Motor-unit behaviour of abductor hallucis and medial gastrocnemius during concentric, eccentric and isometric heel-rise conditions

Introduction: Heel-rise tasks require coordinated control of extrinsic plantar flexors and intrinsic foot muscles. Traditional surface electromyography provides useful information on muscle activation, but amplitude-based measures do not fully describe how individual motor units are modulated during functional movement. Surface electromyography decomposition enables analysis of motor-unit behaviour during both isometric and cyclic dynamic contractions (De Luca et al., 2006, 2015). Previous studies have shown that motor-unit behaviour can differ between concentric, eccentric and isometric contractions (Kallio et al., 2013; Oliveira & Negro, 2021), and recent work has applied this approach to dynamic closed-chain tasks such as squatting (Orantes-Gonzalez et al., 2025). However, little is known about motor-unit behaviour of foot-related muscles during heel-rise tasks, particularly when considering an intrinsic foot muscle, such as abductor hallucis, and an extrinsic plantar flexor, such as medial gastrocnemius. Therefore, the aim of this study was to compare mean motor-unit firing rates across concentric, eccentric and isometric heel-rise conditions, and peak motor unit amplitude across dynamic and isometric conditions in abductor hallucis and medial gastrocnemius.Methods: Thirty-four healthy young adults participated in this study (26 females, 8 males). Four-channel surface EMG sensors (Trigno Galileo, Delsys Inc., Boston, USA) were attached to the skin using hypoallergenic double-sided tape over the abductor hallucis and gastrocnemius medialis muscles of both the dominant and non-dominant lower limbs. Decomposed surface electromyographic recordings were obtained from both muscles during dynamic and isometric heel-rise tasks. In the dynamic condition, participants performed 15 consecutive heel raises guided by a metronome at a rate of 15 heel raises per minute. Dynamic heel raises were segmented into concentric and eccentric phases using angular velocity data. In the isometric condition, participants performed and maintained a heel-rise position for 30 s; angular velocity data were used to identify the stable static holding phase, and motor-unit firing rates were calculated from this selected interval. Mean motor-unit firing rate was calculated for the concentric, eccentric and isometric conditions, whereas peak motor-unit amplitude was analysed for the dynamic and isometric conditions for each muscle. Task-condition effects on mean firing rate were analysed within each muscle using linear mixed-effects models, with task condition as a fixed effect and participant as a random effect. Pairwise comparisons were used to examine differences between concentric, eccentric and isometric conditions. The Wilcoxon signed-rank test was used to compare peak amplitude between dynamic and isometric conditions. Statistical significance was set at p < 0.05.Results: Within-muscle analyses showed significant differences in motor-unit firing rate between conditions in both muscles. In abductor hallucis, pairwise comparison showed no significant difference (p = 0.427) between the concentric (12.33 ± 3.59 pps) and eccentric (12.71 ± 3.42 pps) phases, however both dynamic phases showed significantly higher firing rates (p < 0.001) than the isometric condition (10.01 ± 3.30 pps). Peak motor-unit amplitudes did not differ significantly (p = 0.256) between dynamic and isometric conditions in abductor hallucis, with median values of 1.9 × 10⁻⁴ V (IQR 1.2 × 10⁻⁴) and 1.8 × 10⁻⁴ V (IQR 1.6 × 10⁻⁴), respectively.In gastrocnemius medialis, pairwise comparison showed significantly different firing rates between all conditions (p < 0.001). The highest firing rate was observed in the concentric phase (14.19 ± 2.23 pps), followed by the eccentric phase (13.16 ± 2.43 pps), with the lowest firing rate observed in the isometric condition (9.83 ± 1.94 pps). Peak motor-unit amplitudes were significantly higher (p < 0.001) in the dynamic condition than in the isometric condition, with median values of 1.1 × 10⁻⁴ V (IQR 5.1 × 10⁻⁵) and 7.4 × 10⁻⁵ V (IQR 3.1 × 10⁻⁵), respectively.Conclusions: Our results showed different within-muscle patterns of motor-unit behaviour across heel-rise conditions. Gastrocnemius medialis showed a clear concentric > eccentric > isometric firing-rate pattern and higher peak motor-unit amplitudes during the dynamic condition. In contrast, abductor hallucis showed similar firing rates during the two dynamic phases and no dynamic–isometric difference in peak amplitudes. These findings may reflect different functional roles during heel-rise tasks, with gastrocnemius medialis more closely related to plantar-flexion torque production and abductor hallucis potentially contributing more consistently to foot stabilisation.Acknowledgements: This study was supported by the grants of Internal grant agency of Palacký University Olomouc IGA_FTK_2024_003 and IGA_FTK_2025_009.