LEADING ARTICLE
Sports Medicine 12 ( I): 1-7. 1991 0112-1642/ 91 /0007-0001 / $03.50/0 © Adis International Limited. All rights reserved. SP0131
Haemodynamic Responses to Weightlifting Exercise David W. Hill and S. Dee Butler Exercise Physiology Laboratory, University of North Texas, Denton, Texas, USA
During dynamic exercise, there is an increased demand for the delivery of oxygenated blood to the working muscles and for the removal of products from these tissues. Increased muscle blood flow is permitted by an increased driving pressure and a decreased local resistance to flow. The magnitude of the haemodynamic response is closely coupled to oxygen uptake. However, during weightlifting exercise, there is dissociation between these metabolic (aerobic) demands and haemodynamic responses. Not only is much of the energy for weightlifting provided by other (anaerobic) mechanisms, but the cardiovascular system is affected by altered autonomic nervous system responses and by mechanical effects associated with the heavy resistance exercise, such as increased vascular resistance in vessels compressed in the active muscles and in the thorax. The purpose of this paper is to describe haemodynamic responses during weightIifting exercise, with attention to differences between these responses and the responses to dynamic/aerobic exercise.
1. Types of Exercise Most studies of haemodynamic responses have differentiated between dynamic and isometric exercise. Weightlifting exercise is not a purely isometric or static activity and, therefore, it is inappropriate to assume that specific responses to isometric exercise necessarily reflect the haemodynamics of weightIifting. Clearly, however, as-
peets of the haemodynamic response to weightlifting do reflect the static component in the activity. Activities must be classified not only in terms of mode of contraction - isotonic versus isometric - but in terms of muscle mass involved, intensity (in terms of metabolic demand and percentage of peak force development), and duration. Activities such as running and cycling are often described as dynamic/isotonic/aerobic/submaximal and yet, especially at higher intensities, have static and anaerobic components. Similarly, traditional methods of weightlifting do not involve only static contraction; weightlifting is a dynamic activity associated with elevated aerobic metabolism, albeit with a significant anaerobic component (Collins et al. 1989; McArdle & Foglia 1969). Discussion of haemodynamics during weightlifting exercise is complicated by the fact that the actual exercise bouts (the 'sets' and, indeed, each repetition within a set) are interspersed with periods of rest. When the rest periods between sets are short, and closely regulated, the training is described as circuit weight-training. In some forms of circuit weight-training, low intensity aerobic exercise is performed between weightIifting sets.
2. Blood Pressure The driving force behind the flow of blood to the active tissues is mean arterial pressure, calculated by integration of pressure over a cardiac cycle, or simply estimated as diastolic pressure plus onethird of the difference between the systolic and dia-
2
Sports Medicine 12 (1) 1991
stolic pressures. Determinants of systolic blood pressure are cardiac output and total peripheral resistance. Heart rate, stroke volume and local vasoconstriction all contribute to the expected increase in mean arterial pressure during exercise. In addition, exercise affects blood viscosity and volume, which plays a minor role in increasing peripheral resistance and decreasing venous return. Thus, cardiovascular responses to dynamic exercise are determined by metabolic demand, associated neurohumoral influences, and reflexes elicited by the activity (Mitchell 1990). 2.1 Dynamic Exercise In dynamic exercise such as walking, jogging, or cycling, the cardiovascular responses are closely tied to the increased oxygen usage. Local vasodilation, along with vasoconstriction in the gut, hepatic and skin vessels, permits an increase in blood flow to the working muscles. The resulting increase in venous return contributes to an increased stroke volume which, coupled with an increase in heart rate, causes an increase in cardiac output (Guyton 1991). Systolic blood pressure rises with the first minute of exercise and levels otT during exercise at about 50 to 75mm Hg above rest in young, healthy males driven by the increased cardiac output necessary to meet the oxygen demand of the activity (an increase of about 5 L/min cardiac output per 1 L/ min increase in oxygen uptake). Diastolic pressure (fourth phase) tends to decrease slightly during walking/running exercise and to remain unchanged or to increase only slightly during cycle ergometry; fifth phase pressure always declines, as far as Omm Hg. Palatini (1988) has recently reviewed the effects of exercise on blood pressure and Mitchell (1990) has reviewed neural control of the circulation in exercise. 2.2 Static Exercise Isometric exercise produces an increase in mean arterial pressure via increases in both diastolic and systolic pressures. Mitchell and colleagues (1977) and Morin and colleagues (1964) attributed much
of the elevation to increased sympathetic activity. Clausen (1977) and Bezucha and colleagues (1982) noted that the increased cardiac output during isometric exercise resulted from increased heart rate, not stroke volume. In fact, stroke volume is virtually unchanged during static contractions (Lewis et al. 1985). The magnitude of the haemodynamic response to isometric contraction is a function of muscle mass (Lewis et al. 1985; Misner et al. 1990; Mitchell et al. 1980; Seals et al. 1983). Lewis and colleagues (1985) reported higher heart rates (134 ± 11 versus 91 ± 4 beats/min) higher cardiac outputs (10.1 ± 0.9 versus 6.8 ± 6mm Hg), higher systolic pressures (193 ± 7 versus 150 ± 6mm Hg), and higher diastolic pressures (114 ± 2 versus 94 ± 4mm Hg) during isometric 2-leg versus handgrip exercise. Misner and colleagues (1990) have even shown that maximal static leg extension with both legs elicits a greater blood pressure response than extension with only one leg. Lind (1983) has argued that the differences in haemodynamic responses to static exercise with different muscle masses could be explained by differences in time to exhaustion. Indeed, pressures do tend to increase during performance of a sustained static contraction (Misner et al. 1990; Seals et al. 1983; 1988). For example, during a sustained contraction at 35% maximal voluntary contraction, mean arterial pressures in 8 men were 110 ± 2mm Hg at 0.5 minutes, 126 ± 3mm Hg at 1.5 minutes, and 132 ± 3mm Hg at 2.5 minutes (Seals et al. 1988). However, Maughan and colleagues (1986) and Nagle and colleagues (1988) have shown that there is no relationship between muscle mass and time to exhaustion at given percentages of maximal voluntary force production. The magnitude of the haemodynamic responses is certainly a function of the percentage of the maximal voluntary force production that is sustained during the activity (Maughan et al. 1986; Seals et al. 1988). Seals and colleagues (1988) evaluated responses to static exercise at 15, 25 and 35% of maximal voluntary contraction. Mean arterial pressures rose during the exercise and, at the 2.5 minute mark, were 106 ± 4, 120 ± 4 and 132 ± 3mm Hg,
Haemodynamic Responses to Weightlifting
