EMG Parameters — Which Number Answers Which Question

Why start with the question#

Konrad's analysis chapter is built around a simple observation: the parameters are cheap, the question is expensive. Software will compute mean, peak, area, median frequency and onsets for every channel in a second — but a mean amplitude cannot answer a timing question, a peak cannot answer a fatigue question, and a single-channel value cannot answer a coordination question. Cram's makes the same point from the clinical side (Ch. 8): decide what you want to know, then choose the placement, the task and the number.

Every parameter below assumes the processing chain from raw EMG to RMS and, where amplitudes are compared across sessions or people, normalisation.

Level 1 — On or off?#

Question: Is the muscle active at all during this task, and when?

Parameter: onset and offset, defined by a threshold on the envelope. Konrad lists three ways to set it:

• Multiple of baseline SD — typically 2–3 standard deviations of the resting baseline, plus a minimum duration (e.g. 50 ms) above threshold so single spikes do not trigger. Popular, but the baseline SD varies between trials and people; with very quiet amplifiers the factor may need to rise to 8 or more.
• Percentage of local peak — e.g. 5 % of the peak within the analysis window; more stable, independent of baseline noise.
• Fixed value — a microvolt level, or better a % MVC level in normalised data.

Whatever the rule: check every onset graphically. Hodges & Bui showed how much the detected onset moves with method and window — a timing conclusion is only as good as its threshold.

Level 2 — More or less?#

Question: Is the muscle more active in condition A than in B — left vs. right, before vs. after, exercise 1 vs. exercise 2?

Parameter: the mean amplitude of the envelope over a defined interval — Konrad's "most important EMG calculation": robust, least sensitive to interval length, best for comparisons. Peak is too variable on single trials; use the average peak (mean of the ten highest values) or peaks of averaged curves. Area (iEMG) only at equal interval lengths.

Rules: same electrodes, same task, same window; within one session raw µV are acceptable for left–right; across sessions or people normalise. And remember Cram's warning: more microvolts is not more force.

Level 3 — When? (timing)#

Question: In which order do the muscles fire, how long after a stimulus, when is the peak?

Parameters: time to peak (from contraction start or event to peak of the envelope), firing order across channels (onset ranking), onset pattern diagrams (on/off bars per muscle across a movement cycle — the classic gait representation), and reaction time after an external trigger.

Rules: short smoothing windows or zero-phase filters (every moving window delays onsets), a synchronised event marker (foot switch, goniometer, video), and identical threshold rules for all channels. Typical clinical questions: does the Vastus Medialis start before the Vastus Lateralis? Does the Gluteus Maximus fire before the Biceps Femoris (Long Head) in hip extension? Does the Multifidus switch on in a feed-forward manner before the arm moves?

Level 4 — How much?#

Question: How much of its capacity is the muscle using?

Parameter: mean amplitude expressed as % MVC (or another reference — see the normalisation article). Only normalised data can say "the Upper Trapezius worked at 15 % of maximum during this typing task". Konrad's Input % — each channel's mean as a share of the summed means — answers the related question "how is the effort distributed across these muscles?"

Rules: the reference contraction must be valid (see MVC test rules); values above 100 % occur in dynamic tasks and are a reason to check the reference, not to clip.

Level 5 — Does it fatigue?#

Question: Does the muscle show signs of local fatigue during a sustained effort?

Parameters: in a static, submaximal contraction at constant load and fixed joint angle, two things happen over time (Konrad, p. 50): the amplitude rises (additional motor units are recruited) and the mean and median frequency of the power spectrum fall (conduction velocity slows; the spectrum shifts left). The slope of median frequency over time — with its intercept — is the classic muscle fatigue index (De Luca). It appears before force actually declines.

Rules: the spectral method needs stationarity — a constant, static contraction. In dynamic tasks (strength training) use the amplitude rise instead. Watch for the opposite pattern (frequency up, amplitude down), which Konrad attributes to load shifting to synergists or reduced antagonist co-activation. Applications: back-endurance tests (multifidus and erector spinae in low-back pain), evaluating training effects.

Level 6 — Coordination#

Question: Do the muscles around a joint or along a chain work together appropriately?

Parameters: all of the above, but on at least two channels and with explicit criteria. Konrad's examples of "good" coordination: symmetrical innervation of synergists (the vasti), synchronised firing order within a chain, feed-forward activation of stabilisers, and antagonist co-innervation that is low and late. Co-activation is quantified from the overlap of agonist and antagonist envelopes; ratios such as VMO/VL or upper/lower trapezius are the clinical shorthand.

Rules: define the criterion before recording; record synergists and antagonists together; watch for crosstalk masquerading as co-activation (a channel that mirrors its neighbour perfectly is suspicious).

The map#