The influence of straining maneuvers on the pressor response during isometric exercise

Experiments were performed to determine to what extent increments in esophageal and abdominal pressure would have on arterial blood pressure during fatiguing isometric exercise. Arterial blood pressure was measured during handgrip and leg isometric exercise performed with both a free and occluded circulation to active muscles. Handgrip contractions were exerted at 33 and 70% MVC (maximum voluntary contraction) by 4 volunteers in a sitting position and calf muscle contractions at 50 and 70% MVC with the subjects in a kneeling position. Esophageal pressure measured at the peak of inspirations did not change during either handgrip or leg contractions but peak expiratory pressures increased progressively during both handgrip and leg contractions as fatigue occurred. These increments were independent of the tensions of the isometric contractions exerted. Intra-abdominal pressures measured at the peak of either inspiration or expiration did not change during inspiration with handgrip contractions but increased during expiration. During leg exercise, intraabdominal pressures increased during both inspiration and expiration, reaching peak levels at fatigue. The arterial blood pressure also reached peak levels at fatigue, independent of circulatory occlusion and tension exerted, averaging 18.5-20 kPa (140-150 mm Hg) for both handgrip and leg contractions. While blood pressure returned to resting levels following exercise with a free circulation, it declined by only 2.7-3.8 kPa after leg and handgrip exercise, respectively, during circulatory occlusion. These results indicate that straining maneuvers contribute 3.5 to 7.8 kPa to the change in blood pressure depending on body position.     

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)     

The effects of athletic training and muscle contractile character on the pressor response to isometric exercise of the human triceps surae

In this study, the influence of athletic training status and the contractile character of the active muscle on the magnitude of the pressor response (PR) to voluntary and electrically evoked isometric plantar flexion was investigated. Subjects were 10 sprint-trained athletes (sprint) (100-m, 200-m and 400-m) [mean (SD) age, 21 (2) years], 14 endurance trained athletes (distance) [22 (2) years] and 8 untrained men (control) [23 (3) years]. Twitch time to peak tension (TPT) in the sprint group [108 (7) ms] was significantly less (P<0.001) than that of the distance group [124 (10) ms]. During voluntary contraction, the mean change in systolic blood pressure (SBP), diastolic blood pressure (DBP) and heart rate (fc) was not significantly different between groups. During electrically evoked contractions, mean changes in SBP, DBP and fc were not significantly different between the sprint, distance and control groups. However, division of the sprint group into 400-m (sprint I) and 100/200-m athletes (sprint II) showed that an increase in DBP of 1.6 kPa (12 mm Hg) in sprint I was significantly less (P<0.05) than the 2.5 kPa (19 mm Hg) increase observed for both the distance and control groups. Prediction of the DBP response from our previously published relationship between TPT and DBP showed close agreement in all subject groups except sprint I; in these subjects the observed DBP response was only 55% of that predicted. Attenuation of the PR in the involuntary experiment suggests that some aspect of sprint training, but not endurance training, modifies the muscle afferent input to the PR in man.     

Resistance-training experience and the pressor response during resistance exercise

The purpose of this study was to examine the effects previous resistance-training experience has on the cardiovascular responses to resistance-training exercises. To investigate this, the intra-arterial blood pressure response of four body builders (BB), six novice weight-trained individuals (NT), and six sedentary controls (SC) were monitored during performance of one-arm overhead presses and one-leg knee extensions. One repetition at the maximal weight possible (1 RM) and sets to voluntary fatigue at 90, 80, 70, and 50% of 1 RM were performed. Across groups, the BB demonstrated a significantly (P less than 0.05) lower peak and a lesser magnitude of response (changes from rest to peak) for systolic and diastolic blood pressures than the the NT and SC groups during both exercises. The BB also demonstrated significantly lower values across groups for peak heart rate and magnitude of heart rate response during arm presses. During knee extensions across groups, peak heart rate but not magnitude of the heart rate response was significantly lower in the BB. The results indicate that previous weight-training experience reduces the pressor response to dynamic resistance exercises.     

