Motor Threshold and TMS Dosing Explained

The motor threshold (MT) is the lowest magnetic pulse strength that reliably makes a small muscle in your hand twitch when the coil is held over the motor cortex. It is a quick, individual measure of how easily your brain responds to TMS, and treatment intensity is then set as a percentage of it: most standard depression protocols use 120% of MT. Because medications, sleep and anatomy can shift the number, MT is measured before the first treatment and re-checked when something relevant changes.

Why TMS needs a personal yardstick

A TMS machine's output dial means little on its own. The same setting can strongly stimulate one person's cortex and barely reach another's, because skulls, scalp thickness and brain excitability differ. The motor cortex offers a convenient test: a pulse that crosses threshold produces a visible or measurable finger or thumb movement, so the team can see the brain's response directly.

The prefrontal target for depression gives no such visible signal. So the field uses the motor cortex as a calibration point and assumes the prefrontal cortex will respond similarly at a similar relative intensity. That assumption is useful but imperfect, which is the theme of the rest of this article.

How MT is measured

The coil is moved over the motor area to find the spot that gives the best hand response, then the intensity is adjusted to find the lowest level that still produces a consistent response. The threshold can be judged by watching for finger movement or by recording muscle activity with electromyography (EMG). FDA's summary for the SAINT system, for example, specifies "visual inspection of finger movement + EMG". Some devices include dedicated tools; NeuroStar's system has a "D-Tect MT Accessory" for locating and measuring MT.

The method matters a little. The 2018 consensus recommendations from the National Network of Depression Centers and the APA task force note that visual observation of a single muscle gives significantly higher MTs than EMG of that muscle. They also note that a multicenter sham-controlled trial using both approaches found no significant differences in side effects or outcome, and that visual observation has been accepted in U.S. practice as a safe substitute for EMG.

From threshold to treatment dose

Once MT is known, each protocol sets intensity as a multiple of it. The percentage is part of what was tested in the trials, so it is not arbitrary.

SAINT's lower percentage is not a weaker treatment in disguise. It delivers 10 sessions a day and corrects intensity for how deep the prefrontal target lies compared with the motor cortex, so the effective dose at the target is calculated differently. Details are in the SAINT guide and the theta-burst explainer.

Why 120%, and what happens below it

The 120% figure became standard because it was the dose used in the trials that led to FDA clearance. Lower intensities are sometimes used to make treatment more comfortable, but the 2018 consensus cautions that antidepressant efficacy below 110% of MT "remains questionable due to limited evidence". A 2025 meta-analysis of TMS in late-life depression found that the benefit appeared mainly in trials using at least 100% of resting MT.

Intensity is also a safety variable. Seizures from TMS are very rare, but they were more common when published safety limits were exceeded. FDA's special-controls guidance lists a motor threshold that "cannot be reliably determined" among the situations that call for close monitoring. The full safety picture is in TMS safety and seizure risk.

Comfort and the first week

Higher intensity means a stronger tapping sensation on the scalp. The consensus authors note that early headache and procedural discomfort may be particularly uncomfortable for people with high MT levels, and that discomfort typically decreases with habituation. Options they list include over-the-counter pain relievers, topical anesthetic applied to the coil site beforehand, better head and neck support, and, with the efficacy caveat above, a temporary reduction in intensity.

Tolerance also differs by protocol. In the THREE-D trial, iTBS was rated slightly more painful than 10 Hz rTMS (3.8 vs 3.4 out of 10). In the ONE-D single-day series, 7 of 32 patients could not tolerate the target 120% MT. A patient-level description of these sensations is on what TMS feels like.

Why MT is re-measured

MT is a snapshot. The 2018 consensus recommends measuring it at baseline and lists situations that may justify re-checking before a session:

• Medication changes. Patients should be told that medication changes could affect MT, and MT may be re-checked when a change could alter seizure threshold. Pharmacology studies explain why: in healthy volunteers, single doses of sodium and calcium channel blockers such as carbamazepine and lamotrigine raised MT, while drugs acting mainly on GABA (vigabatrin, baclofen) and gabapentin did not change it. See combining TMS with medication.
• Sleep. A new or significant change in sleep pattern is listed as a reason to consider re-checking MT. Sleep deprivation is also one of the factors the international safety guideline names as lowering seizure threshold.
• Alcohol and other substances. The same guideline lists alcohol withdrawal and stimulants such as cocaine or MDMA among seizure-threshold factors.

Re-measurement is a recognized part of care, with its own Medicare billing code (CPT 90869, "subsequent motor threshold re-determination with delivery and management"). How often it happens in practice varies by team.

Scalp-to-cortex distance and older adults

The biggest weakness of MT-based dosing is that it is measured over one brain region and applied to another. Magnetic field strength falls off quickly with distance: a 2005 study estimated that each additional millimeter between the coil and the motor cortex requires about 3% more stimulator output to produce the same effect. If the prefrontal cortex lies further from the skull than the motor cortex, a dose calibrated at the motor cortex will under-stimulate the target.

That is a particular concern with age. In an MRI study of 29 depressed adults receiving rTMS, longer distance to the motor cortex correlated with higher MT, while prefrontal distance did not. Antidepressant response did not correlate with either distance, although responders were significantly younger. A later pilot study of 18 depressed patients aged 55 to 75 found that skull-to-prefrontal distance increased with age while skull-to-motor distance did not; adjusting for this, intensities averaged 114% of MT with a range of 103% to 141%, and the higher doses were well tolerated. That was an open, uncontrolled pilot, and its authors called for blinded trials. More on treating later-life depression is in the guide to TMS for older adults.

