Lewis, Cynthia L. PhD, PT; Fragala-Pinkham, Maria A. MS, PT
Down syndrome (DS) was the first genetic disorder attributed to a chromosomal abnormality (trisomy 21) and is the most common genetic form of mental retardation.1 The syndrome is characterized by several clinical symptoms that often include orthopedic, cardiovascular, neurological, cognitive, hormonal, and visual perceptual impairments.
Individuals with this disorder have hypotonia and ligamentus laxity, which contribute to orthopedic impairments. Hypotonia and muscle weakness are theorized to impair upper extremity midline movements and gait, noted by shorter step lengths, increased knee flexion at foot contact, decreased single-limb support, and increased hip flexion posture that can contribute to the higher energy cost of gait in persons with this disorder.2 Children with DS have decreased pulmonary function and physical fitness compared with peers who are typically developing.3
Obesity is common in postpubescent males and females with DS. They have a shorter stature and a higher percentage of body fat, and a greater body mass index than age-matched peers.4 Despite comorbidities, life expectancy for individuals with DS has significantly increased over the past three decades. More than 80% of individuals with DS survive past 30 years of age.1 However, with this increased life expectancy come other secondary disorders, including increased incidences of diabetes mellitus and Alzheimer-like dementia after age 30 or 35 years.5,6 Individuals with DS are more sedentary than their siblings.7,8 Higher incidences of diabetes and obesity and a sedentary lifestyle are primary risk factors for cardiovascular disease.9,10
Researchers have demonstrated the beneficial effects of aerobic conditioning in preventing, delaying the onset, and the management of cardiovascular disease, diabetes, and obesity in populations without DS.9 Aerobic conditioning may also help reduce these risk factors in the population with DS. However, although many characteristics of DS are documented, less well understood are the effects that these characteristics have on exercise performance and aerobic training.
At submaximal workloads, individuals with DS have higher heart rates (HRs), oxygen consumption (Vo2), and minute ventilation (VE) than peers with mental retardation but without DS and peers without impairment.10 However, at maximal effort, individuals with DS have lower mean peak HRs and lower Vo2, suggesting a lower level of aerobic conditioning.11–13 Individuals with DS were found to have two thirds the mean force in hip abductor strength and half the mean force in knee extensor strength when compared with peers with mental retardation but without DS.14 Subjects with DS had a high correlation (r = 0.84) between lower limb strength and oxygen consumption during maximal treadmill stress testing that is not observed in individuals without impairment.15 Thus, skeletal and respiratory muscle weakness may be contributing factors to lower maximal oxygen consumption observed in individuals with DS during treadmill stress testing. Although a treadmill stress test is designed to examine the maximal performance of the cardiovascular system rather than the skeletal muscle system, in persons with DS, muscular fatigue may occur before the cardiovascular system is maximally stressed.
Individuals with DS may benefit from aerobic conditioning, but the program’s frequency, intensity, and duration need to be modified from the general recommendations of the American College of Sports Medicine (ACSM).9 Researchers have demonstrated the improvements in cardiovascular function in sedentary individuals without impairment who participated in aerobic training for 30 minutes at 65% to 75% maximal Vo2 three times per week.9 Cardiovascular benefits with aerobic conditioning include a lower resting HR, lower submaximal HR, respiration rates (RRs), and Vo2, and higher Vo2 at maximal workload.16,17
When persons with DS participated in aerobic training programs following the ACSM guidelines, they improved their endurance but showed no changes in cardiovascular function.18,19 Miller et al18 conducted a 10-week jogging program and Varela et al19 carried out a 16-week upper body ergometry-rowing program with young adults with DS. Subjects participated in aerobic activity 30 minutes at 65% to 75% Vo2 maximum three times per week. Following aerobic training protocols, subjects with DS increased their aerobic endurance but showed no changes in cardiovascular variables such as resting HR or submaximal HR, Vo2, and VE. Subjects, however, walked longer on the treadmill or rowed longer on the upper body ergometer, indicating increased aerobic performance.