The pressor response to involuntary isometric exercise of young and elderly human muscle with reference to muscle contractile characteristics

Changes in heart rate (f _c) and blood pressure (BP) were observed in eight healthy young men aged [mean (SD)] 20(l) years and ten healthy elderly men aged 65 (5) years, during electrically evoked contractions of the ankle plantar flexors and elbow flexors which were sustained for 2 min. There was no significant difference in the f _c response to evoked contraction of the ankle plantar flexors or elbow flexors between young and elderly subjects. During contraction of the elbow flexors, elderly subjects produced an unexpectedly large rise in systolic BP which was significantly greater than that of the young subjects. The exaggerated response seen in the elderly group may be due to a more rigid arterial tree which is thought to occur with advancing age. Electrically evoked contraction of the slower contracting elderly ankle plantar flexors resulted in a significantly diminished diastolic BP response when compared with that of the young subjects. In contrast, during electrically evoked contraction of the elbow flexors, which showed a similar twitch time course in young and elderly subjects, the diastolic BP response was not significantly different between groups. This may reflect differences in the peripheral reflex input to the pressor response in elderly arm and leg muscles which, in turn, may be influenced by relative fast twitch fibre area.     

The pressor response to voluntary and electrically evoked isometric contractions in man

Blood pressure and heart rate changes during sustained isometric exercise were studied in 11 healthy male volunteers. The responses were measured during voluntary and involuntary contractions of the biceps brachii at 30% of maximal voluntary contraction (MVC), and the triceps surae at 30% and 50% MVC. Involuntary contractions were evoked by percutaneous electrical stimulation of the muscle. Measurements of the time to peak tension of maximal twitch showed the biceps brachii (67.0 +/- 7.9 ms) muscle to be rapidly contracting, and the triceps surae (118.0 +/- 10.5 ms) to be slow contracting. The systolic and diastolic blood pressures increased linearly throughout the contractions, and systolic blood pressure increased more rapidly than diastolic. There was no significant difference in response to stimulated or voluntary contractions, nor was there any significant difference between the responses to contractions of the calf or arm muscles at the same relative tension. In contrast the heart rate rose to a higher level (P less than 0.01) in the biceps brachii than the triceps surae at given % MVC, and during voluntary compared with the electrically evoked contractions in the two muscle groups. It was concluded that the arterial blood pressure response to isometric contractions, unlike heart rate, is primarily due to a reflex arising within the active muscles (cf. Hultman and Sj?holm 1982) which is associated with relative tension but independent of contraction time and muscle mass.     

Blunted pressor and intramuscular metabolic responses to voluntary isometric exercise in multiple sclerosis.

To test the hypothesis that a lower mean arterial pressure (MAP) response during voluntary isometric exercise in multiple sclerosis (MS) is related to a dampened muscle metabolic signal, 9 MS and 11 control subjects performed an isometric dorsiflexor contraction at 30% maximal voluntary contraction until target failure (endurance time). We made continuous and noninvasive measurements of heart rate and MAP (Finapres) and of intramuscular pH and P(i) (phosphorus magnetic resonance spectroscopy) in a subset of 6 MS and 10 control subjects. Endurance times and change in heart rate were similar in MS and control subjects. The decrease in pH and increase in P(i) were less throughout exercise in MS compared with control subjects, as was the change in MAP response. Differences in muscle strength accounted for some of the difference in MAP response between groups. Cardiovascular responses during Valsalva and cold pressor tests were similar in MS and control subjects, suggesting that the blunted MAP response during exercise in MS was not due to a generalized dysautonomia. The dampened metabolic response in MS subjects was not explained by inadequate central muscle activation. These data suggest that the blunted pressor response to exercise in MS subjects may be largely appropriate to a blunted muscle metabolic response and differences in contracting muscle mass.     

