Several types of seizures affect the muscles, ranging from brief jerks lasting a fraction of a second to full-body convulsions that go on for minutes. The medical term for these is “motor onset seizures,” and they fall into two broad groups: focal motor seizures, which start in one area of the brain and typically affect muscles on one side of the body, and generalized motor seizures, which involve both sides of the brain and affect muscles throughout the body. Here’s how each type works and what it looks like.
Tonic Seizures: Sudden Stiffening
Muscle tone is the low-level tension your muscles maintain at rest to keep you upright and stable. In a tonic seizure, that tone ramps up dramatically. The body, arms, or legs become suddenly stiff or rigid as muscles contract and hold. Tonic seizures are short, usually less than 20 seconds, and the stiffening can affect just one limb or the entire body depending on whether the seizure is focal or generalized. A generalized tonic seizure most often forces the body into extension (arching backward), though it can also lock the flexor muscles, pulling the body forward. Because these seizures can strike while standing, falls and injuries are common.
Clonic Seizures: Rhythmic Jerking
Clonic seizures produce repetitive, rhythmic jerking movements. What’s actually happening inside the muscles is a rapid cycle: flexor and extensor muscles contract simultaneously in a brief burst, then both groups go silent, then contract again. This on-off pattern repeats at a frequency of roughly 0.2 to 5 times per second, creating the characteristic twitching or jerking appearance. Each individual jerk lasts less than 100 milliseconds.
The jerking can vary in intensity across different body parts at the same time. Facial muscles might twitch rapidly with small movements while the arms produce infrequent but intense jerks, and the legs contract so quickly they appear almost continuously tense. Clonic seizures can be focal, affecting one group of muscles, or generalized, involving both sides of the body symmetrically.
Tonic-Clonic Seizures: The Two-Phase Convulsion
Formerly called grand mal seizures, tonic-clonic seizures are the type most people picture when they think of a seizure. They unfold in two distinct phases. The tonic phase comes first: the body stiffens, sometimes beginning with a cry or scream caused by air being forced past tense vocal cords. This phase may also cause the skin to turn bluish as breathing is temporarily disrupted.
The clonic phase follows, with progressive rhythmic jerking that gradually slows before stopping. Afterward, the person enters a recovery period marked by deep sleepiness, confusion, or agitation. Some people experience a more complex version called a myoclonic-tonic-clonic seizure, which adds a round of arm jerking before the stiffening begins. This pattern is commonly seen in juvenile myoclonic epilepsy.
Myoclonic Seizures: Lightning-Fast Jerks
Myoclonic seizures are extremely brief, often lasting less than a tenth of a second. They produce sudden, involuntary twitches or jerks in a single muscle or muscle group. The key difference between myoclonic and clonic seizures is rhythm: clonic seizures repeat in a steady, predictable pattern, while myoclonic jerks are irregular and often occur as isolated events or in unpredictable clusters.
There are two forms. Positive myoclonus involves a sudden muscle contraction, like a jerk of the arm or shoulder. Negative myoclonus is the opposite: a brief interruption of muscle tone lasting less than half a second that causes a sudden lapse in movement. It can look surprisingly similar to a quick jerk because the limb drops and bounces back so fast. Myoclonic seizures don’t typically cause a loss of awareness, so the person usually knows what happened.
Atonic Seizures: Sudden Loss of Muscle Control
Atonic seizures do the opposite of tonic seizures. Instead of muscles tightening, they suddenly lose all tone and go limp. The Cleveland Clinic compares it to a puppet whose strings are released: without muscle tension holding the body up, a person collapses. These episodes are sometimes called “drop attacks” because the person drops to the ground without warning.
Atonic seizures are brief, often just a few seconds, but they carry a high risk of injury from uncontrolled falls. Someone might drop whatever they’re holding, their eyelids might close, or their head might slump forward. In the generalized form, the entire body loses tone at once. In the focal form, just one limb or one side of the body may go limp while awareness is preserved. A related type, the myoclonic-atonic seizure, starts with a quick myoclonic jerk of the shoulders and arms before the atonic drop.
Epileptic Spasms
Epileptic spasms involve brief but sometimes painful contractions of the muscles along the trunk, arms, and legs. Each spasm typically lasts one to two seconds and often involves a characteristic pattern: the arms extend or abduct outward, the hips flex, and the head nods forward. Spasms frequently occur in clusters rather than as isolated events. When they appear in infants, the condition is called infantile spasms. Spasms can be focal, generalized, or of unknown onset, and awareness is usually retained.
Other Focal Motor Seizures
Beyond the major categories, several focal seizure types produce distinctive muscle activity:
- Hyperkinetic seizures cause exaggerated, thrashing movements like agitated kicking or pedaling motions. The muscle activity is intense and appears frantic.
- Automatism seizures produce repetitive, semi-purposeful movements like lip-smacking, hand rubbing, or wandering. These often occur with impaired awareness and may not look like a seizure to bystanders.
- Dystonic seizures cause sustained twisting postures, where muscles pull a limb or the body into an abnormal position and hold it there.
Why Muscles Hurt After a Seizure
After a motor seizure, the body enters what’s called the postictal state. The neurons that fired in massive, synchronized bursts during the seizure are now exhausted and recovering. Common symptoms include muscle soreness, weakness, fatigue, and confusion. This recovery period averages 5 to 30 minutes but can stretch to a full day or, in some cases, several days. Most people feel back to normal within 24 hours.
The soreness makes sense when you consider what the muscles just went through. During a tonic-clonic seizure, for example, nearly every major muscle group contracts forcefully and repeatedly for an extended period. That’s the equivalent of an intense, uncontrolled workout with no warm-up. In prolonged or repeated seizures, the muscle damage can go beyond simple soreness. Severe cases can cause a condition where muscle fibers break down and release their contents into the bloodstream, which can turn urine dark and, in serious situations, damage the kidneys. The risk of kidney injury in these cases ranges from 10% to 50%, making prolonged seizures a medical emergency.
How Motor Seizures Differ From Non-Epileptic Events
Not every episode of uncontrolled movement is an epileptic seizure. Psychogenic non-epileptic seizures (PNES) can look similar but originate from psychological rather than electrical causes in the brain, and they require different treatment. A few movement patterns help distinguish the two. Epileptic motor seizures typically involve eye opening or widening at onset, while PNES more often involve forceful eye closure or resistance to having the eyes opened. Asynchronous limb movements, where the arms and legs move out of sync with each other, are more common in PNES. Duration matters too: epileptic seizures rarely last longer than five minutes, while PNES episodes frequently do.
Some features, like pelvic thrusting, side-to-side head shaking, and body rocking, appear in both epileptic and non-epileptic events, so they can’t reliably tell the two apart on their own. Distinguishing between them typically requires video monitoring combined with brain wave recording.