http://informahealthcare.com/dre ISSN 0963-8288 print/ISSN 1464-5165 online Disabil Rehabil, Early Online: 1–6 ! 2014 Informa UK Ltd. DOI: 10.3109/09638288.2014.935492 RESEARCH PAPER Does muscle size matter? The relationship between muscle size and strength in children with cerebral palsy Siobha ´n L. Reid 1 , Christian A. Pitcher 1 , Sı ˆan A. Williams 1 , Melissa K. Licari 1 , Jane P. Valentine 2,3 , Peter J. Shipman 4 , and Catherine M. Elliott 2,3 1 School of Sport Science, Exercise and Health, The University of Western Australia, Perth, Australia, 2 Department of Paediatric Rehabilitation, Princess Margaret Hospital, Perth, Australia, 3 School of Paediatrics and Child Health, The University of Western Australia, Perth, Australia, and 4 Department of Diagnostic Imaging, Princess Margaret Hospital, Perth, Australia Abstract Purpose: To investigate the muscle size–strength relationship of the knee flexors and extensors in children with spastic cerebral palsy (CP) in relation to typically developing children (TD). Methods: Eighteen children with spastic Diplegia, Gross Motor Function Classification System I– III (mean 7 y 5 mo SD 1 y 7 mo) and 19 TD children (mean 7 y 6 mo SD 1 y 9 mo) participated. Muscle volume (MV) and anatomical cross-sectional area (aCSA) were assessed using MRI. Measures of peak torque (PT) and work of the knee flexors and extensors were assessed isometrically and isokinetically using a Biodex dynamometer, and normalised to bodymass (Bm). Results: Children with CP were weaker than their TD peers across all torque variables (p50.05). MV and aCSA of the knee flexors (MV: p ¼ 0.002; aCSA: p ¼ 0.000) and extensors (MV: p ¼ 0.003; aCSA: p50.0001) were smaller in children with CP. The relationship between muscle size and strength in children with CP was weaker than the TD children. The strongest relationship was between MV and isometric PT/Bm for TD children (r ¼ 0.77–0.84), and between MV and isokinetic work (r ¼ 0.70–0.72) for children with CP. Conclusions: Children with CP have smaller, weaker muscles than their TD peers. However, muscle size may only partially explain their decreased torque capacity. MV appears to be a better predictor of muscle work in children with CP than aCSA. This is an important area of research particularly in regard to treatment(s) that target muscle and strength in children with CP. ä Implications for Rehabilitation  This research adds to the evidence that children with CP have smaller, weaker knee flexor and extensor muscles than their TD peers. However, unlike their TD peers, muscle size does not necessarily relate to muscle strength.  The weak correlation between MRI-derived muscle volume and isometric peak torque suggests children with CP are underpowered relative to their muscle size.  For children with CP, muscle volume appears to be the best predictor of isokinetic muscle torque output. Therefore, when assessing the capacity of a muscle, it appears preferable to measure total muscle volume and torque development through a range of motion (isokinetic strength). Keywords Cerebral palsy, children, muscle strength, muscle strength–size relationship, muscle volume History Received 2 October 2013 Revised 7 May 2014 Accepted 4 June 2014 Published online 3 July 2014 Introduction Cerebral palsy (CP) is the most common physical disability in childhood [1]. Impairments present in individuals with CP include abnormal muscle tone, muscular weakness, muscular contracture and patterns of abnormal co-activation, all of which can result in functional limitations of both the lower and upper extremities [2,3]. As such, many children with CP have difficulty with tasks such as propelling wheelchairs, walking independently, negotiating steps and many activities of daily living [4]. It is well known that children with CP have a significant deficiency in muscular strength [5]. Wiley and Damiano, in 1998, confirmed that children with CP were indeed weaker than their typically developing (TD) peers in all major muscle groups of the lower limb, and numerous studies have since confirmed this [6–9]. The maintenance of muscle strength in children with CP is important as a direct relationship exists between lower limb strength and gross motor function [5,10]. Yet, the neuromuscular and biomechanical basis for the observed weaknesses remains poorly understood. Clinically, it has long been assumed that the deficient strength was the result of spasticity. However, the origin of weakness is Address for correspondence: Asst/Prof. Siobha ´n Reid, School of Sport Science, Exercise and Health (M408), University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia. Tel: +61 8 6488 8781. Fax: +61 8 6488 1039. E-mail: siobhan.reid@uwa.edu.au Disabil Rehabil Downloaded from informahealthcare.com by University of Western Australia on 07/07/14 For personal use only.