Aerobic conditioning programs produced improvements in endurance but not in cardiovascular function in subjects with DS. The high correlation of strength to Vo2 during treadmill stress testing would suggest that a combined strength training and aerobic conditioning program may be more effective for enhancing cardiovascular function in individuals with DS than aerobic training alone. Muscle weakness and muscle fatigue may be limiting factors in aerobic training programs in subjects with DS that are not observed in subjects without impairment.
The purpose of this single case study was to investigate the effects of a six-week home treatment program for a 10.5-year-old girl with DS. The program combined aerobic conditioning and strength training for 30 to 60 minutes daily at 70% to 80% intensity six days per week.
The subject was a 10.5-year-old girl with DS. As a third grade student, she received consultative physical therapy services once monthly through the school system. She participated one hour per week in sport skills activities through the Special Olympics Organization that included swimming, basketball, and soccer. She had cognitive impairments but was able to follow two-step commands. Her parents’ concerns were that she was overweight and fatigued easily. She was unable to keep up with her siblings on family outings. The parents’ goals for the program were increased functional endurance and weight loss. The subject’s interests were music and dancing.
As an infant, she had surgery to repair a ventriculoseptal defect, and, according to the cardiology consultation report, she did not have any residual cardiac problems. At the time of the study, she had no medical problems and was on no medication. Physician approval and parental consent were received for the subject to participate in a home exercise program.
The study was a single case study design. Assessments were done before and after intervention. Her program consisted of a physical therapy consultation to monitor progress and update the program once per week and a daily home exercise program for six weeks. Her home exercise program took place initially four to five days per week increasing to six days per week. By the third week, she performed aerobic activities three days per week alternating with strength training three days per week. She continued to receive once-monthly physical therapy consultation with the school system for academic-related concerns.
The subject was assessed in the following areas: cardiovascular function, body dimensions, flexibility, gross motor skills, anaerobic power test, and muscle strength and endurance.
Oxygen consumption was measured to assess aerobic performance during a submaximal treadmill stress test following the protocol of Rose et al.20 The subject had one practice session on the treadmill one week before testing to familiarize her with walking on a treadmill. She performed three-minute stages starting at 23 m/min (0.8 mi/hr) and increasing 14 m/min (0.5 mph) at each successive stage.20 She was encouraged to complete each stage and progress to the next stage. The testing ended when she indicated that she did not want to go to a higher stage or could no longer walk at the current stage before running. After testing, she did a cool down period at slower speeds.
Expired gases were analyzed using a Medgraphs Metabolic Cart. A Hans Rudolph Mask of appropriate size was used for collecting expired air. Individuals with DS have decreased facial muscle tone, and using a mouthpiece to collect expired gases to prevent expired air from escaping can be challenging. The subject practiced walking on the treadmill while wearing the mask to become familiar with the procedure before testing.
Height and weight were measured and body mass index was calculated. Body mass index was determined by dividing her body weight in kilograms by her height in meters squared (kg/m2) and was used to indicate overall body composition.9
Flexibility was assessed by the sit and reach test, ankle dorsiflexion with knee extension, Apley scratch test, and goniometric measurements of hip internal rotation.
Gross Motor Assessment.
Gross motor skills were assessed using the Gross Motor Scales of the Bruininks-Oseretsky Test of Motor Proficiency (BOTMP). The Gross Motor Scales consist of four subtests to determine running speed and agility, balance, strength, and bilateral coordination. A composite score was calculated from these four subtests.
A modification of the Margaria-Kalamen power test was used to assess anaerobic capacities or muscular power.21 In the modified power test, the subject ascended a flight of stairs as quickly as possible one step at a time. In the typical Margaria-Kalamen power test, healthy young athletes climb three steps at a time. Power was determined by multiplying the subject’s body weight in kilograms by the change in vertical height in meters divided by time21: P (power) = [F (force), weight in kg] × D (vertical displacement]/T (time). Units are kilogram meters per second (kg·m/sec). The vertical height was the vertical rise from the base to the top of the flight of stairs.
