Stiffen. Babies sometimes stiffen their legs and arms or tighten up their body when you pick them up or they get excited or frustrated. Often this type of stiffening is no big deal.
Children might also stiffen up their bottoms when they’re having a bowel movement, especially if the stool is hard. And colic can make babies pull in and tighten their arms and legs, tense their abdomen, and clench their fists.
Your baby may also be discovering new ways to use their muscles. Some older babies stiffen up to resist doing something they dislike, such as getting a diaper change or being put into their car seat.
All of these situations are perfectly normal. But in other cases, a baby stiffening their body or arms and legs can indicate a problem.
What is hypertonia?
Hypertonia is a condition that makes a baby’s muscles stiff and hard to move. It happens because of damage to areas of the brain and spinal cord that control muscle movements.
While babies need to contract their muscles to move their bodies and sit upright, muscles that are continually contracted are less flexible. Often hypertonia affects a baby’s arm, leg, and neck muscles.
Children with hypertonia move in different ways because they often have a limited range of motion at their joints. They may have trouble walking, balancing, and letting go of objects they’ve grabbed.
There are two types of hypertonia:
- Spastic hypertonia is muscle spasms and exaggerated reflexes that increase when a baby moves.
- Dystonic hypertonia is when muscle tone stays the same whether the baby is moving or still.
Doctors will move a baby’s arms and legs in different directions at various speeds to figure out which type of hypertonia they have.
Symptoms of hypertonia in babies
Muscle stiffness is the main symptom of hypertonia. Where that stiffness occurs and how severe it is can vary based on the cause.
In general, a child with hypertonia might:
- Have trouble moving their joints, especially joints in the arms, legs, or neck
- Have less range of motion
- Lose their balance and fall often
- Feel pain or soreness in their muscles
- Jerk or twitch their arms or legs
In severe hypertonia, the joint can freeze in place. Doctors call this a contracture. Babies with contractures may not be able to move the affected body part much, or at all.
Causes of hypertonia in babies
When a baby wants to move, signals from their nervous system (brain and spinal cord) travel to their nerves, which direct their muscles to contract or relax. An injury to the brain or spinal cord can prevent those signals from reaching the nerves.
In hypertonia, the baby’s muscles don’t get the signal to relax. They stay contracted.
Possible causes of hypertonia in babies include:
- A lack of oxygen during birth
- Head injury
- Brain tumor
- Exposure to a chemical that affects the brain
- Problems that happened when the brain developed in the womb
Other reasons why your baby may stiffen their body
Hypertonia isn’t the only possible reason for a baby to stiffen their body. A few other medical conditions also cause this symptom.
Infantile spasm. These rare seizures look like a sudden stiffening of the baby’s arms, legs, or head, which may also jut forward. Infantile spasms often start when a baby is 4 to 7 months old. Each spasm lasts just a second or two, and babies can have up to 100 of them per day. Treatment with medication is important to prevent serious complications like epilepsy, developmental delay, and intellectual disability.
Cerebral palsy. This group of brain disorders affects a baby’s muscle control. Cerebral palsy has a few types. Some cause uncontrollable movements. Others lead to poor balance. The most common type is spastic cerebral palsy, which causes muscle stiffness.
Seizures and epilepsy. Epilepsy is a brain disorder in which nerve cells fire in an abnormal pattern, leading to seizures. During a seizure, a baby’s arms and legs might jerk, twitch, or stiffen. An electroencephalogram and other tests can help your doctor determine whether your baby is having seizures.
When to call the doctor if your baby stiffens their body
The occasional stiffening of an arm or leg probably isn’t anything to worry about. But if it happens often, you’ll want to seek help.
Whenever you have a question or concern about your baby’s development, it’s wise to call the doctor. In the unlikely event that stiffness does signal a condition like cerebral palsy or epilepsy, getting it checked out can start you on the path to a diagnosis and treatment.
Babies have lots of strange behaviors that can look concerning but are normal, like throwing themselves backward when they’re upset. But if your child does have a problem or developmental delay, you’ll want to get help and support ASAP.
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