respectively. The cardiovascular response to isometric exercise is mediated by both central and peripheral mechanisms (Goodwin et al. 1972; Mitchell 1990; Mitchell et al. 1980). Feedforward central drive (reduced parasympathetic activity) causes tachycardia; chemoreceptor feedback from the active tissues to the medulla (increased sympathetic activity) causes vasoconstriction in vessels serving noncontracting muscles. Unlike during dynamic exercise, where rhythmic contraction of muscles promotes venous return, during sustained static contractions there is mechanical compression of the vessels in the active muscles and a marked increase in local resistance. 2.3 Weightlifting Exercise Weightlifting includes a combination of isotonic/dynamic and isometric/static contractions. MacDougall and colleagues (1985) have noted that the haemodynamic response to weightlifting exercise reflects the nature of the activity - it is a series of essentially static contractions, performed dynamically. Haemodynamic responses vary as a function of muscle mass involved, intensity, workrest interval, age of subjects, duration of exercise, and method of measurement. Wescott and Howes (1983) determined blood pressure responses using auscultatory sphygmomanometry in young « 38 years) and older (> 38 years) individuals performing 10 I-arm biceps curls at various intensities - 10 repetition maximum (RM) [heavy], IORM minus 5 pounds (2.3kg) [moderate] and IORM minus 10 pounds (4.5kg) [light]. Men tended to have higher systolic responses at each relative workload than women, and older individuals had higher systolic values than younger individuals. For all subjects, blood pressure showed a progressive increase from rest to the highest load - 123/15 ± 3/2mm Hg at rest, 143/ 74 ± 5/2mm Hg at light intensity, 151/74 ± 5/ 2mm Hg at moderate intensity and 165/75 ± 5/ 3mm Hg at heavy intensity. Increases over resting systolic pressure were 16, 22 and 34%. Diastolic pressures were unchanged at any workload. It was noted that the diastolic measures were obtained
3
immediately after, not during, each exercise set (see section 4). Freedson and colleagues (1984) used a pressure transducer/femoral artery catheter assembly to compare intra-arterial blood pressure response to free weight and hydraulic resistance bench press exercise. Seven subjects performed 10 repetitions of free weight bench press at each of 25% and 50% of maximal isometric strength. They also performed fast and slow (3 and 5 seconds, respectively) hydraulic resistance bench press. All 4 exercise bouts elicited high peak and mean systolic pressure responses. The magnitude of the response was dependent on the force produced with mean systolic pressures of 190 ± 10 versus 125 ± 7mm Hg during the slow versus fast hydraulic bench press, and 169 ± 7 versus 145 ± 4mm Hg in the 50 versus 25% free weight bench press. However, peak systolic pressure was equally high (232 ± 23 to 245 ± 14mm Hg) in all but the low resistance free weight bench press (169 ± 13mm Hg). Mean diastolic pressures were higher in the 50% free weight and slow hydraulic lifts (l08 ± 6 and 117 ± 2mm Hg, respectively), than in the lighter free and hydraulic lifts (87 ± 5 and 76 ± 2mm Hg, respectively). The same pattern held for peak diastolic pressures, with values of 154 ± 17 and 160 ± 10mm Hg reported in the high resistance free weight and slow movement hydraulic resistance exercises. These results suggest that resistance (force production) is a strong predictor of exercise blood pressure response. In addition, the fact that the high speed (i.e. lower force) hydraulic exercise elicited high peak systolic pressure might suggest that this all-out activity was associated with a Valsalva manoeuvre (see section 5). MacDougall and colleagues (1985) evaluated the blood pressure response of 5 healthy body-builders who performed the I-arm curl, I-leg press and 2leg press at 95% IRM to failure and the 2-leg press at 100% IRM. Intra-arterial pressures were measured using a capacitance transducer connected to a catheter and inserted in the brachial artery. Using this set-up, the investigatofS were able to track blood pressure across each repetition and across each set of repetitions. For all lifts except the lRM,
4
Valsalva manoeuvres were avoided until the point of fatigue was approached. Greatest peak systolic and diastolic pressures occurred during the final repetitions of the 2-leg press at 95% IRM, when the group mean was 320/250mm Hg and I subject attained 480/350mm Hg. Thus, the group mean arterial pressure was calculated to be almost 300mm Hg. Pressures for the 2-leg press at IRM were about 260/200mm Hg; for the I-leg press, about 250/ 190mm Hg, and for the I-arm curl, 230/ 170mm Hg. At the initiation of each lift, both systolic and diastolic pressures rose rapidly and remained at this elevated level for the few seconds required to raise the weight. As the weight was lowered, arterial pressure dropped quickly to pre-exercise levels. Thus, it seems that the concentric phase of the lift elicits a greater blood pressure response than the eccentric component. The investigators concluded that muscle mass is a factor in the observed responses, with the larger masses involved causing increased compression of blood vessels and thus directly increasing blood pressure. However, muscle mass could not have been the main cause, or else the difference between 1- and 2-legged values would have been far greater. The mechanical compression, a pressor reflex and a Valsalva manoeuvre all contributed to the observed responses. In the same laboratory, intra-arterial brachial pressures were determined during weightlifting in cardiac patients. Haslam and colleagues (1987) studied 8 male cardiac patients who performed 5 to 15 repetitions of I-arm curls, I-leg presses, and 2-leg presses at 20, 40, 60 and 80% of their I RM. Blood pressures were a function of intensity and muscle mass involved, with highest values obtained during the 10 repetitions of 2-leg presses at 80% of IRM. These values were 215/124 ± 7/ 6mm Hg, which yield a calculated mean arterial pressure of 139 ± 8mm Hg. Heart rate during this set was about 100 beats/min. In these patients, rate pressure product was highest, 249 (± 12) X 102 beats/min· mm Hg during the 2-leg press at 80% IRM, compared to 221 (± 12) x 102 beats/ min· mm Hg measured at 85% maximal oxygen uptake during a cycle ergometer test. Thus, al-
Sports Medicine 12 (1) 1991
though significant elevations in blood pressure were elicited by the weightlifting, the haemodynamic responses not unlike, but of lesser magnitude than, those reported earlier in bodybuilders. Intra-arterial blood pressures have also been evaluated during isokinetic exercise at various contraction velocities (Haennel et al. 1987). Contraction velocity had no consistent effect on responses. Tachycardia was apparent (heart rate exceeded 125 beats/min at all speeds of contraction) and blood pressures were about 175/108mm Hg at 30 o/sec 194/101mm Hg at 90 o/sec and 187/102mm Hg at ISO o/sec. Kelemen and Stewart (1985) have summarised the potential role of circuit weight-training in a cardiac rehabilitation setting, and evaluated haemodynamic responses of cardiac patients to this form of exercise (Kelemen et al. 1984). They reported that patients participating in a 100station circuit weight-training programme were not likely to experience untoward haemodynamic responses. Harris and Holly (1987) also evaluated haemodynamic responses during circuit weight-training. However, as the auscultatory method was used to determine blood pressure, these 'exercise' values actually reflect blood pressure during the first 4S seconds of recovery after the resistance exercise. Ten men performed 3 sets of a IO-station circuit, 3 days per week over 9 weeks. Stations emphasised arm, leg or trunk exercises. Each exercise was performed with an exercise: rest ratio of 45 seconds: 15 seconds. At the completion of each group of exercises (arm, leg, or trunk), 45 seconds rest was provided, and heart rate and blood pressure were determined. Over the course of the study, systolic blood pressure was higher after the leg exercise (ISS ± 12mm Hg) than after trunk exercise (149 ± 14mm Hg) and arm exercise (144 ± II mm Hg). Leg exercise also resulted in the greatest decrease in diastolic pressure compared to baseline (91 ± 6mm Hg); after leg exercise, diastolic pressure was 81 ± 6mm Hg, after trunk exercise, 83 ± 8mm Hg, and after arm exercise, 87 ± 11 mm Hg. While it was concluded that, even in these borderline hypertensive subjects, circuit weight-training does not elicit an exaggerated pres-
5
Haemodynamic Responses to Weightlifting
sor response, it must be noted that these measures took place after exercise. Thus, weightlifting exercise elicits haemodynamic responses that are inappropriately large for the metabolic demand of the task. The responses reflect the interaction between metabolic and mechanical factors. While not suitable for everyday applications, for research purposes beat-to-beat (intra-arterial) measurements are critical for a true understanding of haemodynamic responses during an activity such as weightlifting. However, as Seals and Hagberg (1984) have noted, one is faced with the dilemma of using repeated cuff measurements (with higher reliability) versus a single invasive intra-arterial measurement.
