Aim  Our aim was to investigate muscle architecture and size of the rectus femoris (RF) and vastus lateralis (VL) in children and adolescents with cerebral palsy (CP) compared with age‐matched typically developing participants. Method  Muscle architecture and size were measured with ultrasound imaging in 18 participants with spastic CP (9 females, 9 males; age range 7.5–19y; mean age 12y [SD 3y 2mo]) within Gross Motor Function Classification System levels I (n=4), II (n=2), III (n=9), and IV (n=3) and 12 typically developing participants (10 females, 2 males; age range 7–20y; mean age 12y 4mo [SD 3y 11mo]). Exclusion criteria were orthopedic surgery or neurosurgery within 6 months before testing or botulinum toxin injections to the quadriceps within 3 months before testing. Results  RF cross‐sectional area was significantly lower (48%), RF and VL muscle thickness 30% lower, RF fascicle length 27% lower, and VL fascicle angle 3° less in participants with CP compared to the typically developing participants (p<0.05). Intraclass correlation coefficients were ≥0.93 (CP) and ≥ 0.88 (typical development), indicating excellent reliability. Interpretation  These results provide the first evidence of altered muscle architecture and size of the RF and VL in CP, similar to patterns observed with disuse and aging. These alterations may play a significant role in the decreased capacity for force generation as well as decreased shortening velocity and range of motion over which the quadriceps can act.

Muscle architecture in CP is capable of adapting differentially to the training stimulus. VT was equally effective as traditional ST in improving isokinetic strength of the knee extensors but more effective in improving velocity of movement, muscle power, and walking performance. Differences may be partially attributed to specificity of training effects on muscle architecture, such as the increase in fascicle length after VT. Strengthening interventions involving higher velocity movements should be incorporated into clinical practice.

Gait training was the most effective intervention in improving gait speed for ambulatory children with CP. Strength training, even if properly dosed, was not shown to be effective in improving gait speed. Velocity training, electromyographic biofeedback training, and whole-body vibration were effective in improving gait speed in individual studies and warrant further investigation.

Ultrasound measures of VL muscle thickness, adjusted for age and GMFCS level, were highly predictive of maximum torque and have the potential to serve as surrogate measures of voluntary strength (force-generating capacity) in children and adolescents with and without CP.

Limb movements involving contraction times of 50–200ms occur in many everyday activities, such as gait, which is faster than the time required to generate maximal force. Therefore, the ability to rapidly produce force may be even more important for the performance of functional activities. In this study rate of force development (RFD) and impulse of the knee extensors were examined in 12 ambulatory children with cerebral palsy (CP) (Age:11.9 ± 2.9 yrs) and 11 with typical development (TD) (Age: 11.3 ± 3.0 yrs). The relationship with muscle architecture and functional mobility was also determined. RFD and impulse were calculated during a maximal isometric knee extension contraction. Rectus femoris (RF) cross-sectional area and RF and vastus lateralis (VL) muscle thickness (MT), fascicle length (FL), and fascicle angle (FA) were measured using ultrasound imaging. Gait temporal-spatial parameters, Pediatric Outcomes Data Collection Instrument (PODCI), and Activities Scale for Kids performane version (ASKp) were collected. Although VL MT was the primary determinant of RFD and impulse in CP, FA and FL were also significant predictors in the TD group. RFD and impulse were significantly lower in CP compared to TD (70% decrease) in addition to maximal strength (50% decrease). RFD and impulse were predictive of measures of functional mobility, including gait, transfers, and sports and higher level activities but not temporal-spatial gait measures. Results suggest that the ability to rapidly generate torque may be of greater importance than maximal strength during certain tasks, such as transfers and sports and higher level activities.