Questions to ask about dosing

1. How will my motor threshold be measured: by watching, by EMG, or both?
2. What percentage of MT will be used, and is that the percentage tested for this protocol?
3. If the treatment is uncomfortable, what will be tried before lowering intensity?
4. Should I report changes in my medications, sleep or alcohol use before each session? (The answer should be yes.)
5. When would my MT be re-checked during the course?

Motor threshold is one of the details a physician-supervised team handles routinely; the roles involved are described in who delivers TMS. Targeting, the other half of dosing, is covered in finding the TMS target. For an overview of the whole process, see the TMS guide and the TMS therapy page.

Frequently asked questions

Does a high motor threshold mean TMS will not work for me?

No. A high MT means a higher machine output is needed to reach the treatment dose. It can make sessions feel stronger. It is a dosing detail, not a verdict on whether TMS can help.

Will my finger twitch during treatment?

Usually not. The twitch is used only during the threshold measurement over the motor cortex. During treatment the coil sits over the prefrontal area, where pulses may cause scalp or facial muscle tapping instead.

Can I take my usual medications before MT is measured?

Generally yes, and keeping doses stable through the course is recommended. Tell the TMS team about any change, because some medications can shift motor threshold or seizure threshold.

Is a lower intensity safer?

Lower intensity may feel more comfortable, but the evidence that it works as well is limited. Safety comes mostly from staying within established guidelines and screening for seizure risk factors.

Why does my threshold differ from session to session?

Small variation is normal. Sleep, medications, alcohol, stress and slight differences in coil position can all affect the measurement, which is why teams re-check when something relevant changes.

Sources

1. Consensus Recommendations for the Clinical Application of Repetitive Transcranial Magnetic Stimulation (rTMS) in the Treatment of Depression. Journal of Clinical Psychiatry, 2018. pmc.ncbi.nlm.nih.gov
2. Effects of antiepileptic drugs on motor cortex excitability in humans: a transcranial magnetic stimulation study. Annals of Neurology, 1996. pubmed.ncbi.nlm.nih.gov
3. Simple metric for scaling motor threshold based on scalp-cortex distance: application to studies using transcranial magnetic stimulation. Journal of Neurophysiology, 2005. pubmed.ncbi.nlm.nih.gov
4. How coil-cortex distance relates to age, motor threshold, and antidepressant response to repetitive transcranial magnetic stimulation. Journal of Neuropsychiatry and Clinical Neurosciences, 2000. pubmed.ncbi.nlm.nih.gov
5. Safety and benefits of distance-adjusted prefrontal transcranial magnetic stimulation in depressed patients 55-75 years of age: a pilot study. Depression and Anxiety, 2004. pubmed.ncbi.nlm.nih.gov
6. Daily left prefrontal transcranial magnetic stimulation therapy for major depressive disorder: a sham-controlled randomized trial. Archives of General Psychiatry, 2010. pubmed.ncbi.nlm.nih.gov
7. Efficacy and safety of transcranial magnetic stimulation in the acute treatment of major depression: a multisite randomized controlled trial. Biological Psychiatry, 2007. pubmed.ncbi.nlm.nih.gov
8. Efficacy and safety of deep transcranial magnetic stimulation for major depression: a prospective multicenter randomized controlled trial. World Psychiatry, 2015. pmc.ncbi.nlm.nih.gov
9. 510(k) Summary K173620 – MagVita TMS Therapy System w/Theta Burst Stimulation. U.S. FDA, 2018. accessdata.fda.gov
10. 510(k) Summary K231926 – NeuroStar Advanced Therapy System. U.S. FDA, 2024. accessdata.fda.gov
11. 510(k) Summary K220177 – Magnus Neuromodulation System with SAINT Technology. U.S. FDA, 2022. accessdata.fda.gov
12. Stanford Accelerated Intelligent Neuromodulation Therapy for Treatment-Resistant Depression. American Journal of Psychiatry, 2020. pubmed.ncbi.nlm.nih.gov
13. Repetitive transcranial magnetic stimulation for late-life depression: A systematic review and meta-analysis of randomized controlled trials. Journal of International Medical Research, 2025. europepmc.org
14. Seizures from transcranial magnetic stimulation 2012-2016: Results of a survey of active laboratories and clinics. Clinical Neurophysiology, 2019. europepmc.org
15. Class II Special Controls Guidance Document: Repetitive Transcranial Magnetic Stimulation (rTMS) Systems. U.S. FDA, 2011. fda.gov
16. Safety and recommendations for TMS use in healthy subjects and patient populations, with updates on training, ethical and regulatory issues: Expert Guidelines. Clinical Neurophysiology, 2021. europepmc.org
17. Effectiveness of theta burst versus high-frequency repetitive transcranial magnetic stimulation in patients with depression (THREE-D): a randomised non-inferiority trial. The Lancet, 2018. pubmed.ncbi.nlm.nih.gov
18. Real-world effectiveness of a single-day regimen for transcranial magnetic stimulation using Optimized, Neuroplastogen-Enhanced techniques in Depression (ONE-D). Research Square (preprint), 2024. doi.org
19. Billing and Coding: Transcranial Magnetic Stimulation (A57528). CMS, 2026. cms.gov

Educational information, not medical advice. Whether TMS or any treatment fits you is a decision to make with a psychiatrist who knows your history.