Graded exercise testing of mentally retarded adults: a study of feasibility

There is a striking absence of data on the cardiovascular fitness of mentally retarded adults and what limited data are reported reflect field or submaximal laboratory tests. This study sought to develop a protocol that would allow maximal aerobic testing (VO2max) of mentally retarded adults in the laboratory. Of 21 subjects recruited, 17 (eight men and nine women) were successfully tested. Their mean IQ (+/- SD) was 52.68 +/- 16.3; their weight was 149.76 +/- 35.3 lbs, height 64.4 +/- 4.2 in, and age 29.29 +/- 6.6 yr. The testing occurred in three phases: (1) familiarization with the laboratory environs; (2) training to walk on the treadmill and breathe through the respiratory collection system; and (3) data collection via graded exercise testing. The treadmill protocol consisted of walking at 3 mph at 0% grade for two minutes, followed by 3 mph at a 2.5% grade for two minutes. The speed was then held constant at 3 mph and the grade increased 2.5% every minute until exhaustion. Metabolic data were collected every minute using a Beckman MCC cart connected to the subjects through a Hans-Rudolph valve. Heart rates (HR) were collected with a Quinton electrocardiograph. The mean maximal cardiorespiratory data were as follows (+/- SD):VO2max = 26.3 +/- 8.0 ml X kg-1 X min-1; HRmax = 171 +/- 14 beats/min; VEmax = 62.8 +/- 21.8 L/min; and respiratory quotient (R) = 1.09 +/- .07. The R values obtained were within an acceptable range for valid maximal data. In addition, 15 subjects produced supramaximal work and showed a decline in VO2 during the last minute of exercise.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cardiorespiratory responses of mentally retarded adults to air-brake ergometry and treadmill exercise

The graded treadmill (TM) exercise test is considered the optimal mode of exercise for evaluating the cardiovascular fitness of mentally retarded (MR) individuals. A new mode of exercise, the Schwinn Air-Dyne ergometer (SAE), was evaluated and compared to the TM for determining the cardiovascular fitness of adults. Twelve MR adults performed maximal exercise tests to volitional exhaustion, on separate days, on the SAE and TM. Maximal heart rates, oxygen consumption, minute ventilation, and respiratory quotient were similar for both exercise tests. These results indicate that the SAE is comparable to TM exercise in assessing the cardiovascular capacity of MR adults.     

Oxygen consumption and cardiac response of short-leg and long-leg prosthetic ambulation in a patient with bilateral above-knee amputation: comparisons with able-bodied men

This study measured oxygen uptake (VO2), minute ventilation (VE), and heart rate (HR) in a bilateral above-knee (AK) amputee and in three able-bodied controls during progressive treadmill exercise. Walking conditions for the amputee included using bilateral short-leg (SL) and long-leg (LL) prostheses. A progressive treadmill protocol to maximal capacity was used for the amputee and duplicated by the control subjects. An automated system was used to measure VO2, VE, and HR throughout exercise. Data analysis was restricted to the use of parameter averages and percentages to describe differences between experimental conditions. Maximal VO2 for the amputee averaged 23.3mL/kg-1/min-1 with the LL and 22.8mL/kg-1/min-1 with the SL prostheses, a negligible difference between conditions; however, exercise duration was 27% longer when using the SL prostheses. In addition, when averaged over the first four stages of exercise, VO2, VE, and HR were 24%, 32%, and 14% higher, respectively, when the LL prostheses were used. Treadmill walking by unimpaired controls averaged 47% and 79% more economical than walking with the SL or LL prostheses, respectively. These results demonstrate that the use of currently available AK prostheses requires significant energy expenditure, which limits their use to only the most physically fit individuals.     

