Shaun Mattingly

A patient arrives for a course of transcranial magnetic stimulation and asks why their treatment intensity is not the same as someone else’s. The short answer: the brain’s responsiveness to magnetic pulses varies widely. That variability is why the motor threshold (MT) is central to safe, effective, and reproducible TMS care. Understanding how clinicians find and use the MT explains why some patients respond quickly while others need adjustments to their plan.

How the TMS motor threshold guides personalizing TMS treatment

Motor threshold is the clinical benchmark clinicians use to scale stimulation for each person. It represents the minimum magnetic intensity that elicits a measurable motor-evoked potential (MEP) or visible muscle twitch when the motor cortex is stimulated. Translating that biophysical marker into treatment intensity allows teams to tailor stimulation so that the target brain region receives an individualized dose rather than a fixed, one-size-fits-all setting.

The practical effect: two patients receiving the same nominal percent of device output may experience different cortical activation if clinicians do not base intensity on each person’s MT. Many experts suggest that expressing treatment intensity as a percentage of the motor threshold reduces inter-patient variability, but it remains essential to confirm parameters with a qualified provider and device-specific protocols.

Resting versus active motor thresholds: definitions that matter

Resting motor threshold (RMT) is measured while the target muscle is fully relaxed. Active motor threshold (AMT) is measured during a low-level voluntary contraction. Clinical teams choose one measure or the other depending on the therapeutic protocol and device used. The chosen threshold becomes the reference for setting stimulation intensity (for example, "% of RMT").

Step-by-step: the clinical protocol for measuring motor threshold

Prepare the patient: explain the procedure, confirm contraindications (history of seizures, metallic cranial implants), and obtain consent. Surface EMG leads may be placed over an index finger or thumb muscle to record MEPs.

Find the motor hotspot: using single-pulse stimulation, map the primary motor cortex until the site that evokes the largest MEP in the target muscle is identified.

Determine threshold: deliver single pulses at different intensities to find the minimal stimulator output that produces an MEP of a predefined amplitude (commonly ≥50 µV) in a given proportion of trials (often 50%).

Document carefully: record the coil orientation, distance from the nasion, device model, MT value, and whether RMT or AMT was used. These details ensure reproducibility and support safety monitoring.

That procedure is a core part of the clinical protocol. Accurate MT measurement improves dosing precision and forms the basis for safety decisions during treatment.

TMS coil placement and ensuring treatment accuracy

Locating where to place the coil over the scalp and ensuring consistent placement across sessions are as important as the intensity setting itself. Small deviations in coil position or angle can change the induced electric field at the cortical target. Clinicians commonly use one of three strategies to balance practicality and precision.

Rule-based positioning: simple scalp measurements such as the 5-cm rule (moving anteriorly from the motor hotspot) are quick but can be imprecise across people with different head shapes.

Scalp-based coordinate methods: approaches like the Beam F3 method use proportional landmarks to increase reliability without requiring advanced imaging.

Neuronavigation / MRI-guided targeting: image-guided systems register the patient’s head to their MRI and permit millimeter-level accuracy and reproducibility; this method enhances both localization and documentation.

Ensuring treatment accuracy requires attention to both location and dose. Some centers pair neuronavigation with MT-based dosing to reduce variance in delivered stimulation. Others incorporate coil-position tracking and periodic re-calibration of the MT when patients have major clinical changes or after several weeks of treatment.

Factors that change the motor threshold and how teams adjust

Several physiological and technical factors shift the MT over time or across individuals. Tracking these variables helps clinicians adapt treatment safely.

Medications: certain anticonvulsants or benzodiazepines can raise the MT; clinicians review drug lists before measurement.

Sleep, hydration, and fatigue: transient state changes may alter cortical excitability and therefore MT.

Scalp-to-cortex distance: greater distance typically requires a higher device output to reach the same cortical intensity.

Coil position and orientation: small angular shifts modify the induced field and can make a previously measured MT less accurate for subsequent sessions.