Muscle Strength and Endurance Measurements.
Timed sit-ups in supine and back extensions in prone were used to test trunk muscle strength and endurance. The tests were modified because the child could not perform a standard sit-up or push-up independently even while using a 30-degree wedge. Assisted sit-ups were performed having the subject focus on eccentric lowering of the trunk and upper body. Back extensor strength and endurance were determined by the number of prone extensions over a therapeutic ball that she could complete without stopping for more than a three-second rest. A 10-repetition maximum (10 RM) was employed to assess shoulder flexion and abduction; hip extension, adduction, and abduction; and knee extension bilaterally for upper and lower limb strength and endurance, respectively. A 10 RM is the maximal amount of weight that the subject can lift 10 times.9 A 10 RM was a safe assessment to employ with this subject as less weight is placed on the joints when compared with a one RM or three RM, the maximal amount of weight that can be lifted one time or three times, respectively.
Home Exercise Training Protocol: Aerobic Conditioning
Maximal HR of individuals with DS is less than that of peers without the disorder.13,22 Using the ACSM guidelines of maximal training HR (220 − age = age predicted maximal HR)9 would overestimate the HR of individuals with DS. According to Fernhall et al,22 maximal HR of young adolescents with DS has was found to be approximately 170 to 180 beats per minute. Initial aerobic training intensity was 60% of 180 or a target HR of 101 progressing to 70% at a HR of 126 up to 80% at 144 beats per minute.
The initial aerobic program was performed twice weekly increasing to three times per week.
Initial program required 10 to 15 minutes daily increasing to 45 to 60 minutes daily.
Aerobic activities consisted of ribbon wand exercises, walking through an obstacle course, and stair climbing. The subject was videotaped doing the ribbon wand exercises and dancing, and she used the videotape to guide exercises at home. Music was used for motivation during the movement activities.
Intensity was progressed by increasing the number of repetitions of sit-ups and trunk extensions and increasing the number of repetitions and weight exercises designed to strengthen the limbs.
Initial strength training was performed twice weekly increasing to three times per week by the third week of the study.
Training began at 10 to 15 minutes daily and was increased to 30- to 45-minute sessions.
Strength training included eccentric sit-ups and concentric trunk extensions over a therapeutic ball; knee extension and flexion and hip flexion in sitting; leg lifts in side-lying, prone, and supine positions; squats, toe raises, and heel raises; and hip abduction in the standing position using Theraband for resistance. Upper limb exercises included resistance training using cuff weights and Theraband in diagonal patterns, shoulder abduction/adduction, and shoulder flexion/extension.
The subject and her family kept a daily chart of her exercise program, recording activities performed, intensity, and duration. The child’s age, cognitive level, motivation, and general physical status were considered, and exercises were altered according to these factors. If she became bored with an activity, it was adapted to maintain her interest.
Mean values are reported for HR, RR, and submaximal and peak Vo2. Absolute values are presented for the body mass index, flexibility, BOTMP Gross Motor Scales, Margaria-Kalamen power step test, and strength training. A paired t test was used to determine differences in pre- and posttraining performances where applicable. The p value was set at p = 0.05 for level of significance.
Changes in posttraining submaximal treadmill tests are in Table 1. The subject demonstrated significantly lower HR (p = 0.008) and RR (p = 0.038) posttraining. However, her Vo2 did not significantly change during either the submaximal protocol (Table 1) or highest Vo2 reached (Table 2).
Body mass index did not change during the training period either (Table 2). Flexibility was within normal limits for the sit and reach test, ankle dorsiflexion with knee extension, and Apley scratch test. She had a slight decrease in hip internal rotation at the beginning of the exercise program that was unchanged posttraining (Table 2).
