Impaired force steadiness is associated with changes in force frequency composition in subacute stroke

• Decreased median frequency and increased relative peak power early after stroke.
• Changes in selected frequency parameters found in both paretic and non-paretic legs.
• Power spectra inadequately modulated with increasing contraction level after stroke.
• Force variability and power spectra formed a non-linear relation in the paretic leg.
• Less broadband force output may account for impaired force steadiness after stroke.

We tested the hypothesis that impaired force steadiness early after stroke is associated with changes in frequency composition of the force signal during constant-force task. The power spectra and the relationship between power spectra and force variability during isometric knee extension (10%, 20%, 30%, and 50% of peak torque for 10 s) were studied in the paretic and non-paretic legs of 34 stroke patients (64 ± 14 years, 8–25 days post-injury) and the dominant leg of 20 controls (62 ± 10 years). Power spectrum analysis of the force signal included the median frequency, peak power frequency, relative peak power, and relative power in 0–3, 4–6, and 8–12 Hz bands. Force variability, quantified by coefficient of variation (CV), was increased in patients at 3 of the 4 contraction levels (P ⩽ 0.001). Median frequency across all force levels was decreased and the relative peak power was increased in the paretic and non-paretic legs compared to controls (P ⩽ 0.001). The relative power was increased in 0–3 Hz band and decreased in both 4–6 and 8–12 Hz bands in the paretic leg only (P ⩽ 0.001). Progressively stronger contractions brought about a significant decrease in relative power in the 0–3 Hz band and increase in 8–12 Hz band in controls but not in stroke subjects. The hypothesis was confirmed by significant non-linear correlations between CV and each relative spectral power found in the paretic leg at most contraction levels (0.22 ⩽ R2 ⩽ 0.72, P ⩽ 0.0004) and in the non-paretic leg at 10% only (0.35 ⩽ R2 ⩽ 0.52, P ⩽ 0.0002), but not in controls. Fugl-Meyer lower extremity motor and sensory scores were not related to the frequency measures in stroke subjects (P > 0.05). Limited modulation of frequency spectra and the emergence of non-linear relation between power spectra and force variability suggest that less broadband force output may account in part for impaired force steadiness in paretic and non-paretic legs early after stroke.