Cerebral palsy affects virtually every voluntary muscle in the body to some degree, but the muscles of the legs, hips, arms, hands, and trunk are most commonly involved. The specific pattern depends on the type of CP and which parts of the brain were damaged. In all cases, the problem starts not in the muscles themselves but in the brain’s ability to send proper signals to them, which over time changes the muscles’ structure, tone, and ability to coordinate movement.
How Brain Damage Disrupts Muscle Control
Cerebral palsy results from damage to the parts of the brain responsible for planning, executing, and coordinating movement: the motor cortex, basal ganglia, and cerebellum. The motor cortex sends signals down through pathways called corticospinal tracts, which connect the brain to the spinal cord and ultimately to the muscles. When these pathways are disrupted, the signals that control voluntary muscle action become distorted, weakened, or poorly timed.
This disruption produces different movement problems depending on where the damage occurred. Injury to the corticospinal pathways typically causes spasticity, where muscles are constantly tight. Damage to the basal ganglia tends to produce involuntary, fluctuating movements. Cerebellar damage leads to poor coordination and low muscle tone. Many people with CP have a mix of these patterns.
Lower Limb Muscles: The Most Common Target
The legs bear the heaviest burden in most forms of cerebral palsy. Between 60 and 80% of children with CP develop equinus, a condition where the calf muscles (the gastrocnemius and soleus) are so tight that the child walks on their toes. This makes the calf the single most frequently affected muscle group and the most common site for treatment interventions.
Beyond the calves, a pattern called “crouch gait” reveals how multiple leg muscles malfunction together. Overactive hip flexors pull the hips forward into a bent position, while the hamstrings (the muscles behind the thigh) fire too long during walking and pull the knees into flexion. The body tries to compensate by increasing activity in the quadriceps (front of the thigh), which in turn demands more from the hip extensors. The result is a crouched posture where the child walks with bent hips and knees, never fully straightening the legs.
The inner thigh muscles (adductors) are also commonly overactive, especially in children with more severe involvement affecting both legs. This can cause a “scissoring” posture where the legs cross or press tightly together during walking or standing.
Upper Limb and Hand Muscles
In hemiplegic CP, where one side of the body is primarily affected, the arm and hand on that side typically adopt a characteristic posture. The shoulder pulls inward and rotates, driven by tightness in the pectoralis (chest) and subscapularis (shoulder blade) muscles. The elbow bends due to overactivity in the biceps and brachialis. The forearm rotates palm-down because of a tight pronator teres muscle in the forearm.
The wrist and hand are often the most functionally limiting. The wrist flexors pull the hand into a bent-down position, while the finger flexors curl the fingers into a fist. The thumb frequently folds into the palm, pulled by the adductor pollicis and the thumb’s own flexor muscles. These patterns make it difficult to grasp objects, write, button a shirt, or perform other fine motor tasks. In severe cases, the fingers curl so tightly that keeping the palm clean becomes a challenge.
Trunk and Core Muscles
The muscles that stabilize the spine and pelvis are frequently affected, though this gets less attention than limb involvement. Abnormal muscle tone and muscle imbalances along the trunk are the primary cause of postural deformities in children with CP. These imbalances can pull the spine into a curve (scoliosis), tilt the pelvis to one side, or make it impossible to sit upright without support.
Scoliosis typically appears around age 5 or 6 in children with severe CP, and around age 8 in milder cases. The head may turn persistently to one side. Children with the most severe involvement may lack the core muscle control needed for even basic postures against gravity, making independent sitting impossible. Improving trunk stability through therapy and supportive seating can meaningfully improve a child’s ability to use their arms, eat, and interact with their environment.
How Muscles Change Over Time
CP doesn’t just alter how muscles behave. It physically changes the muscle tissue itself. Research consistently shows that spastic muscles develop longer sarcomeres (the tiny contractile units inside muscle fibers), increased collagen content, and thicker fiber bundles. They also have fewer satellite cells, which are the stem cells responsible for muscle repair and growth.
In practical terms, this means the muscles become stiffer and less elastic over time, not because of the brain signal problem alone, but because the tissue itself remodels. This stiffening leads to contractures, where muscles become permanently shortened and joints lose their range of motion. The most common contracture sites are the ankles (from tight calf muscles), the knees (from tight hamstrings), the hips (from tight hip flexors and adductors), and the wrists and elbows in the upper limbs. These contractures tend to worsen during growth spurts, when bones lengthen faster than the already-compromised muscles can keep up.
Differences by CP Type
Spastic CP
The most common type, accounting for roughly 80% of cases. Muscles are persistently tight and resist being stretched. The pattern can affect one side of the body (hemiplegia), both legs primarily (diplegia), or all four limbs (quadriplegia). In spastic CP, the muscles most affected are those that cross major joints: calf muscles, hamstrings, hip adductors and flexors, biceps, and wrist and finger flexors.
Dyskinetic CP
The second most common subtype. Rather than staying consistently tight, muscle tone fluctuates. At rest, muscles may feel floppy and low-toned. During movement or emotional responses, tone can spike suddenly, producing involuntary twisting or writhing motions in the limbs and trunk. This affects the same muscle groups as spastic CP but in a less predictable way, making it harder to plan and control movements.
Ataxic CP
This type primarily disrupts coordination and balance rather than causing tightness. Muscle tone is generally low (hypotonia), which can make a child appear relaxed or floppy. The muscles of the legs, trunk, and hands are most noticeably affected. Children with ataxic CP walk unsteadily with a wide base and struggle with precise movements like writing. The muscles themselves can generate force, but the timing and coordination of that force is off.
Which Muscles Are Targeted in Treatment
The muscles most commonly treated give a practical picture of which ones cause the most functional problems. The gastrocnemius (calf) is by far the most frequent target for interventions aimed at reducing spasticity, because correcting toe-walking can dramatically improve a child’s ability to get around. Increasing ankle flexibility alone often changes the entire gait pattern.
The hamstrings and adductors are the next most commonly treated, particularly in children with more severe bilateral involvement. Reducing tightness in these muscles can ease scissoring, improve walking posture, and make positioning in a wheelchair more comfortable. At the hip, the psoas (a deep hip flexor) and rectus femoris are frequently addressed to correct the crouched posture.
In the upper limb, the biceps, brachialis, pronator teres, wrist flexors, and thumb adductor are the typical targets. The goal varies: in higher-functioning children, treatment aims to improve hand use and grasp. In children with more severe involvement, the goal may be as straightforward as opening a clenched fist enough to keep the skin healthy.
Regardless of which muscles are most affected, the pattern in CP is rarely static. Muscles that seem mildly involved in early childhood can develop significant tightness and contractures during growth, which is why ongoing monitoring of muscle length and joint range matters throughout childhood and adolescence.