3. Other Haemodynamic Responses Isometric exercise and (circuit) weight-training exercise are associated with tachycardia, with reported heart rates generally usually in the range of 130 to 150 beats/min (e.g. Lewis et al. 1985), although higher values are not unusual for example, 170 beats/min during a 2-leg press to exhaustion (MacDougall et al. 1985) or 158 to 161 beats/min for a 10- to 14-minute 12-exercise task (Collins et al. 1989; Hill ~t al. 1989). The relationship between exercise heart rate and oxygen uptake is different in circuit weightlifting, compared to dynamic exercise. At a given percentage of maximal heart rate, oxygen uptake (as a percentage of maximal), is about half what would be predicted based on heart rate/oxygen uptake ratios reported for dynamic exercise (Collins et al. 1989). The heart rate/oxygen uptake relationship in weightlifting is probably also influenced by the muscle mass involved. In contrast to dynamic exercise, tachycardia in static exercise is not accompanied by an increase in stroke volume (Bezucha et al 1982; Clausen 1977; Lewis et al. 1985). Stroke volume is unchanged regardless of the muscle mass involved (Lewis et al. 1985). Weightlifting exercise results in a relatively large decrease in plasma volume (Collins et al. 1986). Subjects performed 3 circuits of 4 free weight ex-
ercises at 70% of 1RM. Plasma volume, based on changes in haemoglobin and haematocrit, decreased 14%, although the metabolic demand of the 14-minute exercise session was just under 2.0 L/ min. It would be expected that the intensity of dynamic exercise necessary to elicit a plasma volume change of this magnitude would be about 80% to 95% of maximal oxygen uptake. The fluid shift after weightlifting was very transient, and haematocrit was restored to resting levels within 30 minutes after exercise. The authors concluded that the exaggerated plasma volume response might reflect the greater blood pressure response in combination with an increased tissue osmolality consequent to release of anaerobic metabolites.
4. Postexercise Blood Pressure Traditionally, blood pressure is assumed to increase during steady-state dynamic exercise, level off, and then decline gradually during recovery [de Vries (1986), pp. 142-143]. The fact that blood pressure remains slightly below pre-exercise levels well into recovery (Kaufmann et al. 1987) has been used as justification for the beneficial role of exercise in hypertensives. However, it is not clear that blood pressure after weightlifting exercise (or dynamic exercise) does simply decline from peak exercise values to slightly below pre-exercise values. Hill and colleagues (1989) measured blood pressure in 6 men 15 minutes before and during a 60minute period following a bout of weight-training. Subjects performed 3 sets of 4 exercises with free weights (arm curl, bench press, bent arm row, and squat). They used 70% of their IRM, and completed as many repetitions as possible. Prior to exercise, blood pressure by auscultation was 119/86 ± 4/4mm Hg. Blood pressure immediately after exercise (the cuff was in place and being inflated as the final repetition was completed) was 99/62 ± 10/3mm Hg, significantly below pre-exercise levels. Within 1 minute, pressures had rebounded to 116/74 ± 12/8mm Hg; pressure remained unchanged throughout the rest of the 60-minute recovery. Since subjects exercised for an average 14 minutes with a gradually increasing oxygen uptake
Sports Medicine 12 (1) 1991
6
(mean value for the session was 1.96 L/min), cardiac output and mean arterial pressure must have been maintained during the exercise. Thus, the trough represents a postexercise phenomenon. These results highlight the importance of measuring 'exercise' blood pressure during exercise.
The Valsalva maIH>6uvre is commonly performed, even when attempts are made to avoid it, during isometric or heavy resistance weightlifting exercise. It involves apnoea with positive intrapulmonic pressure (e.g. attempted expiration against a closed glottis) [Paulev et al. 1988]. There is an increase in mean arterial pressure following the final inspiration prior to the manoeuvre. Then, during the first 5 to 10 seconds of the-manoeuvre there is a considerable drop in mean arterial pressure brought about by compression of the veins running to the right atrium, a subsequent decrease in venous return and, therefore, reduced stroke volume in the right and then left ventricle. There follows a reflex increase in heart rate which cannot compensate for the decreased stroke volume; so cardiac output falls. However the reflex increase in total peripheral resistance causes increases in both systolic and diastolic pressures. Haemodynamic responses during a Valsalva manoeuvre are similar to the responses during high intensity isometric exercise, namely reduced stroke volume, reflex tachycardia, and increases in both diastolic and systolic blood pressures. When a Valsalva manoeuvre is performed during a weightlifting task, the blood pressure response is exaggerated (MacDougall et al. 1985) and, conversely, when attempts are made to avoid the Valsalva the blood pressure response is somewhat attenuated.
