Effects of Cessation of a Structured and Supervised Exercise Conditioning Program on Lean Mass and Muscle Strength in Severely Burned Children

Article Outline

• Abstract
• Methods
  • Burned Subjects
  • Exercise Testing
  • Strength Measurements
  • 3 Repetition Maximum Test
  • Lean Body Mass Measurements
  • Exercise Training Program
    • Resistive training
    • Aerobic training
  • Home Exercise Prescription
  • Aerobic Home Exercise Prescription
  • Resistive Home Exercise Prescription
  • Children Without Burn Injury
  • Data Analysis
• Results
• Discussion
  • Study Limitations
• Conclusions
• Appendix
• References
• Copyright

Abstract 

Suman OE, Herndon DN. Effects of cessation of a structured and supervised exercise conditioning program on lean mass and muscle strength in severely burned children.

Objective

To determine whether the benefits of exercise by burned children are maintained 3 months after the exercise program is concluded.

Design

Randomized, controlled prospective study.

Setting

Pediatric burn hospital.

Participants

Twenty severely burned children with a 40% or greater total body surface area burn, with main outcome measures completed before exercise training, immediately after 12 weeks of exercise training (intervention), and 12 weeks after training ended.

Intervention

Randomization into a 12-week standard rehabilitation program at home (n=9) or a 12-week standard hospital rehabilitation program supplemented with an exercise-training program beginning 6 months after burn injury (n=11).

Main Outcome Measures

Assessment of lean body mass (LBM) using dual-energy x-ray absorptiometry and of leg isokinetic muscle strength at a speed of 150°/s were done before, after the 12-week rehabilitation and exercise training program, and 3 months after the exercise program was completed (12mo postburn). The effects of exercise on the dependent variables were analyzed by repeated-measures analysis of variance. If we found a significant overall effect of time and/or intervention, we did a post hoc test for multiple comparison (Holm-Sidak). Results are expressed as mean ± standard error.

Results

The mean percentage increase in LBM and muscle strength was significantly greater in the exercise group (6.4%±1.9%, 40.7%±8.6%, respectively) than in the no-exercise group (1.9%±2.6% vs 3.4%±4.5%, respectively). Three months after cessation of the exercise program, LBM remained relatively unchanged in the no-exercise group (3.5%±1.8%). In contrast, LBM in the exercise group increased significantly (10.7%±4.8%, P=.03). In addition, muscle strength further increased by 17.9%±10.1% in the exercise group versus 7.2%±3.4% in the no-exercise group, although neither percentage increase was significant (P=.08 for exercise vs P=.61 for no exercise). Absolute values in LBM and muscle strength for both groups at 12 months postburn continued to be below historical or concurrent age-matched, nonburned children.

Conclusions

Participation in an exercise program resulted in a greater improvement in LBM and muscle strength in the exercise group than in the no-exercise group. Three months after the exercise training ended, there were persistent mild-to-moderate increases in LBM and muscle strength. Absolute levels continued to be below previously reported nonburned, age-matched values, however, which underscores the need for continued exercise to improve LBM and muscle strength in severely burned children.

Key Words: Burns, Child, Exercise, Muscles, Rehabilitation

SEVERE BURNS RESULT in persistent and extensive skeletal muscle catabolism and weakness,¹ which is worsened by prolonged physical inactivity.², ³ The current standard of care consists of rehabilitation exercises of occupational therapy (OT) and physical therapy (PT), which can be done in a hospital setting or in a patient’s home. There are problems with compliance because these exercises are typically done without expert supervision, and often lack structure. Muscle catabolism and weakness persist, however, despite therapy. The physical frailty associated with severe burns is often confounded by cardiac and systemic shock, hypermetabolism, respiratory injury, sepsis, postburn seizures, compromised bone formation, major surgeries, malnourishment, disturbed growth patterns, and psychosocial issues.¹, ⁴, ⁵, ⁶, ⁷ Additionally, low physical work capacity and muscle strength are major obstacles to a burn victim’s return to school and performance of activities of daily living (ADLs).

