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1Department of Geriatrics, University of Arkansas for Medical Sciences, Little Rock, Arkansas 72205; and Departments of 2Physiology and Pharmacology and 3Clinical Physiology, Huddinge University Hospital, Karolinska Institutet, S-17177 Stockholm, Sweden
Submitted 29 September 2003 ; accepted in final form 2 December 2003
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ABSTRACT |
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 TOP ABSTRACT METHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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16 min of maximal contractile activity over the 5-wk course, muscle volume increased 7.7% (P < 0.05). Muscle strength decreased 24-32% (P < 0.05) in response to UL. Group ULRE showed maintained (P > 0.05) strength. Ankle plantar flexor muscle volume of the unloaded limb decreased (P < 0.05) in both groups (UL 10.5%; ULRE 11.1%). In neither group did the right weight-bearing limb show any change (P > 0.05) in muscle volume or strength. The results of this study provide evidence that resistance exercise not only may offset muscle atrophy but is in fact capable of promoting marked hypertrophy of chronically unloaded muscle. flywheel resistance exercise; skeletal muscle atrophy and hypertrophy; spaceflight; unilateral lower limb unloading
Indeed, resistance exercise regimens did attenuate or offset skeletal muscle atrophy (3, 4, 47) and strength decrements (3, 4, 6, 7, 28, 29, 47) when employed during periods of chronic, simulated spaceflight. However, studies that prescribed high-load resistance-training programs to individuals subjected to bed rest or unilateral lower limb suspension used either isometric actions or exercise systems that depend on the pull of gravity and examined responses to relatively short, i.e., 14- to 21-day, interventions (2, 3, 6, 7, 28, 29, 47). Time-consuming, high-volume low-intensity resistance exercise programs employed during long-duration spaceflight simulation have mitigated yet not prevented the negative effects of chronic unloading on skeletal muscle function (37, 43). Because of the inherent problems of simulating and performing weight training in space, we introduced a mechanical, non-gravity-dependent exercise system that utilizes the inertia of rotating flywheel(s) to provide resistance (14, 16, 48). A recent 5-wk resistance exercise regimen, which employed this system configured for monoarticulate knee extensor actions, produced marked muscle hypertrophy in ambulatory men and women (49).
The present study examined the efficacy of this resistance exercise paradigm, to serve as a countermeasure to muscle and strength loss during ground-based unloading. In an effort to simulate the effects of weightlessness on muscle, individuals were subjected to unilateral lower limb unloading (1, 10, 11, 15, 21, 30, 44, 47) with or without concurrent resistance exercise over 5 wk.
On the basis of previous findings, we predicted that quadriceps muscle volume would decrease almost 10% in response to this challenge and that this effect would be accompanied by a decrease in maximal voluntary strength of even greater magnitude (1, 10, 21, 30, 47). Furthermore, it was hypothesized that the proposed resistance exercise regimen would mitigate or offset the effects of unloading on quadriceps muscle size and function. The experimental design also allowed us to compare knee extensor and ankle plantar flexor muscle loss. Because quadriceps muscles are involved in the exercise mode employed here, they but not the ankle plantar flexor muscle group were believed to show maintained size and strength when subjected to this challenge. Our data suggest that resistance exercise of concurrently unloaded skeletal muscle not only maintains volume but in fact produces hypertrophy.
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METHODS |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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72 h before any postmeasurement. Also, we took advantage of the unloading intervention and as an additional aim assessed the influence of resumed weight bearing on muscle use. Hence, surface electromyographic (EMG) activity was measured in group UL during a standardized exercise task performed postunloading. Subjects. Subjects were 21 healthy men and women, age 30-56 yr, with physical characteristics and fitness levels comparable to those of the astronaut corps (Table 1). At the time of being recruited, the subjects did not participate in any regular training programs and refrained from intense or systematic exercise throughout the study. A medical examination excluded women who were on contraceptives or hormone replacement therapy and individuals who were hypertensive or reported past or present knee pathology. Subjects were then matched by gender, body size, age, and knee extensor strength, such that individuals were assigned to either ULRE or UL. Group UL consisted of seven men and four women, whereas group ULRE was made up of seven men and three women. All subjects complied with the prescribed unloading and resistance training protocols (see below).
