This study was designed to determine whether children with cerebral palsy (CP) showed more co-activity than comparison children in non-prime mover muscles with regard to the prime mover during maximum voluntary isometric contraction (MVIC) of four lower-extremity muscles. Fourteen children with spastic diplegic CP (10 males, four females; age range 4-10y), seven children with spastic hemiplegic CP (five males, two females; age range 5-10y), and 14 comparison children (eight males, six females; age range 4-11y) participated in the study. Gross Motor Function Classification System levels of the children with CP were as follows: eight children at Level I, seven children at Level II, five children at Level III, and one child at Level I V. Surface electromyographic recordings were made simultaneously from the vastus lateralis (VL), medial hamstrings (MH), tibialis anterior, and lateral gastrocnemius (LG) muscles during maximal voluntary contraction. Children with CP showed higher co-activity than the comparison children in both antagonist and adjacent muscles. This was particularly true when VL, MH, or LG muscles were engaged in MVIC. These findings may contribute to the weakness and abnormal movement patterns seen in CP, and they have implications for treatment.