6. Cartiio,tuc"lar Adaptation, Spitler (1980) reported borderline hypertensive status in 10 bodybuilders, and it is commonly believed that regular performance ofweightlifting exercise is associated with elevated resting blood pressure (Hunter & McCarthy 1982). However, no
longitudinal study presented evidence of a hypertensive training effect. In fact, Colliander and Tesch (1988) reported similar resting blood pressures in 31 bodybuilders and 37 age-matched medical students (123/11 ± 2/1 versus 127/71 ± 2/1mm Hg, respectively) and somewhat reduced heart rate and blood pressure responses to similar absolute work rates on a cycle ergometer. Heck and Dean (1987) evaluated blood pressure responses of bodybuilders, weight-training novices, and sedentary controls who performed lRM and exercise to fatigue at 50, 70, 80 and 90% of 1RM of the I-arm dumbbell press and I-leg extension. Bodybuilders' arm and leg exercise pressures were about 160/121 and 154/113mm Hg, respectively. Pressures were higher in the novice (about 185/144 and 184/141, respectively) and sedentary (about 178/143 and 179/137, respectively) subjects. It can be concluded that, while steroid use might elevate blood pressure (Spitler 1980), training in the form of weightlifting does not affect resting blood pressure in normotensives and attenuates the acute pressor response to weightlifting exercise.
7. Concl"s;o1ll Weightlifting exercise involves dynamic and static components, and elicits haemodynamic responses that are inappropriately large for the metabolic demand. Factors such as performance of the Valsalva manoeuvre and initiation of reflexes by mechanical compression of veins also contribute to the response. When parasympathetic activity is withdrawn and sympathetic activity increased, there is tachycardia and increased cardiac output, increased total periphera:i resistance, and increase in both systolic and diastolic blood pressures. The magnitude of this pressor response is attenuated by prior training. While intra-arterial measures have the advantage of providing direct recordings of pressures even during weightlifting exercise, auscultatory methods have the advantage of ease of repeatability and are less threatening to the subject. Because of the exaggerated pressor response to
Haemodynamic Responses to Weightlifting
weightlifting exercise, there has been concern (not supported by the literature), that weightlifting and hypertension might be causally linked. However, while caution must be used to minimise the pressor response, some forms of weight-training have been reported to be safe even for hypertensives and cardiac patients.
References Bezucha GR, Lenser MC, Hanson PO, Nagle FJ. Comparison of hemodynamic responses to static and dynamic exercise. Journal of Applied Physiology 53: 1589-1593, 1982 Clausen JP. Circulatory adjustments to dynamic exercise and effect of physical training in normal subjects and in patients with coronary artery disease. In Sonneblick & Lesch (Eds) Exercise and heart disease, Grune and Stratton, New York, pp. 39-75, 1977 Collins MA, Cureton KJ, Hill OW, Ray CA. Relationship of heart rate to oxygen uptake during weightlifting exercise. Medicine and Science in Sports and Exercise 21: 178-185, 1989 Collins MA, Hill OW, Cureton KJ, DeMello JJ. Plasma volume change during heavy-resistance weight lifting. European Journal of Applied Physiology and Occupational Physiology 55: 4448, 1986 deVries HA. Physiology of exercise for physical education and athletics, 4th ed., Wm. C. Brown Co, Dubuque, pp. 142-143, 1986 Fleck SJ, Dean LS. Resistance training experience and the pressor response during resistance exercise. Journal of Applied Physiology 63: 116-120, 1987 Hill OW Collins MA, Cureton KJ, DeMello JJ. Blood pressure respo~se after weight training exercise. Journal of Applied Sport Science Research 3: 44-47, 1989 Hunter GR McCarthy JP. Pressor response associated with highintensity' anaerobic training. Physician and Sportsmedicine II: 151-162, 1982 Kaufman FL, Hughson RL, Schaman JP. Effect of exercise on recovery blood pressure in normotensive and hypertensive subjects. Medicine and Science in Sports and Exercise 19: 1720, 1987 Kelemen MH, Stewart KJ. Circuit weight training; a new direction for cardiac rehabilitation. Sports Medicine 2: 385-388, 1985 Kelemen MH, Stewart KJ, Gillilan RE, Valenti SA, Manley J, et a1. Circuit weight training in a cardiac rehabilitation program.
7
Abstract. Medicine and Science in Sports and Exercise 16: 128, 1984 Lewis SF, Snell PO, Taylor WF, Hamra M, Graham RM, et a1. Role of muscle mass and mode of contraction in circulatory responses to exercise. Journal of Applied Physiology 58: 146151, 1985 Lind AR. Cardiovascular adjustments to isometric contractions: static effort. In Shepherd & Abboud (Eds) Handbook of physiology, the cardiovascular system, section 2, vol. III, Peripheral circulation and orpn blood flow, Williams and Wilkins Co, pp. 947-966, Baltimore, 1983 MacDougall JD, Tuxen 0, Sale G, Moroz JR, Sutton JR. Arteria1 blood pressure response to heavy resistance exercise. Journal of Applied Physiology 58: 785-790, 1985 Maughan RJ, Harmon M, Leiper JB, Sale 0, Delman A. Endurance capacity of untrained males and females in isometric and dynamic muscular contractions. European Journal of Applied Physiology and Occupational Physiology 55: 395-400, 1986 Mitchell JH Payne FC, Saltin B, Schibye B. The role of muscle mass in 'the cardiovascular response to static contractions. Journal of Physiology (London) 309: 45-54, 1980 Mitchell JH, Reardon WC, McLoskey I. Reflex effects on circulation and respiration from contracting skeletal muscle. American Journal of Physiology 223: H374-H378, 1977 Morin Y, Turmel L, Fortier J. Methyldopa: clinical studies in arterial hypertension. American Journal of Medical Science 4: 633-640, 1964 Nagle FJ, Seals DR, Hanson PO. Time to fatigue during isometric exercise using different muscle masses. International Journal of Sports Medicine 9: 313-315, 1988 Palatini P. Blood pressure behaviour during physical activity. Sports Medicine 5: 353-374, 1988 Seals DR, Hagberg JM. The effect of exercise training on human hypertension: a review. Medicine and Science in Sports and Exercise 16: 207-215, 1984 Seals DR, Washburn RA, Hanson PO, Painter PL, Nagle FJ. Increased cardiovascular response to static contraction of 1arJer muscle groups. Journal of Applied Physiology: Respiratory, Environmental and Exercise Physiology 54: 434-437, 1983 Seals DR, Chase PB, Taylor lA. Autonomic mediation of the pressor responses to isometric exercise in humans. lournal of Applied Physiology 64: 2190-2196, 1988 Westcott W, Howes B. Blood pressure response during weight training exercise. National Strengh and Conditioning Association Journal 5: 67-71, 1983 Correspondence and reprints: Dr David W. Hill, Department of Kinesiology, P.O. Box 13857, University of North Texas, Denton, TX 76203-3857, USA.
