Autism spectrum disorder (ASD) is defined by its core features (1). According to the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text-Revision (DSM-IV-TR), the core features include (a) qualitative impairments in social interaction, (b) qualitative impairments in communication, and (c) restricted repetitive and stereotyped patterns of behavior, interests, and activities.
Although an ASD diagnosis is defined by these 3 core features, other issues, more physical or health related, are associated with an ASD diagnosis (9,25). Examples of physical impairments include fine and gross motor problems (22), movement/motor skills deficits (11), balance problems (19), gait pattern differences (5), dysfunctional posture and muscle tone (7), and hypotonia (18). Another aspect that may be reflective of issues in general physical condition is muscle weakness in ASD (12). Anecdotal reports and limited research suggest that children with ASD are weaker than typically developing children. Hardan et al. (12), for example, examined grip strength in 40 individuals diagnosed with autism without intellectual disabilities and 41 healthy controls. These investigators found that grip strength in participants diagnosed with an ASD was significantly weaker than in the controls.
According to Bhat et al., muscle weakness and abnormal muscle tone in ASD may play a role in the limitations in daily activities, such as locomotion and reaching. They also stated that one of the earliest motor signs of an ASD may be weakness in pronation and supination as in turning a doorknob or twisting a bottle cap (3). Studies examining motor impairments in children with ASDs suggest the presence of low muscle tone (3). It is possible that the functional effects of muscle weakness in ASD could be widespread.
A study by Kern et al. (14) found that handgrip strength in participants diagnosed with an ASD was related to the severity of the disorder. In that study, 37 children with ASD were evaluated using the Childhood Autism Rating Scale (CARS) (26) and then tested for hand muscle strength using a handgrip dynamometer. The results showed that the more severely affected the child was based on the child's CARS score, the weaker the handgrip strength.
Importantly, studies have shown that the handheld dynamometer is a valid tool for measuring overall muscle strength and for the assessment of muscle mass (8,16). In addition, Theou et al. (27) found that handgrip strength correlates with an overall frailty index.
There is a paucity of research in muscle strength in children with ASD, even though muscle weakness may be a factor in the difficulties they encounter in ordinary activities of daily living. In addition, there is little research that examines how best to determine muscle weakness in ASD. The present study examines handgrip strength in children with ASD as compared with gender-, race-, and age-matched neurotypical children. The purpose of this study was to examine the extent and consistency of muscle weakness in ASD as compared with the matched neurotypical controls.
Experimental Approach to the Problem
Children with a diagnosis of ASD and neurotypical controls were recruited from the community to participate in the study. After explaining the study and obtaining informed consent from the parent(s), each child was evaluated using the CARS and then tested for hand muscle strength. Statistical analysis was then conducted to compare handgrip strength between the 2 groups.
A total of 33 participants diagnosed with an ASD and 33 gender-, age-, and race-matched controls were prospectively recruited from the community. Autism spectrum disorders include participants diagnosed with autism, pervasive developmental disorder–not otherwise specified (PDD-NOS), and Asperger's disorder. [This study took place before the change to the DSM-IV criteria.] Each child in the ASD group had been previously diagnosed by a professional. In the state of Texas, the only professionals who are allowed to diagnose ASD are licensed clinical psychologists or medical doctors. To further evaluate each child's diagnostic accuracy, each child was observed by one of the investigators (J.K.K.) who has years of experience in evaluating children with ASD. The CARS was also completed on each child. The CARS has been shown in previous research to have a high concordance with clinical diagnosis using DSM-IV. Perry et al. (20), for example, studied a sample of 274 preschool children (aged 2–6 years) clinically diagnosed as falling in 1 of the 5 groups: autistic disorder, PDD-NOS, mental retardation, delayed, and other, and found that the CARS had a high concordance with clinical diagnosis using DSM-IV (including excellent sensitivity and specificity). Similarly, Rellini et al. (23) found complete agreement between the DSM-IV and CARS. They also found that the number of false negatives in distinguishing individuals with autistic disorders from other cases of developmental disorders was 0% with CARS.
In the current study, children with ASD were required to have a CARS score ≥ 24 (to include children with PDD and one child with Asperger's disorder). The neurotypical children were required to have a CARS score ≤ 17. The lowest possible score on the CARS is 15.
A developmental history and medical questionnaire was completed on each child by the child's parent. The typically developing children had no history of any learning disability, such as attention deficit/hyperactivity disorder. However, 1 child had a history of mild reading problems that had resolved. The typically developing children had no history of any medical problems and were not on any medications.
