In vivo muscle architecture and size of the rectus femoris and vastus lateralis in children and adolescents with cerebral palsy.

Children with cerebral palsy (CP) have one of the most sedentary lifestyles across pediatric disabilities.1 Decreased activity leads to weakness, disuse muscle atrophy, and muscle shortening, which can further limit activity. The cycle of inactivity and disuse in children and adolescents with CP is often perpetuated by progressive development of contractures and weakness (atrophy), leading to further declines in activity and worsening disability over the lifespan.2–4 Because the function of a muscle is reflected in its structure, it would be reasonable to expect that muscle structure would be altered in CP in a manner that would parallel these secondary impairments. Muscle architecture refers to the arrangement of muscle fibers within a muscle, or bundles of fibers known as fascicles, and includes fascicle lengths and fascicle angles. Muscle fiber or fascicle length is proportional to the maximal excursion of the muscle and velocity of contraction and is indicative of the number of sarcomeres in series. Measures of muscle size, particularly physiological cross-sectional area, are directly proportional to the maximum force-generating capacity of the muscle and the number of sarcomeres in parallel.5 Fascicle or pennation angle is the positive angle between muscle fibers and the aponeurosis of the muscle. The larger the fascicle angle, the more contractile material can be packed within a set volume, thereby increasing the muscle’s capacity to produce force.6 The classic work of Williams and Goldspink in animal models clearly demonstrates that immobilization of muscle in shortened positions results in a decrease in the number of sarcomeres in series.8 Similarly, human models of disuse such as unloading,9,10 bed rest,11,12 and even aging13 result in decreases in muscle size, fascicle length, and fascicle angles, indicating a loss of sarcomeres in series and in parallel.5 Although it is plausible that children and adolescents with CP would demonstrate similar alterations in muscle architecture and size secondary to disuse, immobilization, and lower activity levels, research in this area is very limited. Until recently, measurement of muscle architecture required highly invasive techniques, such as intraoperative measurements or whole-muscle dissections in human cadavers.5,14 With the advent of musculoskeletal ultrasound imaging, muscle architecture can be measured directly and non-invasively. Research on muscle architecture in CP has been focused primarily on the gastrocnemius muscle. Results are inconclusive, with reports of decreased fascicle lengths15–17 and angles18 as well as no differences18–20 in CP compared with control groups or the uninvolved side in the case of hemiplegia. Some of the inconsistencies have been attributed to whether absolute versus fascicle lengths normalized to fibular length were reported. Ohata et al.21 measured combined rectus femoris (RF) and vastus intermedius muscle thickness as a measure of quadriceps thickness in a group of adults with CP who mostly had severe motor impairment. However, comparisons were not made with a typically developing or control population. The quadriceps and gastrocnemius are the primary muscles affected by disuse and unloading secondary to their antigravity nature.10,11 However, no data exist on thickness, cross-sectional area, or muscle architectural parameters, such as fascicle length and angle, of individual quadriceps muscles in CP. Therefore, the purpose of this prospective study was to compare muscle architecture and size of the RF and vastus lateralis (VL) in children and adolescents with CP versus those who are typically developing, to determine whether these parameters differ in CP in a similar pattern to that observed with disuse. We hypothesized that children and adolescents with CP would have decreased fascicle lengths and angles, as well as decreased measures of muscle size compared with those who were typically developing. A secondary purpose was to examine the intrasession reliability of the measures.