She demonstrated significant gains in gross motor skills posttraining (Table 2). Her composite point score changed from 2 to 19 on the Gross Motor Scale of the BOTMP. She demonstrated an almost 60% increase in anaerobic power as measured by the modified Margaria-Kalamen power test (Table 2). Strength gains were observed in all measurements for the trunk and upper and lower limbs (p = 0.014) (Table 2).
Factors contributing to the success of this program were individualized exercises and parental support. Program adherence was 93% as calculated by the number of recommended sessions and the actual number of days that exercises were reported. Strategies used to motivate the subject were allowing her a choice of activities, videotaping her exercises during the once-weekly physical therapy intervention for carry over at home, and using a sticker chart. The child’s mother supervised and assisted her throughout the training program, which contributed to the success of the home exercise program. Her parents did indicate difficulty in continuing this type of program. They were pleased with the success but could not keep up the intensity. After the study ended, with input by the physical therapist, they switched to a treadmill training program five days per week for 30 minutes per session and strength training two days per week for 20 to 30 minutes with a monthly physical therapy consultation. A few months later, they reported continued success with endurance, enhanced community mobility, and weight management.
This subject was able to demonstrate both aerobic and strength improvements by performing a combined aerobic and strength exercise training protocol. Of the three cardiovascular variables, HR, RR, and Vo2, she demonstrated improvements in two areas: lower HR and RR at all stages on the submaximal stress test post training. No differences were found in Vo2. Not all cardiovascular changes occur at the same rate. Adaptation in HR and RR can occur more quickly than changes in Vo2.23 The program was conducted for six weeks. A training program of longer duration may have resulted in enhanced Vo2.
These cardiovascular findings differ from those reported by Miller et al18 and Varela et al.19 In those two studies, subjects were adolescents and young adults with DS who performed a 10-week program of walking/jogging and a 16-week rowing ergometry training regimen, respectively. The frequency was three days per week, and the intensity was initially 55% progressing to 70%. In both of these studies, subjects demonstrated increased endurance posttraining by performing one minute longer on the maximal stress tests, but subjects had no changes in cardiovascular variables. The duration, intensity, and frequency followed in these two research studies were those recommended by the ACSM for subjects without impairment: 30 minutes per day, 60% to 80% intensity, three days per week.
Differences in this study compared with previous research may result from differences in exercise training protocols. In this study, aerobic training intensity was higher, 65% to 80% maximal HR versus 55% to 70%, duration was longer (30 to 60 minutes vs 30 to 45 minutes), but the frequency of aerobic conditioning was the same (three times per week). Possibly the difference in findings in the current study and previous reports was the inclusion of the strength training program on alternate days of the week. The subject in this study increased her 10 RM from three to five pounds in knee extension as well as increasing her 10 RM in hip extension, adduction, abduction, and increased number of repetitions of sit-up and back extensions. Improvements in trunk and lower limb strength could result in a lower energy cost of gait as demonstrated by lower HR and RR and increased performance time on the treadmill.
Because of the high correlation of leg strength to maximal Vo2 (r = 0.84) in subjects with DS, the rate limiter in treadmill and upper body ergometry stress testing may be skeletal muscle fatigue rather than cardiovascular fatigue.15 As previously noted, maximal stress is designed to strain the cardiovascular system before causing muscle fatigue. Individuals with impairment in the respiratory system or musculoskeletal system may fatigue these systems before fully stressing the cardiovascular system during a stress test.
Fernhall et al24 demonstrated that adolescents and young adults with DS had reduced cardiovascular endurance when compared with peers with mental retardation but without DS. Cioni et al25 and Horvat et al26 measured decreased isokinetic torque, average power, and flexion/extension ratios in subjects with mental retardation when compared with subjects without disabilities. However, these measures of strength were even lower in the group with DS than in peers with mental retardation but without DS. In these studies, subjects with DS demonstrated even lower cardiovascular and strength measures when compared with peers with mental retardation but without DS.