The combined effect of the cold pressor test and isometric exercise on heart rate and blood pressure

The purpose of this study was to determine if the cold pressor test during isometric knee extension [15% of maximal voluntary contraction (MVC)] could have an additive effect on cardiovascular responses. Systolic and diastolic blood pressures, heart rate and pressure rate product were measured in eight healthy male subjects. The subjects performed the cold pressor tests and isometric leg extensions singly and in combination. The increases of systolic and diastolic blood pressure during isometric exercise were of almost the same magnitude as those during the cold pressor test. The responses of arterial blood pressure, and heart rate to a combination of the cold pressor test and isometric knee extension were greater than for each test separately. It is suggested that this additional effect of cold immersion of one hand during isometric exercise may have been due to vasoconstriction effects in the contralateral unstressed limb. In summary, the circulatory effects of the local application of cold during static exercise at 15% MVC were additive.     

Bed rest attenuates sympathetic and pressor responses to isometric exercise in antigravity leg muscles in humans

Although spaceflight and bed rest are known to cause muscular atrophy in the antigravity muscles of the legs, the changes in sympathetic and cardiovascular responses to exercises using the atrophied muscles remain unknown. We hypothesized that bed rest would augment sympathetic responses to isometric exercise using antigravity leg muscles in humans. Ten healthy male volunteers were subjected to 14-day 6 degrees head-down bed rest. Before and after bed rest, they performed isometric exercises using leg (plantar flexion) and forearm (handgrip) muscles, followed by 2-min postexercise muscle ischemia (PEMI) that continues to stimulate the muscle metaboreflex. These exercises were sustained to fatigue. We measured muscle sympathetic nerve activity (MSNA) in the contralateral resting leg by microneurography. In both pre- and post-bed-rest exercise tests, exercise intensities were set at 30 and 70% of the maximum voluntary force measured before bed rest. Bed rest attenuated the increase in MSNA in response to fatiguing plantar flexion by approximately 70% at both exercise intensities (both P < 0.05 vs. before bed rest) and reduced the maximal voluntary force of plantar flexion by 15%. In contrast, bed rest did not alter the increase in MSNA response to fatiguing handgrip and had no effects on the maximal voluntary force of handgrip. Although PEMI sustained MSNA activation before bed rest in all trials, bed rest entirely eliminated the PEMI-induced increase in MSNA in leg exercises but partially attenuated it in forearm exercises. These results do not support our hypothesis but indicate that bed rest causes a reduction in isometric exercise-induced sympathetic activation in (probably atrophied) antigravity leg muscles.     

Carotid baroreflex pressor responses at rest and during exercise: cardiac output vs. regional vasoconstriction

The arterial baroreflex mediates changes in arterial pressure via reflex changes in cardiac output (CO) and regional vascular conductance, and the relative roles may change between rest and exercise and across workloads. Therefore, we quantified the contribution of CO and regional vascular conductances to carotid baroreflex-mediated increases in mean arterial pressure (MAP) at rest and during mild to heavy treadmill exercise (3.2 kph; 6.4 kph, 10% grade; and 8 kph, 15% grade). Dogs (n = 8) were chronically instrumented to measure changes in MAP, CO, hindlimb vascular conductance, and renal vascular conductance in response to bilateral carotid occlusion (BCO). At rest and at each workload, BCO caused similar increases in MAP (average 35 +/- 2 mmHg). In response to BCO, neither at rest nor at any workload were there significant increases in CO; therefore, the pressor response occurred via peripheral vasoconstriction. At rest, 10.7 +/- 1.4% of the rise in MAP was due to vasoconstriction in the hindlimb, whereas 4.0 +/- 0.7% was due to renal vasoconstriction. Linear regression analysis revealed that, with increasing workloads, relative contributions of the hindlimb increased and those of the kidney decreased. At the highest workload, the decrease in hindlimb vascular conductance contributed 24.3 +/- 3.4% to the pressor response, whereas the renal contribution decreased to only 1.6 +/- 0.3%. We conclude that the pressor response during BCO was mediated solely by peripheral vasoconstriction. As workload increases, a progressively larger fraction of the pressor response is mediated via vasoconstriction in active skeletal muscle and the contribution of vasoconstriction in inactive beds (e.g., renal) becomes progressively smaller.     