A call to action towards an evidence‐based approach to using verbal encouragement during maximal exercise testing

By definition, maximal exercise testing inherently requires participants to give a maximal effort. This is an important practical issue as submaximal efforts can produce invalid test results. Verbal encouragement is commonly used to motivate participants to maintain or increase effort investment during maximal exercise testing. Accordingly, studies have reported significant increases in time to exhaustion of between 8% and 18% during VO_2max and multistage shuttle run tests, and a significant 30·5 m mean increase in 6‐min walk test distance. Significant improvements during shorter tests, such as the Wingate and 2‐min walk tests, have not been observed however. Although participants typically perceive verbal encouragement positively during maximal exercise testing, around one‐third have neutral or negative perceptions. Despite the ubiquity and importance of verbal encouragement during maximal exercise testing, surprisingly little research has investigated the characteristics of effective encouragement with respect to its content, timing and frequency. The only randomized controlled trial to investigate one of these issues observed that verbal encouragement delivered every 20 s increased time to exhaustion during VO_{2 max} testing, but not every 60 or 180 s. Of particular concern is that several exercise testing guidelines have incorporated specific guidelines for the use of verbal encouragement, but not provided any theoretical or empirical justification, presumably because of the limited research to inform practice. Recent empirical research does provide some important insight into participant preference for the content and timing of verbal encouragement during maximal exercise testing; however, much more research is clearly required to establish comprehensive evidence‐based guidelines.     

Respiratory gas exchange during treadmill exercise testing: reproducibility and comparison of different exercise protocols.

Ten subjects performed 4 maximal exercise tests to evaluate reproducibility and effects of treadmill inclination on submaximal and maximal oxygen consumption. They performed a standard Bruce protocol twice, and 1 protocol with progressive speed increase with constant, or without, inclination. At maximal exercise there was no significant difference between the protocols in oxygen consumption, respiratory gas exchange ratio, minute ventilation, plasma lactate, serum potassium or heart rate. Exercise time and treadmill distance were shorter than Bruce protocol with inclination, and considerably prolonged without. Reproducibility for Bruce protocol was good for group comparison of oxygen consumption throughout exercise. The individual variations for oxygen consumption were small at maximal exercise, but were considerable at rest and at the lowest exercise steps, this was slightly improved by analysing longer sampling time. Thus, measurement of oxygen consumption is reliable for group analysis, but interpretation must be careful in individuals unless maximal exercise is obtained. Treadmill inclination may be adjusted according to individual preferences.     

Reliability of cardiorespiratory measurements during wheelchair ergometry

The purpose of this research was to evaluate the stability of measures of heart rate (HR) and oxygen uptake (VO2) during repeated 30-minute bouts of constant work-rate wheelchair ergometry. Ten able-bodied subjects (seven male; three female) completed three sequential, single-stage wheelchair ergometer propulsion tests, to exhaustion, at least 48 hours apart, to determine the reliability of measurements of HR and VO2. Power output was determined as the resistance required to elicit 75% of the peak V02 attained during a peak graded exercise wheelchair ergometer test, at a propulsion velocity of three miles per hour and a flywheel roll distance of 6.32 meters. The HR and VO2 measurements were averaged over the last 30 seconds of the first (T1) and second (T2) thirds of the tests and at volitional exhaustion (T3). Significant differences were not observed at any of the data points except for HR at exhaustion. The HR at exhaustion was lower for the third test than for the second test. Intraclass correlation coefficients for HR (R=0.92, 0.95, and 0.86) and VO2 (R=0.95, 0.96, and 0.97) were high across the three tests, at all of the data points, respectively. Coefficients of variation were generally low. The results of this study indicated that, with the exception of HR during exercise sustained longer than approximately 30 minutes, VO2 and HR measurements can be made with high reliability during sustained wheelchair ergometer propulsion.     

Reproducibility of treadmill exercise in patients with intermittent claudication

Summary. The reproducibility of pain limited exercise time (ET) and heart rate (HR) have been evaluated in repeated treadmill exercise tests in 24 patients with arterial occlusive disease of the lower limbs and intermittent claudication. The protocol includes a walking speed of 1 m/s and load increments of 10 watt/min and the two tests were separated by 1–14 days. The total ET was systematically increased by about 30 sec (.P<0–05) at the second exercise test. The reproducibility of ET at the onset of leg pains was 34%, whereas the corresponding value at maximal leg pain was 16%. HR at the termination of the test did not differ systematically between the two tests and the reproducibility of HR was 61% and 5-5% at the onset of pain and maximal pain, respectively. It is concluded that a slight improvement in walking capacity at a second exercise test must be taken into account when evaluating different types of therapy in patients with intermittent claudication and, furthermore, that ET, walking distance or work load, should preferably be evaluated at or near maximal leg pain.     