None of the study participants had previously received carnitine-based therapy or previous methionine or lysine supplementation. Although none were encountered, this study was designed to exclude children who had a history of Fragile X disorder, tuberous sclerosis, phenylketonuria (PKU), Lesch-Nyhan syndrome, seizure disorder, cerebral palsy, fetal alcohol syndrome, or any history of maternal illicit drug use. Detailed information was collected on each participant regarding age, race, gender, year of birth, and a history of prior chelation therapy. Table 1 summarizes the demographic information for the participants examined in the present study. The study protocol received Institutional Review Board (IRB) approval from Liberty IRB, Inc. The study complied with the American College of Sports Medicine's policies and standards in the treatment of participants and in obtaining informed consent. All parents signed a consent and Health Insurance Portability and Accountability Act form and all received a copy. All children were in the presence of a parent during their participation in the study.
Childhood Autism Rating Scale
Study participants were evaluated, using the CARS, by a single study investigator (J.K.K.) who observed the participants and interviewed the parent(s). The CARS test is a 15-item behavioral rating scale developed to identify autism and to quantitatively describe the severity of the disorder (26). For a CARS evaluation, a total score of about 25 is considered to be the minimum cutoff CARS score for an ASD diagnosis (6). The CARS is a well-established measure of autism severity. The internal consistency reliability alpha coefficient is 0.94, the interrater reliability correlation coefficient is 0.71, and the test-retest correlation coefficient is 0.88. The CARS scores have high criterion-related validity when compared with clinical ratings during the same diagnostic sessions, with a significant correlation of 0.84 (26).
Hand Muscle Testing
The muscle strength testing was completed immediately after the CARS by a study investigator to derive their Max Hand Muscle Score using a pneumatic, adjustable squeeze pinch-gauge/dynamometer (Pneumatic Pinch Gauge Dynamometer with 3 different sized interchangeable squeeze bulbs; Baseline Evaluation Instruments, White Plains, NY, USA). Participants were tested using the smallest hand grasp bulb because several of the children could only get the smallest bulb to register. Thus, the smallest ball was used for the study to be consistent. Each child was given as many tries as needed to register their maximum grasp reading (about 4 tries) measured in kilopascals for each hand. The dynamometer has a maximum force indicator (reset) that remains at the maximum reading until reset. Multiple tries are standard for the use of this instrument. Only the maximum reading was recorded and used for analysis. Special emphasis was placed to ensure that the participant positioned the bulb in the palm of the hand and held the bulb in space to ensure that pressure was not applied by the study participant against a fixed surface. In addition, each study participant was strongly encouraged by a study investigator to give maximum effort. Although no interrater reliability testing on the handgrip dynamometer was conducted for this study, all of the children were tested by one investigator (J.K.K.) using the same type of dynamometer made by the same manufacturer and 92% of the children were tested using the same piece of equipment. Because the dynamometer has a maximum force indicator (reset) that remains at the maximum reading until reset, the reading of the dynamometer is not subjective.
There was no preparation or training background for either group. There was no attempt to influence in areas such as hydration, diet, or sleep. All testing was conducted between 10 and 5 during the day.
This instrument is a reliable and valid method for obtaining muscle force or torque measurements in children (2,12,13,15,28) and has been used successfully in autism research (12). As mentioned in the Introduction, the handheld dynamometer has been shown to be a valid tool for measuring overall muscle strength and for the assessment of muscle mass (8,16). Measuring muscle strength in children in general has been found to be reliable (17).
The null hypothesis was that children diagnosed with an ASD would have similar handgrip strength to typically developing children using a handgrip dynamometer. The statistical package in StatsDirect (version 2.4.7) was used, and statistical analyses were undertaken using the paired t-test statistic. In all statistical analyses, a 2-tailed p value = 0.05 was considered statistically significant.
The average handgrip strength in children diagnosed with an ASD was 39.4 ± 17.7 kPa and the average handgrip strength in the neurotypical children was 65.1 ± 26.7 kPa. There was a statistically significant mean difference of −25.7 kPa in handgrip strength between children diagnosed with an ASD in comparison with gender-, race-, and age-matched neurotypical controls (p < 0.0001, 95% confidence interval = −34.7 to −16.7, t = −5.81, df = 32).
The results indicate that handgrip strength in children with ASD is substantially lower compared with that in neurotypical children. The results of this study are consistent with previous research (12).
As mentioned earlier, the handheld dynamometer has been shown to be a valid tool for measuring overall muscle strength and for the assessment of muscle mass (8,16). Thus, the results of the current study suggest that children with ASD have muscle weakness. Conceivably, muscle weakness may contribute to other physical limitations, such as the movement, balance, and coordination problems found in ASD. From a simple measurement of hand strength, it is unclear if other muscles are also affected, but if they are, the functional effects could be widespread. In addition, the muscle weakness could be reflective of a more generalized condition in ASD.
