The aim of this study was to gain insight into the origins of muscle weakness in children with cerebral palsy (CP) by investigating active force in single muscle fibres isolated from the adductor longus (AL) of children with CP. Single skinned muscle fibres (n = 43, from 11 children with CP) were isolated and tested for their maximal active stress, instantaneous stiffness, and the ratio of active stress to stiffness at a sarcomere length (SL) of 2.4 µm, and passive stress at SLs of 2.4, 2.6, 2.8, 3.0, 3.2, and 3.4 µm. The muscle biopsies were tested for their total myofibrillar protein content using a BCA assay, and myosin heavy chain, actin, titin and nebulin content using gel electrophoresis. Muscle biopsies from the vastus lateralis of healthy adults (HA;​ n = 19 fibres from 4 subjects) were used for comparison. The maximal active stress and the instantaneous stiffness, expressed as active elastic modulus, were lower in the CP fibres compared to the HA fibres (79 ± 4 vs. 170 ± 11 kPa, and 5479 ± 381 vs. 8943 ± 402 kPa, respectively). The maximal active stress to stiffness ratio was not different between CP and HA fibres (0.016 ± 0.001 vs. 0.020 ± 0.002). Actin, titin and nebulin content was reduced in the CP compared to the HA samples. Passive stress was also reduced in the CP fibres at SLs of 2.8, 3.0, 3.2 and 3.6 µm. These results suggest that the contractile ability of muscles from children with CP is compromised at the fibre level and this is likely the result of a decrease in sarcomeric proteins such as actin, titin and nebulin.