We have previously reported⁸, ⁹ that a 12-week supervised and structured program of resistive exercise implemented 6 months post-burn in severely burned children increases muscle strength and muscle mass. Because ADLs are integrated functions that require muscle strength and endurance, an effective resistance exercise program may contribute to the rehabilitation of severely burned children by increasing their muscular strength and their capacity to do work.¹⁰, ¹¹, ¹²

Although exercise training improves physical function in severely burned children, it is not known whether these functional and structural benefits last for at least 3 months after a 12-week exercise program. Previous studies in nonburned adults have found that strength performance in general is maintained for up to 4 weeks of inactivity.¹³, ¹⁴ In nonburned children, however, an 8-week strength-training program followed by 8 weeks of detraining resulted in a weekly mean strength loss of 3%. Furthermore, and most important, values in the strength of these trained children regressed toward the values of the untrained control group within 8 weeks of inactivity.¹⁵

Therefore, we designed a study in which we assessed lean mass and muscle strength before and after a 12-week supervised and structured exercise program in a group of severely burned children. In addition, we assessed whether any resulting benefits would be maintained, to a greater extent than in subjects in a no-exercise group, 3 months after the exercise program ended.

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Methods 

Burned Subjects 

We randomized severely burned children, ages 7 to 18 years, into either a no-exercise group or an exercise group. We only enrolled patients with 40% or more total body surface area (TBSA) burned, as assessed by the “rule of nines” method,¹⁶ during excision surgery in the acute phase of injury. Patients were excluded if they had 1 or more of the following: leg amputation, anoxic brain injury, psychologic disorders, quadriplegia, or severe behavior or cognitive disorders. The parent or legal guardian gave informed consent during the first day of admission to acute care. Eleven patients were assigned to participate in a 12-week, in-hospital physical rehabilitation program supplemented with an individualized and supervised exercise-training program. The no-exercise group was to receive standard of care. That is, a home-based written set of instructions of PT and OT exercises.¹⁷

All patients received similar standard medical treatment from the time of hospital admission until time of discharge. In addition, both groups were discharged with similar standard medical and rehabilitation care until the 6-month postburn time point.¹⁷

At 6 months postinjury, all patients returned to Shriners Hospitals for Children in Galveston, TX, for exercise testing. After completing the tests, the exercise group began participation in a 12-week in-hospital physical rehabilitation program supplemented with an individualized and supervised exercise-training program. In contrast, the no-exercise group returned home, if they had no surgical needs, to continue the prescribed OT and PT standard regimen, which has been described previously.¹⁷ Patients in the no-exercise group did not receive an exercise prescription from an exercise physiologist at any time during the study period from 6 months postinjury to 9 months postinjury. The institutional review board at our institution approved the study. Our comparison group was comprised of age-matched, healthy, nonburned children.

Exercise Testing 

Exercise assessments were made for all subjects at 6 months and 9 months postinjury. These time points were equivalent to assessments made before and immediately after training in the exercise group. There was no formal training for the no-exercise group, therefore tests were done at 6 and 9 months after injury. Finally, all burned children underwent exercise testing 3 months after cessation of the specific intervention (ie, 1y postinjury).

Prior to strength testing, patients were made familiar with the exercise equipment and were instructed on proper weight-lifting techniques. They sat quietly for approximately 15 minutes before we recorded their resting measurements, after which we measured their vertical height and body weight.

Strength Measurements 

Strength testing was conducted on day 1, before the start of the exercise program, and after 6 weeks of training; we used a Biodex System 3 dynamometer.^a The isokinetic test was performed at an angular velocity of 150°/s on the dominant leg extensors. We chose this speed (vs lower or higher angular speeds) because it was well tolerated by the children of all ages in both groups. The patients were seated and their position stabilized with a restraining strap placed over the mid-thigh, pelvis, and trunk, in accordance with the Biodex System 3 manual. All patients were made familiar with the Biodex test in the same manner. First, the test administrator demonstrated the procedure for the patients, then it was explained to them, after which they practiced the actual movement, without load, by doing 3 submaximal repetitions. More repetitions were not permitted so as to prevent fatigue. The anatomic axis of the knee joint was aligned with the mechanical axis of the dynamometer before the test. After the warm-up repetitions, subjects performed 10 maximal voluntary muscle contractions (full extension and flexion) consecutively without resting between contractions. The test was repeated after 3 minutes of rest to minimize the effects of fatigue.