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A written consent was given after the procedures, risks, and premises associated with participation in the study had been explained. The experimental protocol was approved by the Institutional Review Board at the University of Arkansas for Medical Sciences, Little Rock, AR. The study was conducted in accordance with the Declaration of Helsinki.
Unilateral lower limb unloading. Unilateral unloading of the left lower limb was accomplished by using the model introduced in our laboratory (10, 11) and subsequently modified (21, 30). In brief, for any upright or ambulatory activities, short-length crutches, aided by handgrip and forearm support distal to the elbow (Swereco Rehab, Sollentuna, Sweden) were used (Fig. 1). The right foot was equipped with a shoe having a 10-cm-thick sole. This removed weight bearing from the left unloaded limb, which could adopt a straight position and yet move passively about the hip joint. Because there were no straps attached to the shoe restraining ankle- (or knee-) joint movement, the foot dropped using this particular model. Car transportation was possible because all subjects had access to a car equipped with an automatic shift. Subjects lived at home and maintained their normal work tasks throughout the experimental period. To encourage compliance, all subjects were interviewed by one of the investigators via telephone or in person on a daily basis. The volunteers also interacted at regular social gatherings. Skin temperature and circumference were measured midcalf at least twice weekly during scheduled visits to the laboratory. Because the unloaded calf consistently shows 2-3°C lower skin temperature and 2-3 cm greater circumference than the weight-bearing limb a few hours after resuming a nonsupine position (44), we believe these are valid measures of compliance.
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As prophylaxis to venous thromboembolism, subjects received a daily dose of 325 mg aspirin. In addition, they wore knee-length, medical-graded (class 2) compression stockings (BSN-Jobst, Rutherford College, NC) during any ambulatory activity. Three men assigned to group UL who complained about posterior leg pain 12-21 days into the intervention were admitted to ultrasound-Doppler examinations. In all three cases, clinical assessments excluded suspected deep venous thrombosis; one subject was diagnosed with a minor superficial thrombosis. These individuals were then administered daily injections of low-molecular-weight heparin (Fragmin 2500 IE), and the reported symptoms of pain subsided and all subjects completed the study without any disruption.
Resistive exercise device. For the purpose of training, an exercise system using the flywheel principle (14) and a configuration constructed for the seated knee extension (Ref. 49; YoYo Technology, Stockholm, Sweden) was employed. The frame of the apparatus, crafted in rectangular steel tubing, is equipped with a pivoting lever arm; its center of rotation is aligned with the knee joint. The lever arm, whose length is adjustable, has a perpendicular shin-padded cross bar mounted at its distal end. While seated and slightly reclined and using back support and upper body restraint, the trainee pushes against the cross bar while grasping the handlebars. From a starting position of 90° knee angle, flywheel rotation is initiated through the pull of a sturdy nylon strap, which is anchored to the distal part of the lever arm and loops around its curved cam. The strap enters through a slot of the flywheel shaft where it is fixed. Energy is imparted to the flywheel while the strap unwinds off the shaft. Once the pushing concentric phase has been completed at 160-170° knee angle, the strap then rewinds by virtue of the kinetic energy of the rotating flywheel. While gently resisting the force produced by the pull of the rotating flywheel during the initial eccentric phase, the trainee then aims at bringing the flywheel(s) to a stop at 90° knee angle. This action is followed by the immediate initiation of a subsequent cycle. The ergometer is equipped with an electrogoniometer, which is affixed onto and aligned with the rotational center of the machine lever arm, and a miniature compression load cell (model 276A, K-toyo, Seoul, Korea) is mounted such that force is measured through the pull of the strap.
Resistance training protocol. Unilateral resistance exercise with the left limb was performed twice (weeks 1, 3, and 5) or thrice (weeks 2 and 4) weekly. Each session consisted of four sets of seven maximal, coupled concentric and eccentric knee extensions from 90 to 160-170° knee joint angle. One polymer flywheel (440 x 20 mm; 4.2 kg) was used to provide resistance during training. Sets, interspersed by 2-min rest periods, were initiated immediately after two submaximal actions. Any exercise session was preceded by a 5-min warm-up consisting of three sets of seven actions carried out with progressively increased effort. Because the coupled concentric and eccentric cycle is completed in 3 s, any exercise session consisted of roughly 80 s of contractile activity at maximal effort. Less than 70 s per session used low-force actions. Any exercise session, including warm-up and rest periods, was completed in 15 min.