In 1994, Dyer27 conducted a circuit weight-training program in an A-B-A design with 10 children and adolescents with DS. She reported that subjects demonstrated a lower mean resting HR, lower mean resting blood pressure, and lower peak HR to a three-minute step test after a 13-week strength training program. Oxygen consumption was not measured in this study. This strength training program resulted in changes in cardiovascular variables as noted by lower resting HR and blood pressure.
The subject in this study displayed not only gains in cardiovascular variables and strength measures, she also demonstrated improved balance, coordination, and power in gross motor tasks. Her score on the Gross Motor Scale of BOTMP increased from 2 to 19. This scale consists of four subtests that measure running speed and agility, balance, strength, and bilateral coordination. Activities included in the subtests were walking heel-toe on a line and on a balance beam, unilateral stance with eyes open and closed on and off a balance beam, jumping up and down with and without clapping hands and with and without touching heels, standing broad jump, and running speed and agility over a 45-ft distance.
She improved her score on the modified Margaria-Kalamen power test from 14 kg·m/sec to 22 kg·m/sec. Since her body weight did not change, the increased score would require her to ascend the stairs more quickly. Her improved balance, running, and coordination as noted by her increased Gross Motor Scale score and by increased trunk and lower limb muscle strength and endurance as noted by her increased number of repetitions and 10 RM measurements could account for her faster speed in stair climbing for an improved anaerobic power score. However, stair training was also part of the aerobic program, and she may have experienced a practice effect.
Since the program was six weeks in length, the changes in muscle strength and speed are a result of enhanced neural recruitment rather than changes in increased muscle fiber size.28 Typically, prepubescent children do not demonstrate changes in muscle fiber size, and increased strength results from enhanced neural recruitment and timing.
Two measurements were unchanged: flexibility and body weight. Although change in flexibility was not a goal of the program, decreased body weight was. Although she did not lose weight, she did not gain weight either. To demonstrate changes in body weight and body mass index, this training program would have needed to include a program of dietary management combined with the exercise training protocol. Researchers suggest that exercise training alone will not make significant changes in body weight and body mass index without dietary changes.29 Measurement of the percentage of body fat would have provided information on body composition. Since we did not use skinfold measurements to assess body composition, we cannot make any conclusions about body composition changes after intervention.
The resting metabolic rate (RMR) also contributes to the incidence of obesity in individuals with DS. The RMR is the caloric expenditure to maintain homeostasis. The lower the RMR is, the fewer calories that the individual expends in maintaining body temperature, energy cost of respiration, and other physiological functions at rest. With a lower RMR, an individual is more likely to gain weight even with a normal caloric intake. Luke et al8 reported that prepubescent children with DS had significantly lower RMRs than peers without DS. The two groups were matched for age, weight, and percentage of body fat. Although aerobic exercise does not significantly change the RMR, it is higher in individuals with greater muscle mass.30 As previously noted, although prepubescent children can improve their strength, this increase is thought to be enhanced by neural recruitment and not increased muscle mass. Increased muscle mass is related to increased sex hormones, especially testosterone.28 Future research on the effects of strength training on the RMR in adolescents with DS would be important to note for lifestyle management to prevent the high incidence of obesity often seen in postpubescent individuals with DS.
A limitation of this case study is that we did not have information on the subject’s thyroid function. Yearly thyroid screenings from infancy to adulthood are recommended for individuals with DS because of the high incidence of hypothyroidism.31 Signs of hypothyroidism include fatigue, weakness, muscle cramps or muscle aches, weight gain or difficulty losing weight, cold intolerance, depression, irritability, and memory loss. Since these symptoms could affect the effectiveness of a fitness program, it is important for therapists to obtain information about thyroid functioning before initiating a fitness program. In the future, other measures such blood pressure at rest and during exercise and skinfold and girth measurements would provide additional information about physical changes after strength and endurance training.