The influence of exercise/rest schedule on the physiological and psychophysical response to isometric shoulder-neck exercise

Six female subjects, aged 24-34 years, performed shoulder-neck exercise for 1 h or until they were exhausted by holding out their arms horizontally at 60° to the sagittal plane. One continuous and six intermittent protocols were applied, all with a mean load corresponding to the torque of the arms, i.e. about 15% maximal voluntary contraction (MVC). The intermittent protocols varied according to cycle time (10 s, 60 s, 360 s) and duty cycle (0.33, 0.50, 0.67, 0.83). Electromyogram (EMG), mean arterial blood pressure ( _a), heart rate (f _c) and perceived fatigue were monitored at regular intervals during exercise. Blood concentrations of potassium, lactate and ammonia were determined in pre- and postexercise samples of venous blood. Before and up to 4 h after exercise, measurements were made of MVC, pressure pain threshold, proprioceptive performance, and of EMG, _a and f _c during 1-min arm-holding at 25% MVC. Endurance times ranged from about 10 min to more than 1 h, significantly relating to both cycle time and duty cycle. The _a, f _c EMG amplitude and perceived fatigue increased early during all protocols and continued to increase throughout the exercise period. Duty cycle influenced all of these variables, while only _a and fatigue perception were related to cycle time. Cardiovascular and neuromuscular recovery was incomplete for hours after several of the protocols, as indicated for example by a sensitizised response to the 1-min armholding. The protocols differed substantially as regards the relationship between different responses. Thus, ranking of the protocols in terms of physiological strain was different, depending on the criterion variable. The result stresses the relevance of applying a comprehensive selection of variables when evaluating the responses to intermittent shoulder-neck exercise.     

Cardiovascular response to hand-grip isometric exercise in healthy men. Noninvasive study

Noninvasive polygraphic tracings obtained at rest and during isometric hand-grip exercise were analysed in 67 healthy subjects. The purpose of the study was to determine the response of noninvasive polygraphic parameters to isometric exercise. During the third minute of sustained squeezing of a balloon dynamometer (30% of maximal voluntary contraction) a significant increase occurred in heart rate (+16.8 +/- 10.7 beats/min) an increase in both systolic and diastolic blood pressure (+3.4 +/- 1.6 kPa and 2.6 +/- 1.7 kPa respectively), increase in apexcardiographic index 100.a/D (+14.5 +/- 15.0% "D" amplitude), decrease of diastolic amplitude time index square root 2-c/(2-0) X (a/D) (-20.1 +/- 26.5), shortening of pulse transmission time (-0.006 +/- 0.005 s) and prolongation of cardiac cycle length corrected for left ventricular ejection time (+0.011 +/- 0.010 s) discussed. All these changes were statistically significant.     

Endogenous endothelin attenuates the pressor response to acute environmental stress via the ETA receptor

Clinical studies have documented an abrupt rise in plasma endothelin-1 (ET-1) coincident with an increase in mean arterial pressure (MAP) during the response to acute stress. We therefore examined the ET(A) and ET(B) receptor-dependent effects of ET-1 on the pressor response to acute environmental stress in ET-1-dependent hypertension. Stress was induced by administration of air jet pulses (3 min) in ET(B) receptor-deficient (ET(B) sl/sl) rats fed normal salt (NS; 0.8% NaCl), high salt (HS; 8% NaCl), and HS plus the ET(A) receptor antagonist ABT-627 (5 mg.kg(-1).day(-1)) on successive weeks. MAP was chronically monitored by telemetry. Total pressor response (area under the curve) was significantly reduced in ET(B) sl/sl rats maintained on a HS vs. NS diet [-6.8 mmHg (SD 18.7) vs. 29.3 mmHg (SD 8.1) x 3 min, P < 0.05]. Conversely, the total pressor response was augmented in both wild-type [34.2 mmHg (SD 29.2) x 3 min, P < 0.05 vs. NS] and ET(B) sl/sl rats [49.1 mmHg (SD 11.8) x 3 min, P < 0.05 vs. NS] by ABT-627. Blockade of ET(B) receptors in Sprague-Dawley rats caused an increase in basal MAP that was enhanced by HS and lowered by mixed ET(A)/ET(B) receptor antagonism; none of these treatments, however, had any effect on the pressor response. These data demonstrate that increasing endogenous ET-1 suppresses the pressor response to acute stress through ET(A) receptor activation in a genetic model of ET-1-dependent hypertension. These results are consistent with reports that ET-1 can attenuate sympathetically mediated responses.     