Peak torque values were calculated with the Biodex software system and we selected the highest value of the 2 trials. Peak torque was corrected for gravitational moments of the lower leg and the lever arm. We used a similar procedure to assess the muscle strength in children without burn injuries. Peak torque values were again calculated with the Biodex software system and the highest value of the 2 trials was selected. Peak torque was corrected for gravitational moments of the lower leg and the lever arm.

3 Repetition Maximum Test 

Only the patients in the exercise group were tested, after a 30-minute rest, to determine the amount of weight or load to use as baseline loads in the first week of the 12-week program. Subjects were tested in the following order of exercises: bench press, leg press, shoulder press, leg extension, biceps curl, leg curl, and triceps curl. The 3 repetition maximum (3-RM) load was determined as follows. After being instructed in the correct weightlifting technique, patients warmed up with lever arm and bar (or wooden dowel) and became familiar with the movement. They then attempted to lift a weight 4 times (ie, 4 repetitions). If the fourth repetition was achieved successfully and with correct technique, subjects were permitted 1-minute rest. After resting, they were instructed to lift a progressively increased amount of weight or load at least 4 times. If they lifted a weight that allowed successful completion of 3 repetitions of the task, but were unable to perform a 4th repetition because of fatigue or an inability to maintain the correct technique, the test was terminated and the amount of weight lifted from the successful set was recorded as their individual 3-RM.

Lean Body Mass Measurements 

On day 2 (6-mo, 9-mo, end of detraining), lean body mass (LBM) measurements were taken for both groups by dual-energy x-ray absorptiometry (DXA), using the QDR 4500A software.^b Scans were taken with the patient laying supine on the scanning table. We followed the previously described protocol¹⁸, ¹⁹ for obtaining a whole body scan. Briefly, DXA with pediatric software can measure the attenuation of 2 x-ray beams, of which 1 is high energy and the other low energy. These measurements are then compared with standard models of thickness used for bone and soft tissue. Subsequently, the calculated soft tissue is separated into LBM and fat mass. LBM is reported in grams.

Exercise Training Program 

All subjects were sedentary before starting the exercise program and had never participated in an exercise-training program. Training sessions consisted of resistance and aerobic exercises.

We used 8 basic resistive exercises: bench press, leg press, shoulder press, leg extension, biceps curl, leg curl, triceps curl, and toe raises. At no time did the exercise group use the Biodex dynamometer. All exercises were done using variable resistance machines or free-weights, and were done 3 days a week (Monday, Wednesday, Friday). During the first week, the patients became familiar with the exercise equipment and were instructed in proper weightlifting techniques. The weight or load lifted was set at 50% to 60% of their individual 3-RM. During the second week, the lifting load was increased to a range of 70% to 75% (4–10 repetitions) of their individual 3-RM and was maintained for weeks 2 through 6. After this, training intensity was increased to a range of 80% to 85% (8–12 repetitions) of the 3-RM for weeks 7 through 12.

Each training session also included aerobic conditioning exercises on a treadmill or cycle ergometer and was also done 3 days a week. Each session lasted 20 to 40 minutes and participants exercised at 70% to 85% of their previously determined individual peak oxygen consumption (Vo₂peak). All sessions were preceded by a 5-minute warm-up period on the treadmill at an intensity of less than 50% of each individual Vo₂peak. Heart rate and oxygen saturation were monitored with a pulse oximeter.^c Rated perceived exertion was obtained at a regular interval. All exercise sessions and exercise prescriptions were supervised by an exercise specialist and were conducted according to the guidelines set by the American College of Sports Medicine (ACSM) and the American Academy of Pediatrics (AAP).²⁰, ²¹, ²² No strength training activities were permitted outside the supervised training session; however, both groups were allowed to pursue their normal daily activities. Patients in the exercise program were required to have participated in at least 33 of the 36 total workout sessions to be considered to be in compliance with the program.

