Children with cerebral palsy (CP) often have stiff muscles primarily due to a neurological condition called **spasticity**, which is characterized by increased muscle tone or hypertonia. This stiffness arises because the brain injury that causes CP disrupts the normal signals from the brain to the muscles, leading to excessive muscle contraction and resistance to stretch[1][4].
To understand why this happens, it is important to know that cerebral palsy results from damage to the brain areas responsible for controlling movement and muscle tone, often the **corticospinal tract** or other motor pathways. This damage is non-progressive but occurs early in life, typically around birth or shortly after[1][6]. The brain injury impairs the ability to regulate muscle activity properly, causing muscles to be continuously or excessively activated even when they should be relaxed.
**Spasticity** is the most common type of muscle tone abnormality in CP and is defined as a velocity-dependent increase in muscle tone. This means that when a muscle is stretched quickly, it reacts with an exaggerated contraction, making the muscle feel stiff and rigid[1][4]. This abnormal muscle tone is due to an imbalance between excitatory and inhibitory signals in the central nervous system. Normally, the brain sends inhibitory signals to muscles to prevent excessive contraction, but in CP, these inhibitory pathways are damaged or weakened, leading to overactive muscle contraction[4].
The stiff muscles in children with spastic CP are often described as having **hypertonia**, which contrasts with hypotonia (low muscle tone) seen in other CP types. Hypertonia causes muscles to be tight and resistant to movement, which can affect posture, gait, and the ability to perform voluntary movements[1]. For example, children with spastic diplegic CP have stiffness mainly in their legs, while those with spastic hemiplegic CP have stiffness on one side of the body[1].
At the muscle tissue level, the chronic overactivity and stiffness lead to structural changes such as **muscle fibrosis**, contractures (permanent muscle shortening), and weakness. These changes worsen the stiffness and reduce the muscle’s ability to stretch and function normally[5]. The extracellular matrix (ECM) in muscles becomes altered, contributing to increased stiffness and reduced elasticity[5].
The stiffness also affects gait and movement patterns. For instance, children with spastic CP may develop a **stiff-knee gait**, where the knee does not bend properly during walking due to overactivity of muscles like the rectus femoris. This abnormal gait increases the risk of tripping and falls and requires compensatory movements[3].
Treatment approaches for managing muscle stiffness in CP focus on reducing spasticity and improving muscle function. One common method is the use of **botulinum toxin injections**, which temporarily block the release of acetylcholine at the neuromuscular junction, causing muscle relaxation and reducing stiffness for several months[4]. Other treatments include physical therapy, oral medications like baclofen, and in severe cases, surgical interventions to lengthen muscles or release tight tendons[4].
In summary, children with cerebral palsy have stiff muscles because the brain injury disrupts normal motor control pathways, leading to excessive muscle activation (spasticity) and increased muscle tone (hypertonia). This neurological dysfunction causes muscles to contract abnormally, resulting in stiffness, reduced mobility, and secondary muscle changes such as fibrosis an
