Why do muscles shake during exercise?
Muscles may begin to shake during heavy strength exercises, static positions, balance exercises, or movements that require a high degree of control. The shaking may be mild and almost invisible, or noticeable enough for the arms, legs, or entire body to tremble.
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This usually happens because the nervous system and muscles are working hard to maintain force and stability. As the load approaches your current capacity, the body must recruit more motor units, continuously adjust force, and compensate for fatigue. Force production then becomes less consistent, which may result in visible small movements or shaking.
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Muscle force is controlled by the nervous system
A muscle does not contract entirely on its own. Movement begins with signals from the brain and spinal cord that travel through motor nerves to the muscle fibres.
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A motor nerve and the muscle fibres it controls are called a motor unit. When the body needs to produce more force, additional motor units are activated, while already active units may send signals more frequently.
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Force is regulated through factors such as:
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• How many motor units are activated
• How quickly nerve signals are transmitted
• How the motor units work together
• Muscle length and joint position
• The size of the load
• How fatigued the muscle is
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With a light load, the body can regulate this relatively smoothly. When the load becomes high or the muscle becomes fatigued, force production often becomes more irregular.
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Shaking consists of small variations in force
Even when you attempt to hold a body part completely still, muscle force varies slightly from moment to moment. These small fluctuations are also present in rested muscles, but they often become more noticeable during high loading or fatigue.
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When force is not produced entirely consistently, a joint may move a few millimetres backwards and forwards. This is perceived as shaking.
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Shaking therefore does not usually mean that the muscle is switching off. It often shows that the body is making rapid corrections to maintain the position or complete the movement.
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Fatigue makes force less stable
As a muscle continues working, conditions inside the muscle fibres change. Energy is used, metabolic by-products accumulate, and the muscle fibres’ ability to produce force may temporarily decline.
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The nervous system attempts to compensate by activating additional motor units or increasing its signalling to the muscle. This continuous adjustment can make force production less consistent.
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Fatigue-related shaking often occurs:
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• Near the end of a heavy set
• During prolonged isometric holds
• During high-repetition sets
• When rest periods between sets are short
• After several hard exercises for the same muscle group
• When training close to exhaustion
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The closer you come to the muscle’s temporary capacity, the more difficult it becomes to keep force completely stable.
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Isometric exercises often produce shaking
An isometric exercise involves producing muscular force without obvious movement of the joint. Examples include a plank, wall sit, static calf raise, or holding a weight still.
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These exercises may cause considerable shaking because the muscle must maintain consistent force over time. There is no clear movement phase that provides brief periods of unloading.
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Common exercises in which shaking may occur include:
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• Plank
• Side plank
• Wall sit
• Static lunge
• Hollow hold
• Isometric knee extension
• Holding a dumbbell with the arm straight
• Standing on one leg
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Shaking during these exercises is often a normal sign that the muscles and nervous system are working close to their current capacity.
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Unfamiliar exercises require more coordination
A new exercise may cause shaking even when the load is not particularly heavy. This happens because the nervous system has not yet developed an efficient strategy for controlling the movement.
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When learning a new exercise, the brain must continuously adjust:
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• Joint position
• Muscle activation
• Balance
• Direction of movement
• Distribution of force
• Movement speed
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This control often becomes more efficient with practice. Shaking may therefore decrease even if you later use a heavier load.
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This is part of neuromuscular learning. The body does not only become stronger; it also becomes better at organising force.
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Stabilising muscles may be challenged
In many exercises, the largest muscle is not necessarily the one that shakes the most. Smaller stabilising muscles around the joints may work hard to maintain the correct body position.
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This may be noticeable during exercises such as:
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• Single-leg squats
• Lunges
• Shoulder presses
• Push-ups
• Plank variations
• Exercises performed on unstable surfaces
• Controlled lowering exercises
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If the demand for stabilisation is greater than you are accustomed to, the body may make many rapid corrections. Shaking may then occur even when the main muscles are not completely exhausted.
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Balance exercises require many small corrections
When you stand on one leg or on an unstable surface, the body continuously moves slightly around its centre of gravity. The nervous system uses information from vision, the vestibular system, and sensory receptors in the muscles and joints to keep you upright.
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These corrections happen very quickly. If the task is demanding, they may become larger and more visible.
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Shaking during balance exercises may therefore result from:
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• Small changes in the centre of gravity
• Limited experience with the task
• Fatigue in the ankle and hip muscles
• Reduced visual information
• An unstable surface
• High concentration and muscle tension
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Shaking does not necessarily mean that your balance is poor. It may also show that the body is actively working to maintain the position.
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Heavy loading increases the demand on motor units
During heavy lifting, the body must activate a large proportion of the available motor units. The force requirement is high, and small variations in neural signalling become easier to see.
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This may be particularly noticeable:
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• During the final part of a heavy repetition
• Around a biomechanically difficult point
• During lifts close to maximal strength
• When technique begins to deteriorate
• During slow repetitions under heavy loading
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A heavy lift may therefore look less smooth than a light lift even when the person has good technique. Shaking should still be considered together with control and movement quality.