Home Exercise Prescription 

On completion of the exercise program, or at 9 months postburn, all subjects were given written instructions (home exercise prescription), which described the activities that patients in the exercise group had been performing and which made recommendations for continued participation in aerobic conditioning and strength training. The prescription given the no-exercise group did not describe past activities, but did include recommendations for physical activities. Exercise prescriptions for both groups were similar for both aerobic and strength training. The patient and his/her caregivers were given written instructions on the weight-lifting techniques, on techniques for assessing heart rate and rated perceived exertion, and also on the frequency and duration of exercise. Patients and families were questioned on compliance with program, but the detraining component was of an intent-to-treat design. Details of the home exercise prescription-training program are shown in appendix 1.

Aerobic Home Exercise Prescription 

We recommended that the prescribed aerobic conditioning exercises be performed 3 to 5 days a week for 20 to 40 minutes a session, with 5-minute warm up and cool down periods included. Suggested activities included using the treadmill, bicycle, rowing machine, cycle ergometer, and elliptical machine; swimming; participating in organized sports; and/or walking and jogging, depending on equipment availability and patients’ interests. Patients and families were instructed on how to assess exercise intensity by using a rated perceived exertion scale throughout each exercise session. An exercise specialist provided all exercise prescriptions, which were in line with guidelines developed by the ACSM and AAP.²¹, ²³, ²⁴

Resistive Home Exercise Prescription 

We recommended that the prescribed resistive conditioning exercises be performed 2 to 3 days a week with a rest day between each session. All patients were instructed to begin with 2 sets of 6 to 10 repetitions and progress to 3 sets of 8 to 12 repetitions over the 12-week period. Eight basic strength training exercises were included: bench or chest press, shoulder press, biceps curl, triceps extension, leg press, leg extension, hamstring curl, and toe raises. All exercises were to be performed, depending on availability, using variable resistance machines, free weights, or body weight. Before leaving the hospital exercise center, all patients and their families were instructed on proper form and weight-lifting techniques. A 3-RM was determined for each exercise and a beginning workload was set at 50% to 60% of this value. Patients were instructed to increase the workload when the final 2 repetitions of each set were no longer difficult. In addition to the gym-based exercise, patients were given a set of diagrams describing similar exercises that could be performed at home with resistance bands and were instructed in the use of the bands and the performance of the specific exercises. Light, medium, and heavy resistance bands were given and patients were instructed to follow the same guidelines and progression as those performing gym-based activities.²⁰, ²¹, ²², ²⁵

Children Without Burn Injury 

We recruited 26 age-matched, nonburned, apparently healthy children to compare their LBM and muscle strength with that of the children with burns. Similar data have also been previously reported.⁸, ⁹ Assessments of LBM and muscle strength was similar to that for the burned children but were only done at 1 time point.

Data Analysis 

All data are expressed as mean ± standard error of the mean (SEM). We analyzed the effects of exercise on the dependent variables by repeated-measures analysis of variance. If there was a significant overall effect of time and/or intervention, we did a post hoc test (Holm-Sidak) for multiple comparisons. Significance was determined and the P value adjusted by the Holm-Sidak method, which accounts for the number of comparisons done. We corrected for differences in LBM by dividing peak torque by LBM.

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Results 

We enrolled 20 burned children (17 boys, 3 girls) in the study and assessed their 1-year complete longitudinal data. Eleven exercise patients and 9 no-exercise patients were tested at 6 and 9 months postburn, and at 3 months after the supervised and structured exercise program ended. The age range for both groups was 7 to 18 years (13.4±1.8y for the no-exercise group vs 11.8±1.5y for the exercise group, P=.30). There were no differences at 6 months postburn between the groups in age, percentage of TBSA, vertical height, standing weight, and body surface area. At 9 months postburn, both groups had similar levels of absolute vertical height and standing weight, although the change in weight from 6 to 9 months postburn was significant only in the exercise group (difference in means: 0.3kg for the no-exercise group vs 2.8kg for the exercise group). At 12 months postburn, body weight and vertical height did not differ significantly between groups (P=.80, P=.78, respectively) (table 1).

Table 1. Demographic Characteristics

NOTE. Values are mean ± SEM. No significant differences were noted in age, % TBSA, height, or weight at 6 months postburn between the exercise and no-exercise groups.

Abbreviations: F, female; M, male; NA, not applicable; NC, no change.

