.

The effects of prolonged cycling on neuromuscular parameters were studied in nine endurance-trained subjects during a 5-h exercise sustained at 55% of the maximal aerobic power. Torque during maximal voluntary contraction (MVC) of the quadriceps muscle decreased progressively throughout the exercise (P < 0.01) and was 18% less at the end of exercise compared with the preexercise value. Peak twitch torque, contraction time, and total area of mechanical response decreased significantly (P < 0.05) after the first hour of exercise. In contrast, changes in M-wave characteristics were significant only after the fourth hour of the exercise. Significant reductions (P < 0.05) in electromyographic activity normalized to the M wave occurred after the first hour for the vastus lateralis muscle but only at the end of the exercise for the vastus medialis muscle. Muscle activation level, assessed by the twitch interpolation technique, decreased by 8% (P < 0.05) at the end of the exercise. The results suggest that the time course is such that the contractile properties are significantly altered after the first hour, whereas excitability and central drive are more impaired toward the latter stages of the 5-h cycling exercise.     

The relationship between creatine kinase kinetics and exercise intensity in human forearm is unchanged by age

Using (31)P magnetic resonance spectroscopy, creatine kinase (CK) reaction kinetics was assessed in the forearm flexor digitorum profundus muscle of healthy young (n = 11, age 34.7 +/- 5 yr) and older (n = 20, age 73.5 +/- 8 yr) subjects at rest, intermittent exercise at 20% maximum voluntary contraction (MVC), and 40% MVC. Exercise resulted in a significant increase in the average ratio of inorganic phosphate (P(i)) to phosphocreatine (PCr) from resting values of 0.073 +/- 0.031 (young) and 0.082 +/- 0.037 (older) to 0. 268 +/- 0.140 (young, P < 0.01) and 0.452 +/- 0.387 (older, P < 0. 01) at 40% MVC. At 40% MVC, intracellular pH decreased significantly, from resting values of 7.08 +/- 0.08 (young) and 7.08 +/- 0.11 (older) to 6.84 +/- 0.19 (young, P < 0.05) and to 6.75 +/- 0.25 (older, P < 0.05). Average values of the pseudo-first-order reaction rate k((PCr-->ATP)) at rest were 0.07 +/- 0.04 s(-1) in the young and 0.07 +/- 0.03 s(-1) in the older group. At both exercise levels, the reaction rate constant increased compared with the resting value, but only the difference between the resting value and the 20% MVC value, which showed an 86% higher reaction rate constant in both groups, reached statistical significance (P < 0.05). No difference in the reaction rate constant between the young and older groups was observed at either exercise level. As with k((PCr-->ATP)), the average phosphorus flux through the CK reaction increased during exercise at 20% MVC (P < 0.05 in the older group) but decreased toward resting values at 40% MVC in both groups. The data in our study suggest that normal aging does not significantly affect the metabolic processes associated with the CK reaction.