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If the weight can no longer be controlled, it may be better to end the set rather than continue forcing repetitions.
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Slow training can make shaking more noticeable
Slow repetitions increase the length of time that the muscle must produce force. This may cause fatigue to develop more quickly, particularly when the load is moderate or heavy.
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When you lower a weight slowly or resist a movement, the nervous system must regulate force very precisely. Small variations then become easier to see.
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Shaking may commonly occur during:
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• Slow eccentric training
• Tempo squats
• Controlled push-ups
• Slow leg presses
• Slow lowering during calf raises
• Low-speed rehabilitation exercises
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This does not necessarily mean that the exercise is too heavy. However, the shaking should not become so severe that you lose control or substantially change the movement.
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Low blood sugar can cause more generalised shaking
Shaking during exercise does not always come only from the working muscle. If the body has little available energy, you may experience more generalised shaking together with weakness, hunger, cold sweating, or dizziness.
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This may occur because of:
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• A long period since the last meal
• Hard training with insufficient energy intake
• Prolonged training sessions
• Diabetes or blood-glucose-lowering medication
• Alcohol combined with low food intake
• A large energy deficit
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If shaking occurs together with clear feelings of illness or weakness, the session should be stopped. People with diabetes should follow their individual plan for testing and treating low blood sugar.
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Local shaking in a fatigued muscle is different from generalised shaking accompanied by illness or signs of energy deficiency.
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Caffeine and stimulants may increase shaking
Caffeine increases alertness and may improve performance in certain situations. High doses may also cause restlessness, palpitations, and shaking.
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The effect varies considerably between individuals. Some people tolerate large amounts of caffeine, while others begin to shake after one strong coffee or a serving of pre-workout.
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Caffeine-related shaking may be more likely with:
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• A high dose of caffeine
• Insufficient sleep
• Stress or nervousness
• Training on an empty stomach
• Combining several stimulant products
• Low caffeine tolerance
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If shaking becomes unusually pronounced after coffee, energy drinks, or pre-workout, reducing the dose may be helpful.
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Stress can make the muscles more unsettled
Nervousness and stress activate the sympathetic nervous system. This may increase muscle tension, heart rate, and physical alertness. The result may be more shaking during an exercise that is already demanding.
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This is common before competitions, heavy lifts, tests, or exercises in which a person is afraid of failing.
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Stress-related shaking may be influenced by:
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• High expectations
• Performance pressure
• Fear of pain or injury
• Rapid breathing
• High muscle tension
• Caffeine
• Insufficient sleep
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Calmer breathing, a better warm-up, and using a load that feels controllable may reduce this type of shaking.
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Insufficient sleep can reduce control
Sleep affects reaction time, concentration, force production, and coordination. After insufficient sleep, an exercise may feel heavier, and the body may have more difficulty keeping force stable.
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This may appear as:
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• Shaking that begins earlier
• Poorer balance
• More technical errors
• Lower motivation
• Greater perceived effort
• Slower reactions
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One night of poor sleep is usually not dangerous, but reducing the load may be sensible if the warm-up feels unusually heavy and control is poorer than normal.
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Dehydration may affect performance
Dehydration can reduce physical capacity and increase heart rate and perceived effort. With greater fluid loss, coordination and muscular function may also be affected.
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Dehydration may be more relevant if you:
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• Train for a prolonged period
• Exercise in hot conditions
• Sweat heavily
• Have consumed little fluid
• Have experienced vomiting or diarrhoea
• Consumed alcohol the previous day
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Mild local muscle shaking is nevertheless rarely caused by dehydration alone. It usually results from a combination of loading, fatigue, and neuromuscular control.
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Shaking does not necessarily mean the exercise is more effective
Some people interpret pronounced shaking as evidence that an exercise is especially effective. This is not always correct.
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Shaking shows that the nervous system and muscles are working to control force, but by itself it provides little information about how effectively the exercise stimulates strength, muscle growth, or function.
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An exercise may be effective without causing any shaking. Another exercise may cause considerable shaking because it is unfamiliar, unstable, or difficult to control, without necessarily being the best option for the training goal.
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Training effectiveness is better evaluated through:
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• Appropriate loading
• Sufficient training volume
• Gradual progression
• Good technique
• Relevant movements
• Consistency
• Recovery
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Shaking is therefore a physiological phenomenon, not a mark of training quality.
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Shaking does not automatically mean the muscle is weak
A strong person may also shake during maximal loading, prolonged holds, or unfamiliar balance tasks. Shaking does not directly show how strong a muscle is.
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A person may shake because:
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• The load is close to maximal capacity
• The muscle is fatigued from previous sets
• The exercise is unfamiliar
• The demand for stability is high
• The movement is being performed very slowly
• The person is stressed or has consumed a large amount of caffeine
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Weakness may be part of the explanation, particularly during a new exercise or after an injury, but shaking alone cannot be used as a test of muscle strength.
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When should you reduce the load?
Mild shaking during demanding exercise is usually harmless. However, the load should be adjusted if the shaking causes you to lose control.