Measurement of total LBM, obtained by DXA, revealed a mean increase of 6.4%±1.9% (baseline value, 36.90±5.51kg; P=.005) in the exercise group after 12 weeks of training (fig 1). In contrast, the mean total LBM from 6 to 9 months in the no-exercise group remained relatively unchanged (1.9%±2.6%; baseline value, 34.57±4.10kg; P=.565). Three months after cessation of the exercise program, LBM remained relatively unchanged in the no-exercise group (3.5%±1.8%). In contrast, LBM in the exercise group increased significantly (10.7%±4.8%, P=.03). Group mean values of LBM are presented in table 2.

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• Fig 1. 

  Mean percentage change in total LBM after 12 weeks of exercise training and also 12 weeks after the end of exercise training. NOTE. Values are mean ± SEM. Total LBM values are reported in kilograms. Abbreviations: EX, exercise; NoEx, no exercise. *P<.05 for 6 to 9 months mean percentage change (within group). †P<.05 for 9 to 12 months postburn mean percentage change (within group).

Table 2. Leg Muscle Peak Torque and Total LBM Results

NOTE. Values are mean ± SEM.

There was a significant increase in strength (reflected by peak torque) after 12 weeks of the exercise intervention in the exercise group (40.7%±8.6%; baseline value, 31.30±6.30Nm), but not in the no-exercise group (3.4%±4.5%; baseline value, 47.05±9.21Nm). Three months after cessation of the exercise program, peak torque was further increased in the exercise group (17.9%±10.1%) versus in the no-exercise group (7.2%±13.4%; fig 2), although neither percentage increase was significant (P=.08 for exercise vs P=.61 for no exercise). Similarly, absolute values in peak torque for both groups at 12 months postburn did not differ significantly (P=.55). Group mean values obtained for peak torque are reported in table 2.

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• Fig 2. 

  Mean percentage change in knee extensor peak torque at 150°/s after 12 weeks of exercise intervention and 12 weeks after cessation of the exercise program. NOTE. Values are mean ± SEM. Absolute peak torque measured in newton-meters was used in the calculation of percentage changes. *P<.05 for 6 to 9 months percentage change.

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Discussion 

Our results indicate that there was an increase in muscle strength and LBM in the exercise group after 12 weeks of exercise, whereas in the no-exercise group, both muscle strength and LBM remain relatively unchanged. Three months after the cessation of supervised and structured training, LBM increased significantly only in the exercise group. In contrast, further increases in muscle strength in both groups were not significant. When expressed in absolute values, between-group comparisons of muscle strength and LBM at any time point did not differ significantly.

To our knowledge, there have been no previous reports of the effects of detraining or cessation of training in burned children after they have participated in a resistance exercise-training program. Our results in this study are in agreement with reported strength gains in nonburned children who trained using various resistance exercise protocols. Reported improvements in strength have ranged from 13% to 74%.²⁶, ²⁷ These increases differ from our mean increase of 40%, and may be the result of differences between the studies in the length of the program, frequency of training, and mode of testing (isokinetic vs isotonic). Our results are also in agreement with results of a previous study by our group in which there was a significant increase of 44% in muscle strength after a similar resistance training program.⁸ Whereas the benefit of exercise training on muscle strength and LBM in burned children has been once more substantiated, our detraining results are in partial disagreement with those reported in the literature concerning nonburned pediatric patients. The few published studies we found reported decreases in muscle strength due to detraining.¹⁵, ²⁸

Sewall and Micheli²⁸ reported increases in leg muscle strength of 30% in response to a 9-week progressive resistive strength training program in pre-pubescent children. In contrast, leg muscle strength decreased only 1.3% after 9 weeks of detraining.²⁸ Unfortunately, muscle strength was not assessed in the control group after training, and it is possible that a similar drop could have occurred in that group. In addition, while strength during shoulder flexion increased 96% in the exercise group versus 18% in the no-exercise group, 9 weeks of detraining resulted in an increase in strength of 66%. Again, a lack of details prevent our knowing if the increase was calculated using the start of the 9-week exercise program or the end of the program as the preassessment and the end of the detraining period as the postassessment. In the other study of children, Faigenbaum et al¹⁵ reported an increase in leg muscle strength of 54% in response to an 8-week exercise program, but a decrease of 28% in lower-body strength in children after an 8-week detraining period. Our results support the notion that there is not a significant increase in muscle strength after training. Because there is not a decrease, however, the net effect is a relative stability of strength values compared with the end of exercise training.