There is a paucity of research that examines muscle weakness in ASD and possible treatments. The authors found one study that examined the usefulness of physical therapy in autism and it focused on the reduction of stereotypic behaviors in children with ASD, not strengthening (21). Vonder Hulls et al. (29) surveyed 18 occupational therapists who use aquatic therapy in treating young children with autism. The majority of clinicians reported a substantial increase in muscle strength and other benefits.
There is one study in persons with intellectual disabilities that shows that strength training can have significant positive effects on handgrip strength and muscle strength in general (4). Two studies examined other forms of therapy, high-frequency low-magnitude vibration (24) and aquatic physical therapy (10), and found that they can improve muscle strength in children with disabilities.
More research is needed to determine the extent of muscle weakness in ASD and its ramifications. Studies that examine the possible benefits of muscle strengthening are also lacking.
Measuring handgrip strength, in adults and children, has been shown to be reliable (2,13,15,28) and has been used successfully in autism research (12); however, measuring handgrip strength in children with autism is more difficult because their level of awareness, attention, and ability to cooperate can present as confounding factors. However, it was observed by the study investigator (J.K.K.) who did the muscle testing that the children were making a sincere effort to obtain a maximum score. And, as mentioned earlier, the results of this study are consistent with previous research (12).
In the current study, children with ASD were required to have a CARS score ≥ 24. The recommended cutoff is 25 (6). The score of 24 was one child with Asperger's disorder. However, even with the inclusion of milder cases, the findings were still very significant (p < 0.0001).
The results suggest that ASD is a medical disorder in which physical aspects of the disorder, such as muscle weakness, need to be considered. Understanding the extent of each child's muscle weakness may provide more insight into the child's physical limitations and plan of care.
As mentioned earlier, muscle weakness and abnormal muscle tone in ASD may play a role in the limitations in daily activities, such as locomotion and reaching (3). Muscle weakness may be a factor in the difficulties that children with ASD encounter in ordinary activities of daily living, such as in turning a doorknob or twisting a bottle cap. It is possible that the functional ability of these children could improve with muscle strengthening.
The authors propose that muscle strength testing is warranted for children with ASD, particularly in instances where strength may play role in the limitations that the child is experiencing. The present study provides support for the use of handgrip strength as a tool for the assessment of targeted treatment in ASD and suggests that research in the possible benefits of muscle strengthening in ASD is warranted.
The authors report no declarations of interest. Research was conducted at the Institute of Chronic Illnesses, Inc. This research was funded by a grant from the Autism Research Institute, nonprofit CoMeD, Inc., and by the nonprofit Institute of Chronic Illnesses, Inc., through a grant from the Brenen Hornstein Autism Research & Education (BHARE) Foundation.
1. American Psychiatric Association. Diagnostic criteria for autistic disorder. In: Diagnostic and Statistical Manual of Mental Disorders (4th ed.). Washington, DC: American Psychiatric Association, 2000.
2. Berry ET, Giuliani CA, Damiano D. Intrasession and intersession reliability of handheld dynamometry in children with cerebral palsy. Pediatr Phys Ther 16: 191–198, 2004.
3. Bhat AN, Landa RJ, Galloway JC. Current perspectives on motor functioning in infants, children, and adults with autism spectrum disorders. Phys Ther 91: 1116–1129, 2011.
4. Calders P, Elmaghoub S, de Mettelinge TR, Vandenbroeck C, Dewandele I, Rombaut L, Vandevelde A, Cambier D. Effect of combined exercise training on physical and metabolic fitness in adults with intellectual disability: A controlled trial. Clin Rehabil 25: 1097–1108, 2011.
5. Calhoun M, Longworth M, Chester VL. Gait patterns in children with autism. Clin Biomech (Bristol, Avon) 26: 200–206, 2011.
6. Chlebowski C, Green JA, Barton ML, Fein D. Using the Childhood Autism Rating Scale to diagnose autism spectrum disorders. J Autism Dev Disord 40: 787–799, 2010.
7. De Jong M, Punt M, De Groot E, Minderaa RB, Hadders-Algra M. Minor neurological dysfunction in children with autism spectrum disorder. Dev Med Child Neurol 53: 641–646, 2011.
8. Febrer A, Rodriguez N, Alias L, Tizzano E. Measurement of muscle strength with a handheld dynamometer in patients with chronic spinal muscular atrophy. J Rehabil Med 42: 228–231, 2010.