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Reduce the weight, resistance, or duration if:
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• Technique clearly deteriorates
• You cannot control the joint position
• The movement becomes jerky and uncontrolled
• You involuntarily hold your breath for a prolonged period
• You develop pain
• You become dizzy or unwell
• The shaking continues for a long time after the exercise
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The aim is not always to eliminate all shaking, but to keep the task within a level that can be completed safely and with control.
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Rest periods may reduce shaking
If shaking is caused by acute muscle fatigue, taking a break will often help. Energy metabolism partially normalises, the load on the nervous system decreases, and force production may become more consistent during the next set.
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The length of the rest period depends on the training goal:
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• Shorter rests may be suitable for light muscular endurance training
• Moderate rests may be suitable for muscle growth
• Longer rests may be useful for heavy strength training and explosive lifts
• Balance training may require a break when control deteriorates
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If shaking becomes stronger with each set, the rest periods may be too short or the total training load may be too high.
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Better technique can make movement smoother
As you become more experienced with an exercise, the nervous system learns to activate the correct muscles at the appropriate time. The movement requires less unnecessary tension, and force can be produced more smoothly.
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This may reduce shaking even before muscle strength has increased substantially.
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To improve control, it may help to:
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• Temporarily reduce the load
• Slow the movement without training to exhaustion
• Use a shorter range of motion initially
• Practise the exercise more frequently with a light load
• Focus on maintaining a stable joint position
• Avoid taking every set to complete exhaustion
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Technical practice should be demanding enough to promote learning, but not so heavy that the movement becomes chaotic.
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Shaking after exercise may be caused by fatigue
Some people continue to shake slightly after the set has finished. This may occur because the muscles are still fatigued, the nervous system remains activated, or the body has a high heart rate and elevated adrenaline levels.
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Short-term shaking after a hard session may be normal, particularly after:
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• Heavy leg exercises
• Interval training
• High-intensity circuit training
• Training close to exhaustion
• Competition or maximal testing
• High caffeine intake
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The shaking should usually decrease with rest, food, fluids, and a gradual cool-down.
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If the shaking is severe, lasts for a long time, or occurs together with confusion, fainting, chest pain, or substantial weakness, the situation should be assessed more carefully.
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When is shaking usually harmless?
Exercise-related shaking is usually harmless when it:
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• Occurs during a demanding exercise
• Begins near the end of a set
• Resolves after a rest period
• Does not cause pain
• Does not continue at rest
• Improves when the load is reduced
• Occurs on both sides during symmetrical training
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This pattern is consistent with normal neuromuscular fatigue and high demands for movement control.
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Shaking may also be normal when you are learning a new exercise or training balance and stability.
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When should shaking be investigated?
Shaking that occurs only during hard exercise is usually not concerning. It should be assessed more carefully if it also occurs at rest, becomes progressively worse, or occurs together with other symptoms.
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Contact a healthcare professional if you experience:
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• New shaking at rest
• Shaking mainly on one side
• Persistent weakness
• Numbness or altered sensation
• Problems with walking or coordination
• Pronounced palpitations
• Fainting or severe dizziness
• Shaking after starting a new medication
• Unexplained weight loss or heat intolerance
• Shaking that affects daily activities
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These symptoms may have causes unrelated to exercise, including medications, thyroid disorders, neurological conditions, or low blood sugar.
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Common misconceptions
Shaking does not mean that the muscle is being damaged. It is often caused by fatigue and less consistent force regulation.
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More shaking does not automatically mean better training. Training effectiveness depends on loading, progression, and relevance, not on how much the body shakes.
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Shaking does not necessarily mean that you are weak. Strong people may also shake during heavy loading or demanding control tasks.
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Shaking is not always caused by dehydration. Neuromuscular fatigue and motor control are often more important explanations.
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You do not always need to stop the exercise. Mild shaking may be acceptable as long as technique and control remain good.
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Summary
Muscles shake during exercise because the nervous system must continuously adjust force to maintain a position or complete a movement. When the load is high, the muscle becomes fatigued, or the exercise requires considerable balance and coordination, force production becomes less consistent. This may cause visible small movements. Shaking is common during heavy sets, isometric exercises, slow repetitions, and unfamiliar stability tasks. It is usually harmless as long as control remains good and the shaking resolves after a break. The load should be reduced if technique deteriorates, pain develops, or you become unwell. Shaking that also occurs at rest or is accompanied by persistent weakness, numbness, or coordination problems should be assessed by a healthcare professional.
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Sources
• Enoka RM, Duchateau J. Muscle fatigue: what, why and how it influences muscle function. Journal of Physiology. 2008;586(1):11–23.
• Taylor JL, Amann M, Duchateau J, Meeusen R, Rice CL. Neural contributions to muscle fatigue: from the brain to the muscle and back again. Medicine & Science in Sports & Exercise. 2016;48(11):2294–2306.
• Allum JHJ, Dietz V, Freund HJ. Neuronal mechanisms underlying physiological tremor. Journal of Neurophysiology. 1978;41(3):557–571.
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