Muscle spasticity is a condition where muscles become abnormally tight and resist being moved, especially when they’re stretched quickly. It happens when damage to the brain or spinal cord disrupts the signals that normally keep muscle tension in check. The result is muscles that feel stiff, pull limbs into awkward positions, and sometimes jerk or spasm on their own. Spasticity affects roughly 35% of stroke survivors, over 90% of people with cerebral palsy, and between 37% and 78% of those with multiple sclerosis.
Why Spasticity Happens
Your muscles constantly communicate with your spinal cord through a feedback loop. Tiny sensors inside each muscle, called spindles, detect how fast and how far a muscle is being stretched. They send that information to the spinal cord, which decides how strongly the muscle should contract in response. In a healthy nervous system, the brain sends signals down the spinal cord that keep this reflex in check, preventing muscles from overreacting to normal movements.
When a stroke, spinal cord injury, or other condition damages these descending pathways, the spinal cord loses its restraining instructions. Several built-in braking systems at the spinal level stop working properly. One of the most important is reciprocal inhibition, the mechanism that normally relaxes one muscle group when the opposing group contracts. When this fails, both sides of a joint can fire at once, creating co-contraction and stiffness. Other braking systems that filter incoming stretch signals or limit how strongly motor neurons fire also weaken. The overall effect tips the balance toward excitation: muscles respond too aggressively to any stretch, producing the tightness and resistance that define spasticity.
Spasticity vs. Rigidity
Spasticity is velocity-dependent, meaning it gets worse the faster you try to move the affected limb. A slow, gentle stretch may meet little resistance, while a quick one triggers a strong muscle contraction. Rigidity, by contrast, produces a constant resistance regardless of speed. Rigidity is more commonly associated with conditions like Parkinson’s disease, while spasticity follows damage to pathways that control voluntary movement. This distinction matters because the two conditions respond to different treatments.
Conditions That Cause Spasticity
Any condition that damages the brain or spinal cord’s motor pathways can produce spasticity. Stroke is one of the most common causes, with about 35% of stroke patients developing it. Cerebral palsy has the highest rate, affecting over 90% of individuals. Roughly half of people with traumatic brain injury experience spasticity, and it occurs in about 40% of those with spinal cord injuries. Multiple sclerosis produces spasticity in 37% to 78% of cases, depending on disease severity and location of nerve damage. Less common causes include oxygen deprivation injuries and various neurodegenerative diseases.
What Spasticity Feels Like
The hallmark sensation is stiffness that resists movement. Your arm might pull tightly against your chest, or your ankle may point downward and resist being flexed. Muscles can feel perpetually “on,” creating a constant ache or tightness that worsens with cold weather, stress, infections, or even a full bladder. Spasms, sudden involuntary contractions, can strike without warning. They range from a brief twitch to a forceful jerk that moves an entire limb.
Clonus is another common feature. It shows up as rhythmic, involuntary jerking, most often at the ankle. If you push the foot upward, the calf muscles contract and relax in a repeating loop, bouncing the foot up and down. Clonus can make standing or walking unstable and is often one of the first signs that spasticity is worsening.
Over time, muscles that stay contracted can shorten permanently, a process called contracture. Once a contracture sets in, the joint loses range of motion even under anesthesia, when the nervous system is no longer driving the tightness. At that point, the problem has shifted from a neural issue to a structural one, and it becomes much harder to treat.
How Spasticity Affects Daily Life
Spasticity commonly interferes with basic activities: getting dressed, bathing, transferring from a wheelchair to a bed, and sleeping. Extensor spasms in the legs, where the knee locks straight and the foot pushes downward, are particularly disruptive for people with spinal cord injuries. These spasms interfere with transfers between surfaces, a task most wheelchair users perform around 14 times a day. Tight hand and wrist muscles can make it difficult to grip utensils, open containers, or maintain hygiene in skin folds.
Interestingly, spasticity isn’t always purely harmful. Some people find that spasms in the legs actually provide postural stability or brief weight support during transfers. This is one reason treatment is tailored to each person rather than aimed at eliminating all muscle tone. The goal is usually to reduce spasticity enough to improve comfort and function without removing tone that might be serving a useful purpose.
How Spasticity Is Measured
Clinicians most commonly use the Modified Ashworth Scale, a 0 to 4 grading system based on how much resistance a limb produces when moved passively. A score of 0 means normal muscle tone. A score of 1 indicates a slight catch or minimal resistance at the end of the movement range. A score of 2 means noticeable resistance through most of the range but the limb can still be moved easily. At a 3, resistance is strong enough that passive movement becomes difficult. A score of 4 means the limb is essentially rigid and cannot be moved. Tracking these scores over time helps gauge whether treatment is working.
Physical Therapy and Stretching
Stretching is the foundation of spasticity management. Regular, sustained stretching helps maintain muscle length and joint range of motion, slowing or preventing contractures. The evidence suggests that more structured approaches, like serial casting (applying a series of casts that progressively stretch a joint into a better position over weeks), tend to be more effective than simple manual stretching alone.
Splints and orthoses worn between therapy sessions help hold joints in a stretched position for longer periods. Techniques like electrical stimulation and therapeutic taping can enhance the benefits of stretching while reducing the risk of injury to muscles, tendons, or skin. Physical therapy programs for spasticity typically combine these approaches, adjusting the mix based on which muscles are affected and how severe the tightness is.
Oral Medications
When physical therapy alone isn’t enough, oral medications can help dial down muscle tone throughout the body. Baclofen is one of the most widely prescribed options. It works by mimicking a natural chemical in the spinal cord that dampens nerve signaling to muscles. Treatment typically starts at a low dose and is increased gradually over several weeks. The most common side effects include drowsiness, dizziness, weakness, and constipation. Because the medication affects the entire body, it can reduce muscle tone in areas that don’t need it, sometimes making a person feel weaker overall.
Tizanidine is another common choice that works through a different mechanism but carries similar side effects, particularly drowsiness. Both medications require careful dose adjustment because stopping them abruptly after prolonged use can trigger withdrawal symptoms, including rebound spasticity and, in rare cases, seizures.
Targeted Injections
For spasticity concentrated in specific muscle groups, injections of botulinum toxin offer a more targeted approach. The toxin blocks the nerve signal at the muscle itself, weakening its ability to contract. Relief typically develops over a week or two after injection and lasts several months, though treatments generally cannot be repeated sooner than 12 weeks apart. Injections work well for focal problems like a clenched fist, a turned-in foot, or a bent elbow, and they’re often combined with stretching or casting to maximize the window of reduced tone.
Surgical Options
When spasticity is severe and widespread, two surgical approaches are commonly considered. An intrathecal baclofen pump is a small device implanted under the skin of the abdomen that delivers baclofen directly into the fluid surrounding the spinal cord. Because the medication bypasses the bloodstream, it can achieve significant spasticity reduction at doses far smaller than oral baclofen requires, with fewer systemic side effects. The trade-off is that the pump requires surgical refills every few months and carries ongoing risk of device-related complications like catheter kinks, infections, or pump malfunctions.
Selective dorsal rhizotomy takes a different approach: a neurosurgeon permanently cuts selected sensory nerve fibers entering the spinal cord, reducing the overactive stretch reflex at its source. This procedure has traditionally been used in children with cerebral palsy who can walk, with the goal of improving gait. More recent case series suggest it may also help non-ambulatory children. Both procedures lower spasticity and can improve motor function, but complication rates tend to be higher with the baclofen pump due to the hardware involved. The choice between them depends on the individual’s functional level, goals, and the pattern of their spasticity.