Legs / Movement

Leg Movements, PLMS & Arousals

Identify the movement, check whether it belongs to a periodic series, then evaluate its association with a scored EEG arousal.

Clinical review pending

AASM-based educational reference. Verify against the current licensed scoring manual and facility protocol. PLMS is a PSG finding; it does not by itself diagnose a movement disorder.

1. Quick thresholds

Leg movement scoring measurements and rules
Measurement	Rule
Candidate movement duration	0.5–10 seconds, inclusive
EMG onset	≥8 µV increase above resting baseline
EMG ending	Start of ≥0.5 seconds during which EMG does not exceed 2 µV above resting baseline
Minimum periodic series	≥4 qualifying movements
Spacing in a periodic series	5–90 seconds, measured onset to onset
Movements in different legs	Onsets <5 seconds apart count as one movement
PLMS–arousal association	Simultaneous/overlapping, or a gap <0.5 seconds between the end of one and start of the other

Amplitude thresholds are measured above resting EMG, not from an arbitrary screen zero.

Identify a candidate leg movement

All required:

Duration ≥0.5 seconds AND ≤10 seconds.

AND EMG rises ≥8 µV above resting baseline.

AND At least part of the movement occurs in an epoch scored as sleep.

Onset: Start the movement at the point where the EMG reaches the onset threshold.

Ending: Find a period lasting at least 0.5 seconds during which the EMG stays no more than 2 µV above resting baseline. The movement ends at the START of that quiet period.

Do not add the 0.5-second confirmation period to the movement duration.

Review both tibialis anterior EMG channels. Confirm a stable baseline and exclude artifact.

Does it form a PLMS series?

≥4 qualifying movements AND each consecutive onset-to-onset interval is 5–90 seconds.

Measure from the START of one movement to the START of the next—not from the end of one to the next start.

Count individual movements, not the number of series.

Once a qualifying four-movement series is established, all four movements belong to the series—not only the fourth.

Three isolated qualifying movements do not form a PLMS series.

These are AASM timing criteria. Do not substitute the different WASM/IRLSSG interval rules.

Left + right leg movements

Movements in two different legs with onsets <5 seconds apart are counted as one movement.

Example: Left starts at 10 seconds. Right starts at 12 seconds. Result: one movement for counting.

Use the first movement’s onset when measuring the interval to the next movement in the sequence.

This is a bilateral counting rule. Do not automatically apply it to every pair of bursts in the same leg.

Exclude respiratory-related movements

Do not score an LM occurring within the exclusion interval from 0.5 seconds before a scored apnea, hypopnea, RERA, or other sleep-disordered-breathing event through 0.5 seconds after it.

Review the entire movement relative to the respiratory event, not just its onset.

Example: A leg burst overlapping the end of an apnea is excluded from PLMS scoring.

Not every leg movement near an oxygen drop is automatically respiratory-related. Identify the scored respiratory event and apply the actual timing rule.

2. PLMS-associated arousals

First confirm a real EEG arousal

An abrupt EEG frequency shift lasting ≥3 seconds AND at least 10 seconds of stable sleep preceding it.

Qualifying frequency changes include alpha, theta, and/or activity >16 Hz, excluding sleep spindles.

During REM: Also require a concurrent increase in chin EMG lasting ≥1 second.

A leg burst, heart-rate increase, or movement on video alone does not establish an EEG arousal.

Link the arousal to the PLMS

The movement must belong to a qualifying PLMS series.

The movement and scored arousal occur simultaneously or overlap; OR there is <0.5 seconds between the END of one and START of the other.

Either event can occur first.

This is not an onset-to-onset measurement.

Association does not prove that the movement caused the arousal.

An isolated LM-associated arousal is not automatically a PLMS-associated arousal.

Timing examples

Assume a qualifying PLMS and an independently valid scored arousal.

PLMS–arousal association timing examples
Situation	Association timing met?
Arousal begins while the PLMS is ongoing	YES
PLMS ends, then arousal begins 0.3 seconds later	YES
Arousal ends, then PLMS begins 0.3 seconds later	YES
A gap of exactly 0.5 seconds	NO
A gap of 1 second	NO

Arousal begins while the PLMS is ongoing

YES

PLMS ends, then arousal begins 0.3 seconds later

YES

Arousal ends, then PLMS begins 0.3 seconds later

YES

A gap of exactly 0.5 seconds

A gap of 1 second

Do not score an arousal with every movement

Each new arousal must satisfy the arousal rules, including the preceding-sleep requirement.

Example: Two PLMS occur close together, each with a brief EEG frequency change. If the second change lacks the required preceding sleep, it is not another scored arousal.

The movements may both qualify, while only one is associated with a valid scored arousal.

3. Index calculations

PLMS Index — PLMI

PLMS count × 60 ÷ total sleep time in minutes

Equivalent: PLMS count ÷ total sleep time in hours.

PLMS Index calculator

PLMS Arousal Index — PLMSArI

Number of PLMS associated with scored arousals × 60 ÷ total sleep time in minutes

PLMS-Arousal calculator

Total Arousal Index

Total scored arousals × 60 ÷ total sleep time in minutes

Arousal Index calculator

Use total sleep time—not time in bed or total recording time. Do not use all leg movements or the number of PLMS series as the PLMS numerator. PLMSArI and total arousal index are different measurements.

Worked example

Total sleep time: 360 minutes = 6 hours. Qualifying PLMS: 120. PLMS associated with scored arousals: 30. Total scored arousals: 90.

PLMI = 120 × 60 ÷ 360 = 20.0/hour
PLMSArI = 30 × 60 ÷ 360 = 5.0/hour
Total arousal index = 90 × 60 ÷ 360 = 15.0/hour

Sleep time must be greater than zero. Counts must be nonnegative whole numbers. PLMS-with-arousals count cannot exceed PLMS count. Keep full precision internally; display one decimal place. Do not diagnose PLMD from the calculated index.

4. What the terms mean

LM: An individual leg movement.
PLMS: Periodic limb movements during sleep: qualifying movements occurring in a periodic series.
PLMS-associated arousal: A qualifying PLMS temporally associated with a separately valid EEG arousal.
PLMD: A clinical diagnosis requiring more than PLMS on a graph, including relevant symptoms and exclusion of other explanations.
RLS: A clinical syndrome assessed from symptoms and history. It is not diagnosed from leg EMG alone.

A high index alone does not establish PLMD. No arbitrary mild/moderate/severe PLMI categories are applied.

RLS — separate clinical topic

5. Quick scoring order

1Check signal quality and resting EMG.
2Identify candidate movements and their boundaries.
3Confirm the sleep-epoch requirement.
4Apply respiratory-event exclusions.
5Combine qualifying bilateral movements for counting.
6Evaluate onset-to-onset spacing and series length.
7Independently score EEG arousals.
8Apply the PLMS–arousal association rule.
9Calculate and review the separate indices.

Review automated detections. A software flag is not a final scored event.

Teaching waveform topics

Other movement patterns

Do not classify these automatically as PLMS.

Advanced measurement reminders

The quiet period confirms an ending retrospectively: the ending remains the start of that quiet period, not its end.

Periodic spacing uses onset to onset. PLMS–arousal association uses overlap or the gap from the end of one event to the start of the other. These are different measurements.

Public summaries and historical protocols do not replace the current licensed manual. Do not import older research-specific exceptions or different WASM criteria.

Sources and reference limitations

Public summaries and historical protocols support this reference but do not replace the full current licensed manual. Do not import older research-specific exceptions or different WASM criteria.