This study has several clinical implications. (1) Findings from this study and those on aerobic conditioning and strength performance suggest that to achieve cardiovascular benefits for children with DS, a combination program of aerobic conditioning and strength training is more effective than aerobic conditioning alone. (2) Additionally, the program has to be of a moderate to high intensity for a minimum of 30 minutes for five to six days per week. The exercise program must go beyond the suggested ACSM guidelines for cardiovascular fitness training in persons without impairment to be effective in individuals with DS. (3) This program worked because of the dedication of the parents and their interest in the child being more physically fit. Without strong parental support and the structure they provided, the home exercise program would not have been effective. At the end of the study, the home program was modified and the parents continued to carry out an aerobic and strength program with their daughter. (4) The program was adapted to the interests and attention level of the subject. She could choose from a variety of activities on a daily basis. Videotaping the weekly physical therapy intervention program assisted the parents and child in following through with the program. (5) Physical activity has to become part of a lifestyle for children and adolescents with DS because of their higher risk of chronic diseases that is worsened by obesity and a sedentary lifestyle.
SUMMARY AND CONCLUSION
In this case study of a child with DS, a home program of combined aerobic conditioning and strength training of moderate to high intensity, 30 to 60 minutes per day for five to six days per week resulted in changes in cardiovascular variables not reported in other studies employing aerobic conditioning only. In individuals with DS, in whom muscle strength and Vo2 is very highly correlated, combined aerobic and strength training protocols may be necessary for the improvement of cardiovascular function. Future research is need to investigate this hypothesis in a larger group of children with DS compared with age- and gender-matched peers without DS. A combined exercise training protocol of longer duration of 10 to 12 weeks and using a maximal stress test may result in enhanced Vo2 in subjects with DS that the authors did not document in this study.
1. Goodman CC, Miedaner J. Genetic and developmental disorders. In: Goodman CC, Boissonnault WG, eds. Pathology Implications for the Physical Therapist. Philadelphia: WB Saunders; 1998:577–616.
2. Parker AW, Bronks R, Snyder CW Jr. Walking patterns in Down’s syndrome. J Ment Defic Res. 1986;30:317–330.
3. Dichter CG, Darbee JC, Effgen SK, et al. Assessment of pulmonary function and physical fitness in children with Down syndrome. Pediatr Phys Ther. 1993;5:3–8.
4. Pitetti KH, Boneh S. Cardiovascular fitness as related to leg strength in adults with mental retardation. Med Sci Sports Exerc. 1995;27:423–428.
5. Folin M, Baiguera S, Conconi MT, et al. The impact of risk factors of Alzheimer’s disease in the Down syndrome. Int J Mol Med. 2003;11:267–270.
6. Krinsky-McHale SJ, Devenny DA, Silverman WP. Changes in explicit memory associated with early dementia in adults with Down’s syndrome. J Intellect Disabil Res. 2002;46:198–208.
7. Sharav T, Bowman T. Dietary practices, physical activity, and body-mass index in a selected population of Down syndrome children and their siblings. Clin Pediatr. 1992;31:341–344.
8. Luke A, Roizen NJ, Sutton M, et al. Energy expenditure in children with Down syndrome: correcting metabolic rate for movement. J Pediatr. 1994;125:829–838.
9. American College of Sports Medicine. ACSM’s Guidelines for Exercise Testing and Prescription, 6th ed. Baltimore: Lippincott Williams & Wilkins; 2000.
10. Luke A, Sutton M, Schoeller DA, et al. Nutrient intake and obesity in prepubescent children with Down syndrome. J Am Diet Assoc. 1996;96):1262–1267.