Again, to our knowledge, LBM and the effects of detraining have not been explored in nonburned or in burned children. Sewall and Micheli²⁸ reported a mean decrease in body weight of .51% during the exercise training period in the exercise group and a gain of 6.7% in the control group. There was, however, a gain of 3.48% over the subsequent 9 weeks of detraining in the group that previously exercised. The control group’s body weight was not assessed during the detraining period. The assumption in the Sewall and Micheli study was that body weight would reflect fat mass, which may not be appropriate. Our study shows that exercise significantly increases lean mass. After 12 weeks of detraining, however, lean mass continued to significantly increase in the exercise group, but not in the no-exercise group. This is important for 2 reasons. One, exercise does increase LBM to a higher level than if there is no exercise, which is a desirable effect in burned children. Two, despite cessation of the exercise stimulus, LBM did not decrease in either group.

We are pleased with the effects of exercise on LBM and, to some extent, on muscle strength in burned children. We expected that once our “formal” supervised training stopped, both structure (ie, LBM) and function (ie, muscle strength) would deteriorate. This was not the case, as mean values in structure and function did not decrease. In fact, the increase of 18% in muscle strength may be of physiologic significance despite a lack of statistical significance (P=.09). It is important to note, however, that although values in burned children had a tendency to increase after the formal training period, the absolute values in muscle strength and LBM were still below values for nonburned children (see table 1).⁸, ⁹

Direct or indirect effects of our “formal” exercise training may be operant during the detraining period and may help explain the lack of a decrease in muscle strength or a continued increase in LBM. These include increased spontaneous physical activity,²⁹ improved nutritional habits,³⁰ improved behavior modification strategies,³¹, ³², ³³ and also a time effect on burn-induced catabolism.¹⁸ In addition, there is the possibility that complete cessation of physical activities (or detraining) did not occur, as some children may have continued to be physically active. We did not formally evaluate physical activity patterns or behavior from the 9- to the 12-month postinjury time point. When briefly questioned as to their physical activities during this time period, all the children acknowledged no formal, structured, supervised, intensity-controlled (ie, percentage of peak aerobic capacity set or heart rate monitored) exercise activities, and no-resistive training activities. Some children in both groups, however, did report increased play and child-like activities. Nevertheless, again it is important to note that in burned children, the absolute values in muscle strength and LBM were still below values for historical or concurrent nonburned children (see table 2).⁸, ⁹

In our review of the literature, we found no studies that reported on changes in strength and lean mass after an exercise-training program for burned children. We have shown that exercise training in burned children improves muscle strength and lean mass relative to a no-exercise group of children. Our results also show that after the structured and supervised exercise program ended, LBM continued to improve, while there was a pattern of continuing increases in muscle strength (P=.09) in the exercise group, but not in the no-exercise group. Based on these findings, we advocate that an exercise program be initiated to improve muscle strength and LBM. We recommend a continued exercise maintenance program, however, either to minimize loss of exercise-induced benefits, or to facilitate further gains in those benefits. This is particularly important when we compare the values of muscle strength and LBM in burned versus nonburned children.⁸, ⁹

Study Limitations 

We recognize that the possibility of a type II error in this study cannot be ruled out and that the number of subjects in both groups is small.

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Conclusions 

We report here the benefits of a supervised and structured exercise program relative to a home exercise prescription. In addition, there is a continued improvement in these benefits 3 months after the structured and supervised exercise program is stopped. Further studies are needed to determine what type of exercise maintenance program is optimal in maintaining or further improving LBM and muscle strength in burned children. Such studies should also assess how to best improve nutritional habits and spontaneous physical activity, or modify behavior to decrease time spent in sedentary activities.

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Appendix 

Appendix 1. Home Exercise Prescription

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• a Biodex Medical Systems Inc, 20 Ramsay Rd, Shirley, NY 11967-4704.
• b Hologics Inc, 35 Crosby Dr, Bedford, MA 01730.
• c Ohmeda Medical, 8880 Gorman Rd, Laurel, MD 20723.