Sports Medicine 12 ( I): 1-7. 1991 0112-1642/ 91 /0007-0001 / $03.50/0 © Adis International Limited. All rights reserved. SP0131
Haemodynamic Responses to Weightlifting Exercise David W. Hill and S. Dee Butler Exercise Physiology Laboratory, University of North Texas, Denton, Texas, USA
During dynamic exercise, there is an increased demand for the delivery of oxygenated blood to the working muscles and for the removal of products from these tissues. Increased muscle blood flow is permitted by an increased driving pressure and a decreased local resistance to flow. The magnitude of the haemodynamic response is closely coupled to oxygen uptake. However, during weightlifting exercise, there is dissociation between these metabolic (aerobic) demands and haemodynamic responses. Not only is much of the energy for weightlifting provided by other (anaerobic) mechanisms, but the cardiovascular system is affected by altered autonomic nervous system responses and by mechanical effects associated with the heavy resistance exercise, such as increased vascular resistance in vessels compressed in the active muscles and in the thorax. The purpose of this paper is to describe haemodynamic responses during weightIifting exercise, with attention to differences between these responses and the responses to dynamic/aerobic exercise.
1. Types of Exercise Most studies of haemodynamic responses have differentiated between dynamic and isometric exercise. Weightlifting exercise is not a purely isometric or static activity and, therefore, it is inappropriate to assume that specific responses to isometric exercise necessarily reflect the haemodynamics of weightIifting. Clearly, however, as-
peets of the haemodynamic response to weightlifting do reflect the static component in the activity. Activities must be classified not only in terms of mode of contraction - isotonic versus isometric - but in terms of muscle mass involved, intensity (in terms of metabolic demand and percentage of peak force development), and duration. Activities such as running and cycling are often described as dynamic/isotonic/aerobic/submaximal and yet, especially at higher intensities, have static and anaerobic components. Similarly, traditional methods of weightlifting do not involve only static contraction; weightlifting is a dynamic activity associated with elevated aerobic metabolism, albeit with a significant anaerobic component (Collins et al. 1989; McArdle & Foglia 1969). Discussion of haemodynamics during weightlifting exercise is complicated by the fact that the actual exercise bouts (the 'sets' and, indeed, each repetition within a set) are interspersed with periods of rest. When the rest periods between sets are short, and closely regulated, the training is described as circuit weight-training. In some forms of circuit weight-training, low intensity aerobic exercise is performed between weightIifting sets.
2. Blood Pressure The driving force behind the flow of blood to the active tissues is mean arterial pressure, calculated by integration of pressure over a cardiac cycle, or simply estimated as diastolic pressure plus onethird of the difference between the systolic and dia-
2
Sports Medicine 12 (1) 1991
stolic pressures. Determinants of systolic blood pressure are cardiac output and total peripheral resistance. Heart rate, stroke volume and local vasoconstriction all contribute to the expected increase in mean arterial pressure during exercise. In addition, exercise affects blood viscosity and volume, which plays a minor role in increasing peripheral resistance and decreasing venous return. Thus, cardiovascular responses to dynamic exercise are determined by metabolic demand, associated neurohumoral influences, and reflexes elicited by the activity (Mitchell 1990). 2.1 Dynamic Exercise In dynamic exercise such as walking, jogging, or cycling, the cardiovascular responses are closely tied to the increased oxygen usage. Local vasodilation, along with vasoconstriction in the gut, hepatic and skin vessels, permits an increase in blood flow to the working muscles. The resulting increase in venous return contributes to an increased stroke volume which, coupled with an increase in heart rate, causes an increase in cardiac output (Guyton 1991). Systolic blood pressure rises with the first minute of exercise and levels otT during exercise at about 50 to 75mm Hg above rest in young, healthy males driven by the increased cardiac output necessary to meet the oxygen demand of the activity (an increase of about 5 L/min cardiac output per 1 L/ min increase in oxygen uptake). Diastolic pressure (fourth phase) tends to decrease slightly during walking/running exercise and to remain unchanged or to increase only slightly during cycle ergometry; fifth phase pressure always declines, as far as Omm Hg. Palatini (1988) has recently reviewed the effects of exercise on blood pressure and Mitchell (1990) has reviewed neural control of the circulation in exercise. 2.2 Static Exercise Isometric exercise produces an increase in mean arterial pressure via increases in both diastolic and systolic pressures. Mitchell and colleagues (1977) and Morin and colleagues (1964) attributed much
of the elevation to increased sympathetic activity. Clausen (1977) and Bezucha and colleagues (1982) noted that the increased cardiac output during isometric exercise resulted from increased heart rate, not stroke volume. In fact, stroke volume is virtually unchanged during static contractions (Lewis et al. 1985). The magnitude of the haemodynamic response to isometric contraction is a function of muscle mass (Lewis et al. 1985; Misner et al. 1990; Mitchell et al. 1980; Seals et al. 1983). Lewis and colleagues (1985) reported higher heart rates (134 ± 11 versus 91 ± 4 beats/min) higher cardiac outputs (10.1 ± 0.9 versus 6.8 ± 6mm Hg), higher systolic pressures (193 ± 7 versus 150 ± 6mm Hg), and higher diastolic pressures (114 ± 2 versus 94 ± 4mm Hg) during isometric 2-leg versus handgrip exercise. Misner and colleagues (1990) have even shown that maximal static leg extension with both legs elicits a greater blood pressure response than extension with only one leg. Lind (1983) has argued that the differences in haemodynamic responses to static exercise with different muscle masses could be explained by differences in time to exhaustion. Indeed, pressures do tend to increase during performance of a sustained static contraction (Misner et al. 1990; Seals et al. 1983; 1988). For example, during a sustained contraction at 35% maximal voluntary contraction, mean arterial pressures in 8 men were 110 ± 2mm Hg at 0.5 minutes, 126 ± 3mm Hg at 1.5 minutes, and 132 ± 3mm Hg at 2.5 minutes (Seals et al. 1988). However, Maughan and colleagues (1986) and Nagle and colleagues (1988) have shown that there is no relationship between muscle mass and time to exhaustion at given percentages of maximal voluntary force production. The magnitude of the haemodynamic responses is certainly a function of the percentage of the maximal voluntary force production that is sustained during the activity (Maughan et al. 1986; Seals et al. 1988). Seals and colleagues (1988) evaluated responses to static exercise at 15, 25 and 35% of maximal voluntary contraction. Mean arterial pressures rose during the exercise and, at the 2.5 minute mark, were 106 ± 4, 120 ± 4 and 132 ± 3mm Hg,
Haemodynamic Responses to Weightlifting