Device and coil model: different systems have different pulse shapes and cooling properties that influence effective dosing.

Because these influences exist, many clinics re-measure the motor threshold at defined intervals or after changes in medications. That practice is part of a safety-first protocol designed to maintain predictable cortical dosing.

How MT-based personalization affects outcomes and safety

Clinical outcomes and adverse event rates are both linked to how well a protocol matches stimulation to the individual’s brain. Personalizing TMS treatment via MT-based scaling reduces the risk of under-stimulation, which may lead to nonresponse, and of excessive stimulation, which can increase discomfort or lower but measurable seizure risk.

When clinicians report outcomes, they often correlate delivered intensity (percent of MT) with response rates. While specifics depend on the protocol and diagnosis, the broader point is clear: individualized dosing improves the interpretability of results and supports safer care. Again, many experts suggest MT-based dosing as a best practice, but final parameter selection should follow device instructions and clinician judgment.

Common protocol examples and cautious framing

High-frequency protocols frequently express intensity as a percentage of the RMT; a higher percentage generally increases cortical activation but also the potential for discomfort.

Theta-burst protocols often use a different reference (sometimes AMT) or different percentage targets; device approvals and published trials define the ranges used in clinical practice.

Protocol selection and adjustments should rely on peer-reviewed protocols and device-specific guidance rather than ad-hoc changes.

Because device models and published protocols vary, clinicians combine empirical evidence with on-site safety procedures. Patients should ask their treatment team which threshold was measured and how it guides stimulation settings.

Practical patient-facing considerations

Measuring an MT is brief and typically well tolerated. Patients can expect a sequence of single pulses while seated, with brief thumb or finger twitches and clicking sounds from the coil. Surface electrodes may be used, and teams explain each step and monitor comfort.

Pre-measurement checklist: remove metal hair accessories, disclose all medications, and discuss seizure history.

During measurement: brief sensory sensations and muscle twitches are normal; monitoring and adjustments minimize discomfort.

After measurement: staff document the MT and explain how it will be used to set treatment intensity and to plan coil placement.

Those practical steps form part of a transparent and safety-oriented clinical process. Patients who understand the role of MT are better equipped to follow instructions and to participate in shared decision-making.

Quality assurance: documentation, re-assessment, and staff training

High-quality TMS programs incorporate formal procedures that standardize MT measurement and coil placement. Documentation typically records the MT value, method (RMT vs AMT), device model and serial number, coil orientation, and the person who performed the measurement. Periodic staff competency checks and calibration of equipment support consistent practice.

Quality-assurance practices that many clinical programs follow include:

Standard operating procedures for MT measurement and coil positioning.

Scheduled re-assessment when clinical or medication changes occur.

Routine device maintenance and logging of treatment parameters.

Clear incident reporting pathways for adverse events.

When to ask questions and seek clarification

If you or a loved one is preparing for TMS, seek clarity about the role of the motor threshold in the planned protocol. Ask whether the clinic will use neuronavigation or scalp-based methods for coil placement, how often MT will be re-measured, and how medication changes are handled.

Clinics that offer services such as TMS therapy for depression typically have established MT procedures and safety checks. If you are comparing programs, request documentation about their MT measurement protocol and about how they maintain consistent coil placement to reduce variability in treatment delivery.

For those starting research or doing background reading, resources describing transcranial magnetic stimulation explain both the technology and real-world considerations about dosing, coil placement, and patient selection.

Final thought and next step

Measuring the motor threshold is not a technicality; it is a foundational clinical step that aligns biological variability with treatment planning, safety monitoring, and outcome expectations. Many experts view MT-based personalization as a core element of modern TMS care, yet the specifics of measurement and target selection vary by protocol and device. Ask your provider which threshold method they use, how coil placement will be ensured across sessions, and how they will monitor and adjust treatment—then make decisions in partnership with that clinical team.