REF-256 · Heidary et al., 2026 · Sensors, 2026; · Paper

Variation of Lower-Limb Muscle Activation Asymmetry in Step Incremental and Constant-Power Pedaling Exercise

Blimited evidenceMeasured on recreational peopleWritten before October 2026, being re-checked

Participants

n=20 recreationally active participants. Bilateral surface EMG of seven lower-limb muscles on a stationary ergometer.

Protocol

Two tests to task failure on a stationary ergometer:

Step incremental cycling (step size and start load not given in the abstract)

Constant-power cycling (intensity not given in the abstract)

Asymmetry quantified with the Normalized Symmetry Index (NSI) — higher values = greater asymmetry between limbs.

Key Results

Condition	Muscle	NSI Start → End	Significance
Incremental	Vastus medialis	44% → 21%	p < 0.001
Incremental	Vastus lateralis	45% → 22%	p = 0.002
Incremental	Rectus femoris	54% → 24%	p < 0.001
Incremental	Biceps femoris	52% → 29%	p = 0.003
Incremental	Tibialis anterior, soleus, gastrocnemius medialis	no significant change	—
Constant-power	All muscles	12–30% average, unchanged over time	all p > 0.05

Authors' reading: surface EMG is sensitive to workload-dependent changes in activation asymmetry, and higher cycling intensities, compared with lower ones, may promote more balanced engagement of the primary cycling muscles. The constant-power NSI values (12-30%) they describe as "consistent bilateral activation".

Mechanism

Inference, not tested: as intensity climbs toward failure, both legs are forced to contribute near-maximally, so the between-leg difference in activation shrinks. At a constant submaximal power there is no rising demand, so whatever asymmetry exists at the start persists. The abstract states the observation and the intensity hypothesis, not the mechanism.

Practical Application

n=20 recreationally active people, bilateral EMG of seven muscles: during a step-incremental ride to failure, left-right activation asymmetry (NSI) roughly halved in the quadriceps and hamstrings (vastus medialis 44→21%, vastus lateralis 45→22%, rectus femoris 54→24%, biceps femoris 52→29%; p ≤ 0.003), while during constant-power cycling to failure NSI stayed flat at 12-30% in every muscle — rising intensity evens out how the two legs work; steady power leaves the pattern as it is.

Inference, not tested: constant-power riding (including ERG mode, which the study did not test) is unlikely to correct an existing left-right imbalance on its own; ramps and ascending efforts recruit both legs more evenly.

Inference, not tested: a rider who trains mostly at steady power could periodically check bilateral balance (dual-sided power meter, or single-leg work) — the study measured EMG asymmetry in one session, not whether any of this matters for performance or injury.

The participants were recreationally active, not trained cyclists — trained riders may start with smaller asymmetry.

Limitations

Recreationally active, not trained cyclists — asymmetry patterns may differ in experienced riders

Constant-power intensity and incremental step size not given in the abstract

No direct ERG mode comparison — constant power on an ergometer used as a proxy

Acute measurement — no chronic adaptation data; no performance or injury outcome

How this entry was checked and changed

Content correction 2026-09-27

Protocol details "+20 W/min" and "70% of individual peak power" -> not in the abstract; removed (abstract gives neither).

Tier 2-3 (Trained/Well-trained) -> Tier 1: participants were recreationally active.

Lead Practical bullet was an ERG inference ("constant power / ERG mode preserves asymmetries") -> now the study's finding with the abstract's NSI values; ERG and corrective-work bullets labelled as inference. Added the abstract's null result for tibialis anterior, soleus and gastrocnemius medialis.