9. Filipek PA, Juranek J, Nguyen MT, Cummings C, Gargus JJ. Relative carnitine deficiency in autism. J Autism Dev Disord 34: 615–623, 2004.
10. Fragala-Pinkham MA, Dumas HM, Barlow CA, Pasternak A. An aquatic physical therapy program at a pediatric rehabilitation hospital: A case series. Pediatr Phys Ther 21: 68–78, 2009.
11. Green D, Charman T, Pickles A, Chandler S, Loucas T, Simonoff E, Baird G. Impairment in movement skills of children with autistic spectrum disorders. Dev Med Child Neurol 51: 311–316, 2009.
12. Hardan AY, Kilpatrick M, Keshavan MS, Minshew NJ. Motor performance and anatomic magnetic resonance imaging (MRI) of the basal ganglia in autism. J Child Neurol 18: 317–324, 2003.
13. Janssen JC, Le-Ngoc L. Intratester reliability and validity of concentric measurements using a new hand-held dynamometer. Arch Phys Med Rehabil 90: 1541–1547, 2009.
14. Kern JK, Geier DA, Adams JB, Troutman MR, Davis G, King PG, Young JL, Geier MR. Autism severity and muscle strength: A correlation analysis. Res Autism Spectr Disord 5: 1011–1015, 2011.
15. Larson CA, Tezak WD, Malley MS, Thornton W. Assessment of postural muscle strength in sitting: Reliability of measures obtained with hand-held dynamometry in individuals with spinal cord injury. J Neurol Phys Ther 34: 24–31, 2010.
16. Leal VO, Mafra D, Fouque D, Anjos LA. Use of handgrip strength in the assessment of the muscle function of chronic kidney disease patients on dialysis: A systematic review. Nephrol Dial Transplant 26: 1354–1360, 2011.
17. Merlini L, Domenico D, Granata C. Reliability of dynamic strength knee muscle testing in children. J Orthop Sports Phys Ther 22: 73–76, 1995.
18. Ming X, Brimacombe M, Wagner GC. Prevalence of motor impairment in autism spectrum disorders. Brain Dev 29: 565–570, 2007.
19. Minshew NJ, Sung K, Jones BL, Furman JM. Underdevelopment of the postural control system in autism. Neurology 63: 2056–2061, 2004.
20. Perry A, Condillac RA, Freeman NL, Dunn-Geier J, Belair J. Multi-site study of the Childhood Autism Rating Scale (CARS) in five clinical groups of young children. J Autism Dev Disord 35: 625–634, 2005.
21. Petrus C, Adamson SR, Block L, Einarson SJ, Sharifnejad M, Harris SR. Effects of exercise interventions on stereotypic behaviours in children with autism spectrum disorder. Physiother Can 60: 134–145, 2008.
22. Provost B, Heimerl S, Lopez BR. Levels of gross and fine motor development in young children with autism spectrum disorder. Phys Occup Ther Pediatr 27: 21–36, 2007.
23. Rellini E, Tortolani D, Trillo S, Carbone S, Montecchi F. Childhood Autism Rating Scale (CARS) and Autism Behavior Checklist (ABC) correspondence and conflicts with DSM-IV criteria in diagnosis of autism. J Autism Dev Disord 34: 703–708, 2004.
24. Reyes ML, Hernández M, Holmgren LJ, Sanhueza E, Escobar RG. High-frequency, low-intensity vibrations increase bone mass and muscle strength in upper limbs, improving autonomy in disabled children. J Bone Miner Res 26: 1759–1766, 2011.
25. Schieve LA, Gonzalez V, Boulet SL, Visser SN, Rice CE, Braun KV, Boyle CA. Concurrent medical conditions and health care use and needs among children with learning and behavioral developmental disabilities, National Health Interview Survey, 2006-2010. Res Dev Disabil 33: 467–476, 2011.
26. Schopler E, Reichler RJ, Renner BR. The Childhood Autism Rating Scale. Los Angeles, CA: Western Psychological Services, 90025–91251, 1994.
27. Theou O, Jones GR, Jakobi JM, Mitnitski A, Vandervoort AA. A comparison of the relationship of 14 performance-based measures with frailty in older women. Appl Physiol Nutr Metab, 36: 928–938, 2011.
28. Ties Molenaar HM, de Kraker M, Zuidam JM, Hovius SE, Stam HJ, Selles RW. Visual feedback and weight reduction of a grip strength dynamometer do not increase reliability in healthy children. J Hand Ther 23: 272–279, 2010.
29. Vonder Hulls DS, Walker LK, Powell JM. Clinicians' perceptions of the benefits of aquatic therapy for young children with autism: A preliminary study. Phys Occup Ther Pediatr 26: 13–22, 2006.