respectively. The cardiovascular response to isometric exercise is mediated by both central and peripheral mechanisms (Goodwin et al. 1972; Mitchell 1990; Mitchell et al. 1980). Feedforward central drive (reduced parasympathetic activity) causes tachycardia; chemoreceptor feedback from the active tissues to the medulla (increased sympathetic activity) causes vasoconstriction in vessels serving noncontracting muscles. Unlike during dynamic exercise, where rhythmic contraction of muscles promotes venous return, during sustained static contractions there is mechanical compression of the vessels in the active muscles and a marked increase in local resistance. 2.3 Weightlifting Exercise Weightlifting includes a combination of isotonic/dynamic and isometric/static contractions. MacDougall and colleagues (1985) have noted that the haemodynamic response to weightlifting exercise reflects the nature of the activity - it is a series of essentially static contractions, performed dynamically. Haemodynamic responses vary as a function of muscle mass involved, intensity, workrest interval, age of subjects, duration of exercise, and method of measurement. Wescott and Howes (1983) determined blood pressure responses using auscultatory sphygmomanometry in young « 38 years) and older (> 38 years) individuals performing 10 I-arm biceps curls at various intensities - 10 repetition maximum (RM) [heavy], IORM minus 5 pounds (2.3kg) [moderate] and IORM minus 10 pounds (4.5kg) [light]. Men tended to have higher systolic responses at each relative workload than women, and older individuals had higher systolic values than younger individuals. For all subjects, blood pressure showed a progressive increase from rest to the highest load - 123/15 ± 3/2mm Hg at rest, 143/ 74 ± 5/2mm Hg at light intensity, 151/74 ± 5/ 2mm Hg at moderate intensity and 165/75 ± 5/ 3mm Hg at heavy intensity. Increases over resting systolic pressure were 16, 22 and 34%. Diastolic pressures were unchanged at any workload. It was noted that the diastolic measures were obtained
3
immediately after, not during, each exercise set (see section 4). Freedson and colleagues (1984) used a pressure transducer/femoral artery catheter assembly to compare intra-arterial blood pressure response to free weight and hydraulic resistance bench press exercise. Seven subjects performed 10 repetitions of free weight bench press at each of 25% and 50% of maximal isometric strength. They also performed fast and slow (3 and 5 seconds, respectively) hydraulic resistance bench press. All 4 exercise bouts elicited high peak and mean systolic pressure responses. The magnitude of the response was dependent on the force produced with mean systolic pressures of 190 ± 10 versus 125 ± 7mm Hg during the slow versus fast hydraulic bench press, and 169 ± 7 versus 145 ± 4mm Hg in the 50 versus 25% free weight bench press. However, peak systolic pressure was equally high (232 ± 23 to 245 ± 14mm Hg) in all but the low resistance free weight bench press (169 ± 13mm Hg). Mean diastolic pressures were higher in the 50% free weight and slow hydraulic lifts (l08 ± 6 and 117 ± 2mm Hg, respectively), than in the lighter free and hydraulic lifts (87 ± 5 and 76 ± 2mm Hg, respectively). The same pattern held for peak diastolic pressures, with values of 154 ± 17 and 160 ± 10mm Hg reported in the high resistance free weight and slow movement hydraulic resistance exercises. These results suggest that resistance (force production) is a strong predictor of exercise blood pressure response. In addition, the fact that the high speed (i.e. lower force) hydraulic exercise elicited high peak systolic pressure might suggest that this all-out activity was associated with a Valsalva manoeuvre (see section 5). MacDougall and colleagues (1985) evaluated the blood pressure response of 5 healthy body-builders who performed the I-arm curl, I-leg press and 2leg press at 95% IRM to failure and the 2-leg press at 100% IRM. Intra-arterial pressures were measured using a capacitance transducer connected to a catheter and inserted in the brachial artery. Using this set-up, the investigatofS were able to track blood pressure across each repetition and across each set of repetitions. For all lifts except the lRM,
4
Valsalva manoeuvres were avoided until the point of fatigue was approached. Greatest peak systolic and diastolic pressures occurred during the final repetitions of the 2-leg press at 95% IRM, when the group mean was 320/250mm Hg and I subject attained 480/350mm Hg. Thus, the group mean arterial pressure was calculated to be almost 300mm Hg. Pressures for the 2-leg press at IRM were about 260/200mm Hg; for the I-leg press, about 250/ 190mm Hg, and for the I-arm curl, 230/ 170mm Hg. At the initiation of each lift, both systolic and diastolic pressures rose rapidly and remained at this elevated level for the few seconds required to raise the weight. As the weight was lowered, arterial pressure dropped quickly to pre-exercise levels. Thus, it seems that the concentric phase of the lift elicits a greater blood pressure response than the eccentric component. The investigators concluded that muscle mass is a factor in the observed responses, with the larger masses involved causing increased compression of blood vessels and thus directly increasing blood pressure. However, muscle mass could not have been the main cause, or else the difference between 1- and 2-legged values would have been far greater. The mechanical compression, a pressor reflex and a Valsalva manoeuvre all contributed to the observed responses. In the same laboratory, intra-arterial brachial pressures were determined during weightlifting in cardiac patients. Haslam and colleagues (1987) studied 8 male cardiac patients who performed 5 to 15 repetitions of I-arm curls, I-leg presses, and 2-leg presses at 20, 40, 60 and 80% of their I RM. Blood pressures were a function of intensity and muscle mass involved, with highest values obtained during the 10 repetitions of 2-leg presses at 80% of IRM. These values were 215/124 ± 7/ 6mm Hg, which yield a calculated mean arterial pressure of 139 ± 8mm Hg. Heart rate during this set was about 100 beats/min. In these patients, rate pressure product was highest, 249 (± 12) X 102 beats/min· mm Hg during the 2-leg press at 80% IRM, compared to 221 (± 12) x 102 beats/ min· mm Hg measured at 85% maximal oxygen uptake during a cycle ergometer test. Thus, al-
Sports Medicine 12 (1) 1991
though significant elevations in blood pressure were elicited by the weightlifting, the haemodynamic responses not unlike, but of lesser magnitude than, those reported earlier in bodybuilders. Intra-arterial blood pressures have also been evaluated during isokinetic exercise at various contraction velocities (Haennel et al. 1987). Contraction velocity had no consistent effect on responses. Tachycardia was apparent (heart rate exceeded 125 beats/min at all speeds of contraction) and blood pressures were about 175/108mm Hg at 30 o/sec 194/101mm Hg at 90 o/sec and 187/102mm Hg at ISO o/sec. Kelemen and Stewart (1985) have summarised the potential role of circuit weight-training in a cardiac rehabilitation setting, and evaluated haemodynamic responses of cardiac patients to this form of exercise (Kelemen et al. 1984). They reported that patients participating in a 100station circuit weight-training programme were not likely to experience untoward haemodynamic responses. Harris and Holly (1987) also evaluated haemodynamic responses during circuit weight-training. However, as the auscultatory method was used to determine blood pressure, these 'exercise' values actually reflect blood pressure during the first 4S seconds of recovery after the resistance exercise. Ten men performed 3 sets of a IO-station circuit, 3 days per week over 9 weeks. Stations emphasised arm, leg or trunk exercises. Each exercise was performed with an exercise: rest ratio of 45 seconds: 15 seconds. At the completion of each group of exercises (arm, leg, or trunk), 45 seconds rest was provided, and heart rate and blood pressure were determined. Over the course of the study, systolic blood pressure was higher after the leg exercise (ISS ± 12mm Hg) than after trunk exercise (149 ± 14mm Hg) and arm exercise (144 ± II mm Hg). Leg exercise also resulted in the greatest decrease in diastolic pressure compared to baseline (91 ± 6mm Hg); after leg exercise, diastolic pressure was 81 ± 6mm Hg, after trunk exercise, 83 ± 8mm Hg, and after arm exercise, 87 ± 11 mm Hg. While it was concluded that, even in these borderline hypertensive subjects, circuit weight-training does not elicit an exaggerated pres-
5
Haemodynamic Responses to Weightlifting
sor response, it must be noted that these measures took place after exercise. Thus, weightlifting exercise elicits haemodynamic responses that are inappropriately large for the metabolic demand of the task. The responses reflect the interaction between metabolic and mechanical factors. While not suitable for everyday applications, for research purposes beat-to-beat (intra-arterial) measurements are critical for a true understanding of haemodynamic responses during an activity such as weightlifting. However, as Seals and Hagberg (1984) have noted, one is faced with the dilemma of using repeated cuff measurements (with higher reliability) versus a single invasive intra-arterial measurement.