Improved upper-body endurance following a 12-week home exercise program for manual wheelchair users

This study determined if a 12-week monitored home exercise program would improve cardiorespiratory endurance in a heterogeneous group of manual wheelchair users, which incorporated subsets of individuals with and without upper-limb impairment. Twenty-seven subjects made up two groups of manual wheelchair users: 20 without upper-limb impairment and 7 with upper-limb impairment. Subjects completed wheelchair ergometer tests using a 1 min JUMP protocol that resulted in volitional exhaustion in 6 to 12 min. Following a recovery period (time > 30 min), subjects completed subsequent constant work rate endurance tests to exhaustion at a power output corresponding to 60% of the maximum attained on the JUMP test. Subjects then underwent 12 weeks of simulated wheelchair rolling exercise using elastic straps positioned to mimic the motion of propulsion. JUMP and constant work rate tests were performed before training and after 6 and 12 weeks of exercise. Oxygen consumption (VO2) increased from rest to peak exercise in both groups and was significantly (p < 0.016) higher at peak for subjects without upper-limb impairment than for those with upper-limb impairment. Heart rate (HR) responses between the groups were similar. No significant differences in peak VO2, anaerobic threshold, or peak HR were observed at 6 or 12 weeks of the training program. Substantial improvement (p < 0.001) in maximum constant work rate tests time (10.37 +/- 2.79 min) was noted at 6 and 12 weeks, with no significant difference between 6 and 12 weeks and no significant intergroup difference. Results of this study indicated that simulated propulsion exercise endurance was improved as a result of the home exercise program.     

Moderate exercise, postprandial lipaemia and triacylglycerol clearance

Moderate intensity exercise reduces postprandial triacylglycerol (TG) concentrations. We tested whether this reflects increased TG clearance. Eight normotriglyceridaemic men, aged 48.3 +/- 7.3 years (mean +/- SD), performed two oral fat tolerance tests (blood samples taken in the fasted state and for six hours after a high-fat meal containing 1.00 g fat, 0.97 g carbohydrate, 58 kJ energy kg-1 fat-free body mass) and two intravenous fat tolerance tests (blood samples in the fasted state and after a bolus injection of Intralipid, 0.1 g fat kg-1 body mass). The afternoon before one oral and one intravenous test, subjects walked briskly for 90 min; no exercise was performed before the control tests. Prior exercise reduced fasting TG concentration similarly in the oral (16 +/- 7 %) (mean +/- SEM) and intravenous (18 +/- 7 %) tests, and reduced postprandial TG concentrations in the oral test by 18 +/- 6 % (all P < 0.05). However, prior exercise did not increase Intralipid clearance (disappearance curve slopes: control, 4.69 +/- 0.49 % min-1; exercise, 4.85 +/- 0.40 % min-1). These data suggest that mechanisms other than increased TG clearance mediate the lower postprandial TG concentrations seen after moderate exercise.     

A reliable isokinetic strength test for arm and leg musculature for mildly mentally retarded adults

This study attempted to establish a reliable test protocol to measure the strength of muscles involved in elbow and knee flexion and extension for mentally retarded (MR) adults. Nineteen mildly MR adults performed strength tests on a Cybex 340 isokinetic dynamometer on two separate days; 12 subjects performed a third trial of knee flexion and extension tests. For the 19 subjects, there were no significant differences in strength measurements--except for peak torque of knee flexion--between the first two test days. There were relatively high correlation coefficients when test parameters were compared for both test days, and best efforts occurred almost as often on test day two as on test day one. For the 12 subjects tested on three test days, there were no significant differences for test parameters among the three test days; however, 25% of best efforts took place on test day three. These findings indicate that when measuring the isokinetic strength of mildly MR individuals, tests should be performed at least two, optimally three, separate times to ensure reliability. The test protocol would be reliable when measuring knee and elbow flexion and extension strength for job assessment or clinical or rehabilitation purposes.     