Force was measured during each exercise session. Subjects received verbal encouragement, and, by producing force and joint angle signals on a personal computer (PC) display, instant performance feedback was provided during exercise. An analog-to-digital converting system (MuscleLab, Ergotest, Langesund, Norway), which was interfaced to the PC, integrated these signals and also served to log performance during training. The average force in a 30° window from 5° above the angle at which the transition from eccentric to concentric action occurred was measured. Training load was calculated as the average concentric and eccentric force, respectively, in that particular window from repetitions 2-7 of each of the four sets. Although eccentric peak force, and hence "eccentric overload," occurs near the deflection point, typically the average concentric force in the 30° window equals or exceeds the average eccentric force (49).
Maximal strength. Unilateral (right and left limb) maximal voluntary isometric force (MVC) was measured in the seated knee extension device (see above) at 90 and 120° knee angle (49) and, in the multijoint seated leg press at 90° knee angle, as described elsewhere (14). Three trials of each test mode and limb were allowed to assess MVC; i.e., average force in a 1-s window once a force plateau had been established. The order (right vs. left limb) for each of the different tests employed was random among subjects. Subjects were restrained by chest and thigh straps (knee extension) and allowed to grasp the handlebars while performing these tests. Force was measured by using the compression load cell, as described above, or using a strain gauge mounted on the footplate and attached via a ball-and- socket joint to the moment arm (leg press). On any day of use, the exercise and measuring systems were calibrated with use of standardized weights certified to 0.01%. The day-to-day coefficient of variation in assessing knee extension or leg press MVC ranges 3-6% (present study, Ref. 49).
Electromyography. EMG activity was measured before and after 60 min reambulation after unloading (group UL only). A task required subjects to sustain an isometric action at 20% of the pre-UL MVC for 10 s at 120° knee angle (see above). Knee extensor EMG activity from the three superficial aspects of the quadriceps femoris was recorded by using disposable bipolar Ag-AgCl surface EMG electrodes (10-mm diameter, 16-mm interelectrode distance; Multi Bio Sensors, El Paso, TX) affixed over the vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF). The reference electrode was placed over the patella. Electrodes were retained throughout these sets of experiments. Likewise, individual amplification level of each EMG electrode channel was maintained across the two measurements. The raw EMG signal was amplified 600 times and then band-pass filtered at cutoff frequencies 6-1,500 Hz. The filtered signal was converted to a root-mean-square signal using AD536 circuit (Analog Devices, Norwood, MA) with a 100-ms time constant. Torque and root-mean-square signals were digitized at 100 Hz and subsequently processed by using the MuscleLab system (see Resistance training protocol) for the MP100WS. The integrated EMG activity for each individual muscle was analyzed.
Imaging techniques and analysis. Magnetic resonance images were collected by use of a 1.5-T Signa scanner (General Electric, Milwaukee, WI). In assessing muscle volume, precautions were taken to minimize any potential influence that acute fluid shifts (13, 18) or soft tissue compression might have, by use of procedures described elsewhere (49). After coronal scout images, axial scans (echo time 9.0 ms, repetition time 2,000 ms, contiguous 10-mm images) were obtained from the femoral head to the knee joint and from the knee joint to the lateral malleolus. The field of view was 480 (thigh) and 240 or 360 mm (calf), respectively.
After electronic data transfer of images, cross-sectional area (CSA) measurements and calculations were performed by use of a public-domain software package (Scion Image Beta 4.0.2 for Windows, Scion, Frederick, MD) on a PC. By using computerized planimetry (Intuos Graphic Tablet System, Wacom, Vancouver, WA), regions of interest, e.g., VL, VM, vastus intermedius (VI), and RF were identified from the displayed images, manually traced by using a mouse-pen, and then automatically computed. Ankle plantar flexor muscle area was assessed by producing a straight line between the anterior boundaries of the lateral and medial aspects of the gastrocnemius. The area containing both the soleus and gastrocnemius muscles was then encircled. This approach allowed accurate measurements of the major portion (>80%) of the soleus and the entire gastrocnemius. To assess quadriceps or calf muscle volume of any individual, 12-15 interleaved, serial images were chosen.