3. Other Haemodynamic Responses Isometric exercise and (circuit) weight-training exercise are associated with tachycardia, with reported heart rates generally usually in the range of 130 to 150 beats/min (e.g. Lewis et al. 1985), although higher values are not unusual for example, 170 beats/min during a 2-leg press to exhaustion (MacDougall et al. 1985) or 158 to 161 beats/min for a 10- to 14-minute 12-exercise task (Collins et al. 1989; Hill ~t al. 1989). The relationship between exercise heart rate and oxygen uptake is different in circuit weightlifting, compared to dynamic exercise. At a given percentage of maximal heart rate, oxygen uptake (as a percentage of maximal), is about half what would be predicted based on heart rate/oxygen uptake ratios reported for dynamic exercise (Collins et al. 1989). The heart rate/oxygen uptake relationship in weightlifting is probably also influenced by the muscle mass involved. In contrast to dynamic exercise, tachycardia in static exercise is not accompanied by an increase in stroke volume (Bezucha et al 1982; Clausen 1977; Lewis et al. 1985). Stroke volume is unchanged regardless of the muscle mass involved (Lewis et al. 1985). Weightlifting exercise results in a relatively large decrease in plasma volume (Collins et al. 1986). Subjects performed 3 circuits of 4 free weight ex-
ercises at 70% of 1RM. Plasma volume, based on changes in haemoglobin and haematocrit, decreased 14%, although the metabolic demand of the 14-minute exercise session was just under 2.0 L/ min. It would be expected that the intensity of dynamic exercise necessary to elicit a plasma volume change of this magnitude would be about 80% to 95% of maximal oxygen uptake. The fluid shift after weightlifting was very transient, and haematocrit was restored to resting levels within 30 minutes after exercise. The authors concluded that the exaggerated plasma volume response might reflect the greater blood pressure response in combination with an increased tissue osmolality consequent to release of anaerobic metabolites.
4. Postexercise Blood Pressure Traditionally, blood pressure is assumed to increase during steady-state dynamic exercise, level off, and then decline gradually during recovery [de Vries (1986), pp. 142-143]. The fact that blood pressure remains slightly below pre-exercise levels well into recovery (Kaufmann et al. 1987) has been used as justification for the beneficial role of exercise in hypertensives. However, it is not clear that blood pressure after weightlifting exercise (or dynamic exercise) does simply decline from peak exercise values to slightly below pre-exercise values. Hill and colleagues (1989) measured blood pressure in 6 men 15 minutes before and during a 60minute period following a bout of weight-training. Subjects performed 3 sets of 4 exercises with free weights (arm curl, bench press, bent arm row, and squat). They used 70% of their IRM, and completed as many repetitions as possible. Prior to exercise, blood pressure by auscultation was 119/86 ± 4/4mm Hg. Blood pressure immediately after exercise (the cuff was in place and being inflated as the final repetition was completed) was 99/62 ± 10/3mm Hg, significantly below pre-exercise levels. Within 1 minute, pressures had rebounded to 116/74 ± 12/8mm Hg; pressure remained unchanged throughout the rest of the 60-minute recovery. Since subjects exercised for an average 14 minutes with a gradually increasing oxygen uptake
Sports Medicine 12 (1) 1991
6
(mean value for the session was 1.96 L/min), cardiac output and mean arterial pressure must have been maintained during the exercise. Thus, the trough represents a postexercise phenomenon. These results highlight the importance of measuring 'exercise' blood pressure during exercise.
The Valsalva maIH>6uvre is commonly performed, even when attempts are made to avoid it, during isometric or heavy resistance weightlifting exercise. It involves apnoea with positive intrapulmonic pressure (e.g. attempted expiration against a closed glottis) [Paulev et al. 1988]. There is an increase in mean arterial pressure following the final inspiration prior to the manoeuvre. Then, during the first 5 to 10 seconds of the-manoeuvre there is a considerable drop in mean arterial pressure brought about by compression of the veins running to the right atrium, a subsequent decrease in venous return and, therefore, reduced stroke volume in the right and then left ventricle. There follows a reflex increase in heart rate which cannot compensate for the decreased stroke volume; so cardiac output falls. However the reflex increase in total peripheral resistance causes increases in both systolic and diastolic pressures. Haemodynamic responses during a Valsalva manoeuvre are similar to the responses during high intensity isometric exercise, namely reduced stroke volume, reflex tachycardia, and increases in both diastolic and systolic blood pressures. When a Valsalva manoeuvre is performed during a weightlifting task, the blood pressure response is exaggerated (MacDougall et al. 1985) and, conversely, when attempts are made to avoid the Valsalva the blood pressure response is somewhat attenuated.