Activity-Based Pacing: Comparison of a Device Using an Accelerometer Versus a Piezoelectric Crystal

The EXCEL VR, an accelerometer-based pacemaker (AC), and the Legend, a pacemaker utilizing a piezoelectric crystal (PZ), were compared under ergometric conditions and during stair climbing to assess the appropriateness of their rate responses. The pacemakers, programmed to the manufacturers' nominal settings in order to compare different technologically based sensors under identical conditions, were strapped over subjects' left mid-pectoral region. Placement of the devices was randomized to control for positional effects. Ten healthy subjects (55-72 years) completed a graded exercise treadmill test to 80% of maximum predicted heart rate (HR). An additional group of ten subjects (50-66 years) completed exercise protocols involving bicycle ergometry and stair climbing. Throughout all tests, pacemaker pulse rates and subjects' intrinsic HR were monitored continuously. For the treadmill exercise, the average correlations between the AC and PZ pacemakers' pulse rate and HR for the group as a whole were r = 0.92 and r = 0.82, respectively. Individual subject comparisons were also made between each pacemaker rate and intrinsic HR. The mean difference from intrinsic rate was 11 ppm for the AC pacemaker and 24 ppm for the PZ pacemaker. In addition, the PZ pacemaker's maximal pulse rate was significantly lower (105 +/- 9.6 ppm) than the other two rates (AC 137 +/- 6 ppm; intrinsic HR 129 +/- 2 beats/min). Throughout the bicycle ergometry testing, the intrinsic HR was higher than the AC and PZ pacing rates. However, the AC's rate was significantly higher than the PZ's rate. When subjects ascended stairs, the intrinsic HR and AC rate were closely correlated, but the PZ rate was significantly lower.(ABSTRACT TRUNCATED AT 250 WORDS)     

Maximal response of wheelchair-confined subjects to four types of arm exercise

Maximal work capacity and aerobic fitness of wheelchair-confined subjects generally have been determined by one of three modes of exercise: wheelchair ergometry, wheelchair exercise on a treadmill (TM), and arm crank ergometry. In the present study, two new types of arm ergometers, the Cybex Upper Body Exercise ergometer (CUBE) and the Schwinn Air-Dyne ergometer (SAE), have been evaluated and compared with older methods for determining aerobic capacities of wheelchair-confined subjects. Seven persons with paraplegia with lesions ranging from T-3 to L-1 and one bilateral amputee each performed four exercise tests to volitional exhaustion, on separate days, utilizing four modes of exercise. These included wheelchair exercise on a treadmill and arm exercise using a Monark arm ergometer (MAE), a CUBE, or an SAE. Peak values for oxygen consumption, heart rate, and blood lactate were similar during each type of exercise. Significant (p less than 0.05) differences were observed only in the peak values for minute ventilation (CUBE greater than MAE, SAE greater than MAE). The similarity of the maximal responses observed in this study indicates that the CUBE and SAE are comparable to TM exercise and to the MAE in assessing the aerobic capacity of wheelchair-confined subjects.     

Physical Therapy (94)

Aerobic fitness testing in patients with chronic low back pain—Which is the best test? (New England Medical Center, Boston, MA) Spine 2000;25:1704–1710. In this study, 30 participants with chronic low back pain performed three symptom‐limited maximal exercise tests: a treadmill, an upper extremity ergometer, and a bicycle ergometer. The tests were administered in randomized order. Heart rate was continuously monitored and oxygen consumption in terms of mL/kg/minute was measured by indirect calorimetry each 30 seconds. The statistical difference among the tests was highly significant (P < 0.0001). The treadmill test yielded the highest peak and predicted oxygen consumption followed by the bicycle and the upper extremity ergometer test, respectively. Conclude that the treadmill test is the best test for measuring aerobic fitness levels in patients with chronic low back pain. It yielded the highest peak oxygen consumption compared with the other tests, coming closest to measuring maximal oxygen consumption. Comment by Karen Crawford, RPT. Aerobic conditioning has been a component of several treatment approaches in the reduction of back pain disability. The specific contribution of aerobic fitness achieved through these approaches is not always known. Chronic pain patients tend to be inactive. As a result of this inactivity muscles become inefficient at using oxygen and this results in a loss of muscle endurance and cardiac output. The purpose of this study was, 1) to determine which testing protocol yields the highest peak values of VO₂, heart rates, and respiratory changes ratio in a sample of patients with CLBP; 2) to determine whether the VO₂ measures between the tests are significantly different from each other; 3) to determine which factors limit performance in each of these tests; and 4) to compare values of peak VO₂ and predicted VO₂ max with aerobic fitness values published for normal subjects. Chronic low back pain was defined as pain persisting for 3 months or more. Patients were excluded from this study if they were taking medications that influenced heart rate or blood pressure or if they had coexisting major medical disease, amputations of one or more extremities, or acute upper or lower extremity muscular skeletal pain that would interfere with exercise testing or acute psychiatric illness. Thirty participants with chronic low back pain performed 3 symptom limited maximal exercise tests: a treadmill, upper extremity ergometer, and a bicycle ergometer. These tests were randomly administered and heart rate was continuously monitored along with oxygen consumption. Of the 3 tests, the treadmill test is the most functional because of its uses in walking, a function of everyday life. This advantage to bicycle testing is that most Americans are unaccustomed to bicycle riding. Many of the patients reported increased burning in the thigh and buttocks for sitting on a hard saddle. Most clinics have access to a treadmill making this a practical test. The treadmill test is, thus, the best measure of cardiovascular performance; deconditioning of these patients expressed itself as early fatigue of peripheral muscles due to inactively, and limiting their performance on the bicycle and UBE test.     