Volume of individual quadriceps muscles was assessed by analyzing images from the most distal one containing RF to the most proximal image not containing the gluteal muscle. Likewise, CSA of all images beginning with the first one where both tibia and fibula appeared and ending with the most distal image containing both heads of the gastrocnemius were measured to assess plantar flexor volume. Muscle volume was determined by multiplying the measured CSA by slice thickness and number of slices being analyzed from any particular limb. We estimate the approach employed measures at least 70-80% of "real" quadriceps and triceps surae muscle volumes (49).
Body fat percentage. Body fat percentage was calculated from multiple (biceps, triceps, suprailiac, and front thigh) skinfolds by use of a Harpenden caliper (34).
Statistics. All group values presented are means ± SD. Data were analyzed with repeated-measures two- or three-way analysis of variance. When significant main effects were found, variables were contrasted post hoc with a least significant test. The relative changes in muscle volume and voluntary force were compared between groups by using an independent t-test. Differences were considered significant when P < 0.05.
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RESULTS |
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Calf temperature and circumference. Overall, calf temperature was 2-3°C lower (P < 0.05) in the unloaded left leg compared with the weight-bearing right leg. Conversely, calf circumference was 2-3 cm larger (P < 0.05) in the unloaded compared with the weight-bearing leg (Fig. 2). The two groups showed similar (P > 0.05) differences across legs.
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Muscle performance. Average force during concentric and eccentric actions increased 11.1 and 10.8% (P < 0.05; Fig. 3), respectively, over the 12 exercise sessions.
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Muscle volume. UL induced a decrease (P < 0.05) in quadriceps muscle volume of the left limb corresponding to 8.8%. Quadriceps muscle volume increased 7.7% (P < 0.05) in group ULRE. In neither group did volume of the right limb change (P > 0.05; Fig. 4). Individual quadriceps muscles showed differential responses to both UL and ULRE (Table 2). The decrease (P < 0.05) in plantar flexor muscle volume of the left leg was comparable in groups UL (10.5%) and ULRE (11.1%). The weight-bearing right leg showed no change in either group (Fig. 5).
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Muscle strength. Knee extensor MVC of the left limb at 90 and 120° decreased (P < 0.05) 24 and 26%, respectively, in group UL. There was a nonsignificant (P > 0.05) 7-8% decrease in group ULRE. In neither group did the weight-bearing right limb show a change (P > 0.05; Figs. 6 and 7). However, and regardless of group, the responses across legs were different (P < 0.05), such that the left limb showed greater strength loss than the right limb over time. Leg press MVC of the left limb showed a 32% decrease (P < 0.05) in group UL. The corresponding decrease (P < 0.05) in group ULRE was 12%. This decay was, however, smaller (P < 0.05) in group ULRE than in group UL. Either group showed maintained (P > 0.05) leg press MVC of the right limb. However, although the response across limbs was different (P < 0.05) in group UL, the change in leg press MVC over time was similar (P > 0.05) for the left and right limbs in group ULRE (Fig. 8).
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Electromyography. Although EMG activity was higher (P < 0.05) for the left compared with the right knee extensor muscles after unloading, 60 min of ambulation produced no change (P > 0.05) in either the left (previously unloaded) or the right (previously weight-bearing) limb (Fig. 9).
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DISCUSSION |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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We took advantage of a ground-based model that is known to successfully unload the knee extensor and ankle plantar flexor muscle groups (10, 30, 47). The 5-wk intervention produced a nearly 9% decrease in quadriceps muscle volume. The estimated rate of muscle loss (i.e., almost 2% per week) is very similar to what has been reported by our laboratory (10, 30, 44) and others (1, 21, 47) that used this paradigm over 16-42 days and accords with data generated in response to 28-42 days of bed rest (12, 19, 22, 42). Evidently, a difference in atrophic response across plantar flexor (10-11%) and knee extensor (9%) muscles was not discernible after 5 wk of unloading. Although these results confirm data obtained after 21-42 days unloading (21, 47), this does not rule out that either muscle is more susceptible than the other to long-duration interventions. Furthermore, the global response also accords with scarce and fragmental spaceflight data (2, 40), albeit there are puzzling reports of an appreciable decrease in VL muscle fiber size after short-term missions. Hence, the 11-24 and 16-36% atrophy reported after 5- or 11-day spaceflight (24) do not corroborate with the very modest whole muscle loss (41) and voluntary force decrement (32) demonstrated in the same crew. If real, these data would also imply a more than threefold greater weekly quadriceps muscle atrophy than that assessed by means of computerized imaging techniques and reported in astronauts after 8- and 17-day spaceflight (2, 40).