Carotid distensibility, baroreflex sensitivity, and orthostatic stress.

In this study, we tested the hypothesis that carotid arteries undergo rapid changes in distensibility on moving from the supine to head-up tilt (HUT) postures and, subsequently, that this change in carotid distensibility (cDa) might be associated with concurrent reductions in cardiovagal baroreflex sensitivity (BRS). Thus the effect of posture on carotid vascular mechanics and cardiovagal BRS with consideration for altered central hemodynamics (i.e., stroke volume; Doppler ultrasound) was examined. Carotid pulse pressure (cPP; Millar transducer) and contralateral B-mode ultrasound images were assessed at the carotid artery during supine and 60 degrees HUT postures. From these measures, cDa was calculated at 5-mmHg pressure increments experienced during the cardiac cycle (n = 6). cPP (n = 9) was not different in the two postures. A smaller stroke volume being ejected into a smaller carotid artery in HUT explained the maintenance of cPP in HUT. Also, compared with supine, cDa was reset to a lower level in HUT (main effect of posture; P < 0.05). Cardiovagal BRS (sequence method) was diminished in HUT vs. supine (P < 0.05). A positive correlation was observed between the tilt-induced changes in maximal cDa (in early systole) and cardiovagal BRS (r2 = 0.75; P < 0.05), but there was little predictive relationship between changes in cPP, systolic vessel dimensions, or average cDa and the corresponding change in BRS. The present results indicate that HUT elicits rapid changes in carotid artery mechanics and further suggest that reductions in the maximal cDa measured in early systole contribute to reduced cardiovagal BRS with HUT.     

Influence of active recovery following prolonged bed rest on static exercise pressor response

The present study investigated the effect of active recovery, following 35 days of horizontal bed rest, on the magnitude and time course of the pressor and heart rate responses to sustained 90 minute submaximal isometric contraction of unilateral knee extensor muscles. Ten healthy male subjects were tested immediately post bed rest (Post BR) and again after 4 weeks of active recovery (Recovery). In both trials subjects sustained an absolute force equal to 30% of Post BR maximal voluntary contraction (MVC). Beat-to-beat heart rate (HR) and mean arterial blood pressure (MAP) were monitored continuously during sustained contraction using the volume-clamp technique. Despite a 24% increase in MVC, there were no significant differences in the magnitudes of HR and MAP responses between Post BR and Recovery trials, suggesting a bed rest-induced attenuation of the static exercise pressor response.     

Differences in muscle sympathetic nerve response to isometric exercise in different muscle groups

The aim of this study was to examine the effects of muscle fibre composition on muscle sympathetic nerve activity (MSNA) in response to isometric exercise. The MSNA, recorded from the tibial nerve by a microneurographic technique during contraction and following arterial occlusion, was compared in three different muscle groups: the forearm (handgrip), anterior tibialis (foot dorsal contraction), and soleus muscles (foot plantar contraction) contracted separately at intensities of 20%, 33% and 50% of the maximal voluntary force. The increases in MSNA relative to control levels during contraction and occlusion were significant at all contracting forces for handgrip and at 33% and 50% of maximal for dorsal contraction, but there were no significant changes, except during exercise at 50%, for plantar contraction. The size of the MSNA response correlated with the contraction force in all muscle groups. Pooling data for all contraction forces, there were different MSNA responses among muscle groups in contraction forces (P = 0.0001, two-way analysis of variance), and occlusion periods (P = 0.0001). The MSNA increases were in the following order of magnitude: handgrip, dorsal, and plantar contractions. The order of the fibre type composition in these three muscles is from equal numbers of types I and II fibres in the forearm to increasing number of type I fibres in the leg muscles. The different MSNA responses to the contraction of different muscle groups observed may have been due in part to muscle metaboreflex intensity influenced by their metabolic capacity which is related to by their metabolic capacity which is related to the fibre type.