6. Cartiio,tuc"lar Adaptation, Spitler (1980) reported borderline hypertensive status in 10 bodybuilders, and it is commonly believed that regular performance ofweightlifting exercise is associated with elevated resting blood pressure (Hunter & McCarthy 1982). However, no
longitudinal study presented evidence of a hypertensive training effect. In fact, Colliander and Tesch (1988) reported similar resting blood pressures in 31 bodybuilders and 37 age-matched medical students (123/11 ± 2/1 versus 127/71 ± 2/1mm Hg, respectively) and somewhat reduced heart rate and blood pressure responses to similar absolute work rates on a cycle ergometer. Heck and Dean (1987) evaluated blood pressure responses of bodybuilders, weight-training novices, and sedentary controls who performed lRM and exercise to fatigue at 50, 70, 80 and 90% of 1RM of the I-arm dumbbell press and I-leg extension. Bodybuilders' arm and leg exercise pressures were about 160/121 and 154/113mm Hg, respectively. Pressures were higher in the novice (about 185/144 and 184/141, respectively) and sedentary (about 178/143 and 179/137, respectively) subjects. It can be concluded that, while steroid use might elevate blood pressure (Spitler 1980), training in the form of weightlifting does not affect resting blood pressure in normotensives and attenuates the acute pressor response to weightlifting exercise.
7. Concl"s;o1ll Weightlifting exercise involves dynamic and static components, and elicits haemodynamic responses that are inappropriately large for the metabolic demand. Factors such as performance of the Valsalva manoeuvre and initiation of reflexes by mechanical compression of veins also contribute to the response. When parasympathetic activity is withdrawn and sympathetic activity increased, there is tachycardia and increased cardiac output, increased total periphera:i resistance, and increase in both systolic and diastolic blood pressures. The magnitude of this pressor response is attenuated by prior training. While intra-arterial measures have the advantage of providing direct recordings of pressures even during weightlifting exercise, auscultatory methods have the advantage of ease of repeatability and are less threatening to the subject. Because of the exaggerated pressor response to
Haemodynamic Responses to Weightlifting
weightlifting exercise, there has been concern (not supported by the literature), that weightlifting and hypertension might be causally linked. However, while caution must be used to minimise the pressor response, some forms of weight-training have been reported to be safe even for hypertensives and cardiac patients.
References Bezucha GR, Lenser MC, Hanson PO, Nagle FJ. Comparison of hemodynamic responses to static and dynamic exercise. Journal of Applied Physiology 53: 1589-1593, 1982 Clausen JP. Circulatory adjustments to dynamic exercise and effect of physical training in normal subjects and in patients with coronary artery disease. In Sonneblick & Lesch (Eds) Exercise and heart disease, Grune and Stratton, New York, pp. 39-75, 1977 Collins MA, Cureton KJ, Hill OW, Ray CA. Relationship of heart rate to oxygen uptake during weightlifting exercise. Medicine and Science in Sports and Exercise 21: 178-185, 1989 Collins MA, Hill OW, Cureton KJ, DeMello JJ. Plasma volume change during heavy-resistance weight lifting. European Journal of Applied Physiology and Occupational Physiology 55: 4448, 1986 deVries HA. Physiology of exercise for physical education and athletics, 4th ed., Wm. C. Brown Co, Dubuque, pp. 142-143, 1986 Fleck SJ, Dean LS. Resistance training experience and the pressor response during resistance exercise. Journal of Applied Physiology 63: 116-120, 1987 Hill OW Collins MA, Cureton KJ, DeMello JJ. Blood pressure respo~se after weight training exercise. Journal of Applied Sport Science Research 3: 44-47, 1989 Hunter GR McCarthy JP. Pressor response associated with highintensity' anaerobic training. Physician and Sportsmedicine II: 151-162, 1982 Kaufman FL, Hughson RL, Schaman JP. Effect of exercise on recovery blood pressure in normotensive and hypertensive subjects. Medicine and Science in Sports and Exercise 19: 1720, 1987 Kelemen MH, Stewart KJ. Circuit weight training; a new direction for cardiac rehabilitation. Sports Medicine 2: 385-388, 1985 Kelemen MH, Stewart KJ, Gillilan RE, Valenti SA, Manley J, et a1. Circuit weight training in a cardiac rehabilitation program.
7
Abstract. Medicine and Science in Sports and Exercise 16: 128, 1984 Lewis SF, Snell PO, Taylor WF, Hamra M, Graham RM, et a1. Role of muscle mass and mode of contraction in circulatory responses to exercise. Journal of Applied Physiology 58: 146151, 1985 Lind AR. Cardiovascular adjustments to isometric contractions: static effort. In Shepherd & Abboud (Eds) Handbook of physiology, the cardiovascular system, section 2, vol. III, Peripheral circulation and orpn blood flow, Williams and Wilkins Co, pp. 947-966, Baltimore, 1983 MacDougall JD, Tuxen 0, Sale G, Moroz JR, Sutton JR. Arteria1 blood pressure response to heavy resistance exercise. Journal of Applied Physiology 58: 785-790, 1985 Maughan RJ, Harmon M, Leiper JB, Sale 0, Delman A. Endurance capacity of untrained males and females in isometric and dynamic muscular contractions. European Journal of Applied Physiology and Occupational Physiology 55: 395-400, 1986 Mitchell JH Payne FC, Saltin B, Schibye B. The role of muscle mass in 'the cardiovascular response to static contractions. Journal of Physiology (London) 309: 45-54, 1980 Mitchell JH, Reardon WC, McLoskey I. Reflex effects on circulation and respiration from contracting skeletal muscle. American Journal of Physiology 223: H374-H378, 1977 Morin Y, Turmel L, Fortier J. Methyldopa: clinical studies in arterial hypertension. American Journal of Medical Science 4: 633-640, 1964 Nagle FJ, Seals DR, Hanson PO. Time to fatigue during isometric exercise using different muscle masses. International Journal of Sports Medicine 9: 313-315, 1988 Palatini P. Blood pressure behaviour during physical activity. Sports Medicine 5: 353-374, 1988 Seals DR, Hagberg JM. The effect of exercise training on human hypertension: a review. Medicine and Science in Sports and Exercise 16: 207-215, 1984 Seals DR, Washburn RA, Hanson PO, Painter PL, Nagle FJ. Increased cardiovascular response to static contraction of 1arJer muscle groups. Journal of Applied Physiology: Respiratory, Environmental and Exercise Physiology 54: 434-437, 1983 Seals DR, Chase PB, Taylor lA. Autonomic mediation of the pressor responses to isometric exercise in humans. lournal of Applied Physiology 64: 2190-2196, 1988 Westcott W, Howes B. Blood pressure response during weight training exercise. National Strengh and Conditioning Association Journal 5: 67-71, 1983 Correspondence and reprints: Dr David W. Hill, Department of Kinesiology, P.O. Box 13857, University of North Texas, Denton, TX 76203-3857, USA.