The reduction in maximal voluntary force amounted to 24-32% in response to unloading, and this effect was comparable when determined by various measures. Thus the decrement was not different for the one- and two-joint knee extension test modes and similar regardless of knee joint angle. Likewise, previous studies have produced results suggesting that neither the in vivo speed-torque (1, 10, 47) nor the torque-angle relationship (21) shows any prominent alterations after unloading.
Clearly, the decrease in voluntary force could not be accounted for by the decrease in muscle volume only. This disproportionate strength loss is not a novel finding and rather confirms several reports that determined the effects of bed rest (12, 19, 22) or unloading (1, 10, 30, 47) on muscle size and function. Collectively, the present results and those of other studies (15, 21) imply that mechanisms other than reduced skeletal muscle protein synthesis (25, 27) and governing protein loss are also responsible for the compromised in vivo muscle function frequently reported after simulated spaceflight. The main finding of this study, however, was the demonstration of the countermeasures effect induced by a novel resistance exercise paradigm. The robust, 8% increase in quadriceps muscle volume is striking given the brief episodes and limited amount of contractile activity that was imposed on unloaded muscle. The twelve exercise sessions consisted of 28 coupled concentric and eccentric actions each, and <16 min were dedicated to maximal contractile activity over the 5-wk intervention. This equals only 0.03% of the unloading time period. At first, this finding may be surprising. However, athletes emphasizing and relying on grand muscle mass and power typically refrain from voluminous low-intensity exercise and rather employ high-force, low-repetition training strategies. Likewise, analogies can readily be found in the animal kingdom among species characterized by extraordinary speed and power. It is also noteworthy that, after reviewing the literature (49), we found no resistance training study reporting similar or greater quadriceps muscle hypertrophy in healthy young or middle-aged ambulatory subjects in response to programs of comparable duration. However, spinal cord-injured patients who underwent surface electrical stimulation of this muscle twice weekly for 8 wk showed a marked increase in muscle size, at a rate that was even greater than what we report here (20). This raises the question of whether skeletal muscle exposed to minimal activity or loading is more apt to grow in response to a high-tension stimulus challenge compared with muscle that is chronically exposed to low tension, e.g., normal weight bearing.
Although the three vastus muscles showed a rather uniform response, i.e., they decreased 9 (VI and VL) and 12% (VM), the biarticulate RF showed maintained volume after unloading. Conversely, in group ULRE, the increase in volume of these muscles followed the same hierarchical pattern, i.e., the hypertrophy of VI and VL was 5-6% and VM increased by 9%. The RF muscle enlargement was almost 17%. One might suggest that the absence of atrophy of RF noted here and elsewhere (30) in response to lower limb unloading simply implies failure of the model to prevent use of this muscle. Thus some movement about the hip joint is allowed and brought about in part by RF use. This, however, cannot serve as the sole explanation, because bed rest provokes only minute, if any, atrophy of this muscle (B. A. Alkner and P. A. Tesch, unpublished observations).
Interestingly, ambulatory subjects who carried out the exercise program prescribed here (49) showed an almost identical progress in training load. Parallel to this progression, the increase in quadriceps muscle volume was also substantial, i.e., 6.1%. However, in frank contrast to the response demonstrated in the training group that was subjected to unloading, the ambulatory trainees experienced marked gains in non-task-specific voluntary strength. This apparent discrepancy in response might reflect inability of chronically unloaded muscle to make use of the increased muscle size brought about by resistance exercise, at least initially and on resuming weight bearing. Similar to the results reported here, resistance exercise failed to offset the decrement in non-task-specific force during 2-wk bed rest (5). Combined, this is further evidence that mechanisms other than muscle atrophy only must be responsible for the muscle dysfunction, shown here and elsewhere (1, 10, 12, 15, 21, 30, 44) in response to unloading. Whether this reduced efficiency, in turn, was governed by neural changes, e.g., reduced recruitment as a result of decreased neural drive (15) or by any other means, and/or decreased cross-bridge interaction (45, 50, 51), can only be speculated on.
