Baby Stiffens Legs and Arms and Grunts (4 Possible Causes & What It Could Mean)

Babies tense, stiffen and grunt at various times during the day, often observable whenever they poop. When it happens occasionally, and your baby is acting normal after it, there is nothing worth worrying about. Infants tense up when they are excited, threatened, frustrated, or feeling overwhelmed or uncomfortable. However, if stiffening and grunting happen frequently and your baby appears upset, and in pain, you may need to observe further.

Parents who worry about their baby tensing up and grunting should visit and talk it out with their pediatrician.

Doctors will provide necessary diagnoses and treatments in cases where medical intervention is required, such as in epilepsy and infantile spasm.

Causes of baby stiffening up and grunting

Babies display strange behavior that sometimes baffles adults.

More often, it puts parents in bouts of worry. But sometimes, those behaviors are normal and do not merit developmental problems at all.

1. Bowel movement

Newborn babies usually stiffen and grunt while learning to pass stool or release trapped gas. 

Their stomach muscles are not yet strong enough to pass stool, and stretching is one way to help them push it out.

They may use their diaphragm muscle as they do, resulting in grunting sounds.

2. Stretching

Yes, babies stretch too! However, it seems to us that they are stiffening up and in pain.

They do this a lot to get their muscles to move and work. In fact, they tend to do a lot more stretching while still in the tummy.

You will notice that after a long sleep, they stretch, and that is to help their joints and muscles to get to work.

3. Digestive problems

When it seems that your baby is stiffening and obviously in distress, the common culprit is often digestive problems.

A newborn boy is in distress as he is having stomach pain as a result of digestive problems

Constipation, gas, reflux, and colic can result in babies arching and grunting. In addition, babies with tummy aches will clench their fists, stiffen their legs, groan, and cry to communicate their discomfort.

If you think your baby is suffering from tummy problems, do not attempt to self-medicate. Instead, seek medical advice for the proper diagnosis and treatment.

4. Overtired baby

Many think that when babies are overtired, they will lapse into a comfortable sleep. But that is contrary to the case.

Instead, overtired babies are more stimulated and may have difficulty sleeping once their window of rest has passed.

A dad is exhausted after trying to help his overtired infant son fall asleep

You will also notice a lot of rigid movement, yawning, and stiffening that comes along with irritability and fussing.

Other less common causes of baby stiffening and grunting

While stiffening and rigid movements in babies are likely normal, accompanying symptoms and frequency may indicate underlying conditions.

Infantile spasm

An infantile spasm is a rare form of epilepsy that can happen during the first two years of the baby’s life.

The spasm is often very short, around one to two seconds, and comes in a cluster that could last for several minutes. It often happens after waking up and get mistaken as a startle or hiccup.

Babies with infantile spasms may have their heads bent forward or back. In addition, their arms are flung outwards that straighten along with their legs.

Their eyes may also widen and roll up along with small movements of other body parts, particularly in the neck area.

Infantile spasm is caused by improper brain development, infection, brain injury, or abnormal brain blood vessels.

Cerebral palsy

Cerebral palsy is the impaired movement and balance that occurs in the brain even before birth.

The signs and symptoms are often manifested in early infancy and preschool years. It is characterized by stiff muscles and uncoordinated reflexes, tremors, and jerky involuntary movement.

When a child has cerebral palsy, they will have speech and developmental delays. Other neurological problems like seizures, hearing loss, urinary incontinence, and mental problems may also happen.

Cerebral palsy results from a gene mutation, infant and maternal infections, fetal stroke and lack of oxygen, or a traumatic head injury.


Hypertonia is a condition that affects the infant’s arms, legs, and neck muscles, making them stiff and hard to move. Hypertonia has two types:

  • Spastic hypertonia is characterized by muscle spasms and exaggerated movements that increase as the baby moves.
  • Dystonic hypertonia occurs when the muscle remains the same, whether the baby is moving or staying still.

Babies with hypertonia may jerk or twitch their arms to alleviate the soreness of their muscles. However, some infants may have trouble using their joints at all.

As they grow older, they will have a limited range of motion and have trouble with balance and coordination.

Hypertonia can occur when the baby has a head injury, asphyxiation or lack of oxygen, brain tumor, or exposure to chemicals that affect the brain.

Seizure and epilepsy

Stiffening of the body and grunting may also indicate epilepsy in infants.

A pediatrician is checking the vitals of an infant boy

It often occurs as a jerking movement of arms and legs, which may seem like a startle reflex at first.

But babies with seizures or epilepsy may lose consciousness, experience a period of rapid eye blinking and staring, or even stop breathing.

Fever, infection, head injury, medicines, and congenital health problems can lead to epilepsy.

The seizure can either be focal, which occurs in one part of the brain, or generalized, which happens on both sides. Treatment for epilepsy depends on the type of seizure and your child’s age.

If you suspect your baby’s stiffening and grunting are something else, do not hesitate to inform your doctor.

Early treatment will help you avoid triggers to help you better manage your child’s condition.


When should I call the doctor about my baby’s stiffening and grunting?

If your infant grunts and stiffens while obviously in distress, even if you think it is constipation, talk to your doctor.

Other warning signs you would not miss are: baby has a blue tongue and skin, weight loss, lethargy, nasal flaring, pauses in breathing, loss of consciousness, stiffening that comes in clusters and frequently occurs, as well as prolonged stiffening.

How can I help my baby if he is overtired?

A bedtime routine, short naps during the day, massage, and the right room temperature will help the baby sleep more soundly.

Read this post if you want to break the cycle of an overtired baby.

Is it normal for a baby to stiffen during a diaper change?

Yes, babies may tense up if they want to resist something that they do not like and is nothing to worry about.


Babies often exhibit strange behaviors that can scare their parents. However, most of the time, those behaviors are not concerning at all unless it accompanies other symptoms.

They may grunt and stiffen to jumpstart their muscles, relieve tension, or resist an action.

As parents, you are the better judge of your child’s behavior. Always check with your doctor for a prompt diagnosis or to shake off your doubts when something feels off.

Your pediatrician can help you sort out what is expected and concerning for your little one.

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Ann Marie is a licensed nurse in the Philippines. She experienced handling and assisting deliveries of newborns into the world. She also trained in labor rooms and pediatric wards while in nursing school - helping soon-to-be mothers and little kids in the process. Though not a mother by nature but a mother by heart, Ann Marie loves to take care of her younger cousins as well as nephews and nieces during her free time.

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