11. Fernhall B, Miller AL. Tymeson GT, et al. Maximal exercise testing of mentally retarded adolescents and adults: reliability study. Arch Phys Med Rehabil. 1990;71:1065–1068
12. Climstein M, Pitetti HK, Barrett PJ, et al. The accuracy of predicting treadmill Vo2 max for adults with mental retardation, with and without Down’s syndrome, using ACSM gender- and activity-specific regression equations. J Intellect Disabil Res. 1993;37:521–531.
13. Pitetti KH, Climstein M, Campbell KD, et al. The cardiovascular capacities of adults with Down syndrome. Med Sci Sport Exerc. 1992;24:13–19.
14. Croce RV, Pitetti KH, Horvat M, et al. Peak torque, average power, and hamstrings/quadriceps ratios in nondisabled adults and adults with mental retardation. Arch Phys Med Rehabil. 1996;77:369–372.
15. Pitetti KH, Boneh S. Cardiovascular fitness as related to leg strength in adults with mental retardation. Med Sci Sports Exerc. 1995;27:423–428.
16. Paffenbarger RS, Kampert JB, Lee IM, et al. Changes in physical activity and other lifeway patterns influencing longevity. Med Sci Sports Exerc. 1994;26:857–865.
17. Blair SN, Kohl HW, Barlow CE, et al. Changes in physical fitness and all-cause mortality: a prospective study of healthy and unhealthy men. JAMA. 1995;273:1093–1098.
18. Miller AL, Fernhall B, Burkett LN. Effects of aerobic training in adolescents with Down syndrome. Med Sci Sports Exerc. 1993;25:270–274.
19. Varela AM, Sardinha LB, Pitetti KH. Effects of an aerobic rowing training regimen in young adults with Down syndrome. Am J Ment Retard. 2001;106:135–144.
20. Rose J, Gamble JG, Lee J, et al. The energy expenditure index: a method to quantitate and compare walking energy expenditure for children and adolescents. J Pediatr Orthop. 1991;11:571–578.
21. Huskey T, Mayhew JL, Ball TE, et al. Factors affecting anaerobic power output in the Margaria-Kalamen test. Ergonomics. 1989;32:959–965.
22. Fernhall B, McCubbin JA, Pitetti KH, et al. Prediction of maximal heart rate in individuals with mental retardation. Med Sci Sports Exerc. 2001;33:1655–1660.
23. Mujika I, Padilla S. Cardiorespiratory and metabolic characteristics of detraining in humans. Med Sci Sports Exerc. 2001;33:413–421.
24. Fernhall B, Pitetti KH, Rimmer JH, et al. Cardiorespiratory capacity of individuals with mental retardation including Down syndrome. Med Sci Sports Exerc. 1996;28:366–371.
25. Cioni M, Cocilovo A, DiPasquale F, et al. Strength deficit of knee extensor muscles of individuals with Down syndrome from childhood to adolescence. Am J Ment Retard. 1994;99:166–174.
26. Horvat M, Pitetti KH, Croce R. Isokinetic torque, average power, and flexion/extension ratios in nondisabled adults and adults with mental retardation. J Orthop Sports Phys Ther. 1997;25:395–399.
27. Dyer SM. Physiological effects of a 13-week physical fitness program on Down syndrome subjects. Pediatr Exerc Sci. 1994;6:88–100.
28. Kramer WJ. Strength Training for Sports: Olympic Handbook of Sports Medicine. Oxford: Blackwell Science; 2001
29. Wilmore JH. Increasing physical activity: alterations in body mass and composition. Am J Clin Nutr. 1996;63(suppl):456S–460S.
30. Tataranmi PA, Ravussin E. Variability in metabolic rate biological sites of regulation. Int J Obes Relat Metab Disord. 1995;19(suppl 4):S102–S106.
31. American Academy of Pediatrics, Committee on Genetics. Health supervision for children with Down syndrome. Pediatrics. 2001;107:442–449.
© 2005 Lippincott Williams & Wilkins, Inc.