Previously, it has been reported that a large portion of the decay in maximal voluntary force elicited by 10-28 days of unloading is regained after only a few days of resumed ambulation (10, 15, 21), and this transient effect is accompanied by an appreciable increase in maximal EMG activity. We addressed this issue in the present study and hypothesized that unloading would impair muscle function such that greater neural input would be required to drive the muscle and more muscle to be involved for a given motor task (15, 44); we speculated that acute reambulation, at least partially, would serve to reset this mechanism. Accordingly, after the intervention, the previously unloaded left knee extensors showed greater overall EMG activity than the right weight-bearing limb while sustaining a given submaximal force. However, the standard reloading procedure, consisting of 60 min of walking with progressively increased weight bearing, did not alter knee extensor EMG response to a modest exercise challenge, suggesting that this loading stimulus was insufficient to modulate the neural input required to produce a certain force.
The findings here of a dissociation between muscle and force loss and failure to transform an increase in muscle volume into enhanced non-task-specific voluntary force lend further support to the idea that short-term spaceflight or lack of weight bearing elicits prominent neural adaptations resulting in reduced muscle performance. This in turn suggests that maintaining muscle mass by any means does not necessarily offset the negative effects of spaceflight on neuromuscular function. This is a problem that needs to be addressed in more detail to aid in the development of exercise countermeasures for space travelers.
It should be appreciated that compliance is a major concern or drawback with use of the unilateral lower limb unloading model and critical to the study results and outcome. Given that the triceps surae muscles are unloaded by the present intervention and not involved in the resistance exercise task employed, the finding of no group difference in atrophic response of this particular muscle group was expected. Also, in accordance with previous observations (44) and in support of similar compliance, calf surface temperature and circumference showed comparable responses in the two groups when monitored over the course of interventions. The unloaded limb was consistently 2-3°C cooler and circumference 2-3 cm larger compared with the weight-bearing limb, and the differences across legs were almost identical for the two subject groups. In further support of successful compliance, in neither group did the weight-bearing right knee extensors show altered muscle strength or volume. Hence, we are confident that the goal to induce comparable and near complete or complete unloading response in the two experimental groups was accomplished and that the observed changes in muscle volume after unloading with or without resistance training resulted from net gain and loss of muscle tissue.
Previously, various resistance training programs carried out by individuals subjected to either bed rest or lower limb suspension have been proven effective in mitigating or preventing muscle (3, 4, 47) and strength losses (3, 4, 6, 7, 28, 29, 47). Unfortunately, these studies collectively employed exercise countermeasure aids that used either gravity-dependent weights or systems that failed to offer resistance during eccentric actions. In the present study, resistance training was performed by use of a mechanical, yet gravity-independent, non-electrically powered exercise system that it is capable of providing eccentric overload (49). Although it appears that inclusion of eccentric actions in resistance programs is an important stimulus and crucial in optimizing muscle growth of weight-bearing (30) or unloaded muscle (26, 35), we have no proof that the unique features of the device used amplified the benefits of resistance exercise shown before. An established exercise promoting knee extensor muscle use was chosen. Albeit, the particular exercise configuration (i.e., monoarticulate knee extension) used here is not intended for use by space crew, we believe the results of this study are applicable and reflect efficacy of this loading feature to counteract muscle loss experienced by space travelers.
In conclusion, the present results suggest that muscle atrophy induced by 5 wk of unilateral lower limb unloading is offset by resistive exercise using flywheel technology performed two or three times weekly and calling for maximal effort. In fact, it appears that the potential for exercise-induced muscle hypertrophy is not reduced in chronically unloaded muscle. Hence, resistance exercise not only is capable of blunting the atrophic response but in fact produces marked hypertrophy. This, however, may not necessarily transfer over to parallel changes in neuromuscular performance. Nevertheless, the results of this study add further strong support to earlier reports advocating that resistance exercise should be employed as a countermeasure on extended space missions.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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