The "Low Intensity Treadmill Exercise" Protocol for Appropriate Rate Adaptive Programming of Minute Ventilation Controlled Pacemakers

The objective of rate adaptive pacemakers that measure minute ventilation by tmnsthoracic impedance is to simulate the physiological relationship of the sensed signal to the sinus node response during exercise, thus achieving an appropriate matching of heart rate with patient effort. The purpose of this study was to determine the physiological relationship between heart rate and minute ventilation (HR/VE) during peak exercise testing in order to develop a database for appropriate rate adaptive slope programming of minute ventilation controlled pacemakers. Due to several clinical limitations of peak exercise testing, it was additionally determined whether the 35-watt “low intensity treadmill exercise” (LITE) protocol can be used as a substitute for peak exercise test using the “ramping incremental treadmill exercise” (RITE) protocol in order to assess the correct HR/VE slope below the anaerobic threshold. The stress tests were performed on a treadmill with the collection of breath-by-breath gas exchange. Linear regression analysis was used to determine the HR/VE slope below and above the anaerobic threshold and during the early, dynamic phase of low intensity exercise with the RITE and LITE protocols, respectively. The results of this testing in 41 healthy subjects demonstrated that the HR/VE relationship throughout treadmill exercise using the RITE protocol was not linear but curvilinear in nature, with a steeper HR/VE slope of 1.54 ± 0.51 below versus 1.15 ± 0.37 above the anaerobic threshold (P < 0.005). The HR/VE slope determined during the early, dynamic phase of the LITE protocol (1.58 ± 0.88) did not differ from the HR/VE slope from rest to anaerobic threshold obtained using the peak exercise RITE test (1.54 ± 0.51; P = 0.79), Rate adaptive pacing should simulate the curvilinear relationship between heart rate and minute ventilation from rest to peak exercise. The HR/VE slope determined during the early, dynamic phase of low intensity exercise represents the HR/VE slope derived from the RITE protocol below the anaerobic threshold. According to the peak exercise database, the slope above anaerobic threshold can easily be calculated as a percentage of the slope below the anaerobic threshold. The LITE protocol can, therefore, be effectively performed as a substitute for peak exercise stress tests to determine the correct pacemaker rate response factor in order to obtain a physiological heart rate to minute ventilation relationship for the appropriate matching of paced heart rate with patient effort.     

Repeatability of measurements of oxygen consumption,heart rate and Borg's scale in men during ergometer cycling

The coefficient of repeatability (COR), expressed as 2-SD of differences, was calculated between two measurements of oxygen consumption (V O2), heart rate (HR) and rating of perceived exertion (RPE) during ergometer cycling by men. The two sets of measurements were performed 5 to 6 weeks apart. Nineteen healthy men performed an incremental maximal exercise test on an ergometer cycle. The load started at 50 W and increased by 5 W 20 s-1 until exhaustion was reached. At 40% of the individual maximum load of the pretest, the load was kept constant for 4 min in order to reach steady state. Gas measurements were recorded continuously by computerized instrumentation. The HR was monitored with electrocardiography (ECG) and the perceived exertion was evaluated using Borg's scale. The COR of V O2 at sub-maximal load was 14% and at maximum load 11%. The values in absolute figures were 209 and 332 ml min-1. The corresponding COR of the HR was 16% at sub-maximum load and 6% at maximum load, and an evaluation of the perceived exertion yielded CORs in absolute values of 4.8 and 1.3, respectively. The COR for V O2, HR and ratings of perceived exertion when cycling on an ergometer cycle thus indicate a better agreement between the measurements at maximum load. The COR of the heart at sub-maximal loads must be kept in mind when using HR for estimation of V O2max. The reported findings should be considered when using tests on an ergometer cycle for evaluating exercise capacity.     

Aerobic power during maximal exercise in untrained and well-trained persons with quadriplegia and paraplegia

The aerobic power during maximal exercise was studied in 58 males with traumatic spinal cord lesions from C4 to L4 (25 well-trained "world-class athletes" and 33 untrained). For comparison we tested five well arm-trained and five arm-untrained able-bodied subjects. During maximal wheelchair exercise the aerobic power (VO2 peak), pulmonary ventilation and blood lactate concentration was higher in subjects with lower levels of spinal cord injury. At each injury level above C6-C7, nearly all trained subjects reached higher VO2 peaks than untrained subjects with the corresponding level of lesion. The mean values for trained paraplegic persons were 2.16 +/- 0.38 l x min-1, corresponding to 33.6 +/- 6.7 ml x kg-1 x min-1. The peak heart rate was lower in the quadriplegic than in the paraplegic group with no or only small difference between trained and untrained subjects at the same level of spinal cord injury. Spinal cord lesions with pareses reduce the total active skeletal muscle mass. This can cause physical inactivity, medical complications and social isolation. As a consequence, cardiovascular disorders as cause of death is higher in this group compared to the general population. Therefore, one aim of rehabilitation is to increase the individual's performance in daily life activities. It has been shown that the normal daily life activities of quadri- and paraplegic individuals with no additional physical training are not intense enough to maintain a satisfactory level of physical fitness.(ABSTRACT TRUNCATED AT 250 WORDS)     

Prediction of cardiac mortality after myocardial infarction: the role of maximal treadmill stress echocardiography.

Patients frequently undergo low-level exercise treadmill testing after acute myocardial infarction (MI) and, in the absence of inducible ischemia, a maximal test several weeks later. This study examines 203 patients who had 2-dimensional echocardiography before and after a maximal Bruce protocol exercise treadmill test performed 4 to 6 weeks after MI. The subjects were followed for a mean of 43 months (range 1 to 77 months). Predictors of cardiac mortality by multivariate or univariate analysis included an ejection fraction < or =40%, diabetes, age > or=70 years, and ischemia by exercise echocardiography but not by electrocardiography. Therefore, standard electrocardiographic monitoring during exercise treadmill testing 6 weeks after MI fails to predict cardiac mortality. The addition of pre-exercise and post-exercise treadmill stress echocardiography to readily available clinical parameters identifies those patients at greatest risk for cardiac death (resting ejection fraction < or=40%) and detects residual exercise-induced ischemia that may be of additional prognostic value.     

Physiological and symptomatic responses to cycling and walking in intermittent claudication

To shed light on the potential efficacy of cycling as a testing modality in the treatment of intermittent claudication (IC), this study compared physiological and symptomatic responses to graded walking and cycling tests in claudicants. Sixteen subjects with peripheral arterial disease (resting ankle: brachial index (ABI) < 0.9) and IC completed a maximal graded treadmill walking (T) and cycle (C) test after three familiarization tests on each mode. During each test, symptoms, oxygen uptake (VO2), minute ventilation (VE), respiratory exchange ratio (RER) and heart rate (HR) were measured, and for 10 min after each test the brachial and ankle systolic pressures were recorded. All but one subject experienced calf pain as the primary limiting symptom during T; whereas the symptoms were more varied during C and included thigh pain, calf pain and dyspnoea. Although maximal exercise time was significantly longer on C than T (690 +/- 67 vs. 495 +/- 57 s), peak VO2, peak VE and peak heart rate during C and T were not different; whereas peak RER was higher during C. These responses during C and T were also positively correlated (P < 0.05) with each other, with the exception of RER. The postexercise systolic pressures were also not different between C and T. However, the peak decline in ankle pressures from resting values after C and T were not correlated with each other. These data demonstrate that cycling and walking induce a similar level of metabolic and cardiovascular strain, but that the primary limiting symptoms and haemodynamic response in an individual's extremity, measured after exercise, can differ substantially between these two modes.