Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2025 Jun 1.
doi: 10.1007/s00421-025-05824-z. Online ahead of print.

Is soleus intrinsic motor neuron excitability contributing to motor deficits in runners with Achilles tendinopathy?

Gabriel L Fernandes  1   2 , Lucas B R Orssatto  3 , Gabriel S Trajano  4
Affiliations

Is soleus intrinsic motor neuron excitability contributing to motor deficits in runners with Achilles tendinopathy?

Gabriel L Fernandes et al. Eur J Appl Physiol. .

Abstract

Objective: Soleus weakness is suggested to contribute to Achilles tendinopathy (AT) in runners. Since muscle force relies on the ability of motor units firing at high frequencies, and intrinsic motor neuron excitability contributes to firing rate modulation, soleus inhibition and hypoexcitability may contribute to AT soleus weakness. This study investigated soleus motor neuron excitability by comparing: (i) estimates of persistent inward currents (Δf and Δf/k), which is known to modulate excitability; (ii) brace height, which is related to neuromodulatory input onto motor neurons; (iii) attenuation slopes, which estimate the influence of inhibitory input onto the motor units; and (iv) and motor unit firing rates between runners with and without mid-portion AT.

Methods: Delta frequency absolute and normalized (ΔF and ΔF/k), brace height, attenuation slope, and peak firing rates were compared between runners with AT (n = 11) and without AT (n = 12). These variables were calculated from ramp triangular-shaped isometric plantar flexor contractions at 20% maximal torque. Soleus motor unit firing rates were assessed using high-density surface electromyography.

Results: No significant differences were found between groups in soleus ΔF (estimated mean difference: - 0.1 pps; 95% CI: - 1.3 to 1.0; p = 0.79), ΔF/k (0.2 pps; - 0.03 to 0.3; p = 0.09), brace height (- 3.9% rTri; - 8.3 to 0.4; p = 0.07), attenuation (0.04 pps/% torque; - 0.04 to 0.1; p = 0.27), or peak firing rates (- 0.4 pps; - 1.6 to 0.9; p = 0.55).

Conclusion: Soleus motor neuron excitability or firing rates may not contribute to plantar flexor weakness in AT. Future studies should investigate other neurophysiological mechanisms and gastrocnemius contributions to AT-related weakness.

Keywords: Achilles tendon; Motor unit; Neuromodulation; Persistent inward current; Running; Triceps surae.

PubMed Disclaimer

Conflict of interest statement

Declarations. Conflict of interest: The authors declare no conflict of interest with the present research.

References

    1. Bates D, Mächler M, Bolker BM, Walker SC (2015) Fitting linear mixed-effects models using lme4. J Stat Softw. https://doi.org/10.18637/jss.v067.i01 - DOI
    1. Beauchamp JA, Khurram OU, Dewald JPA, Heckman CJ, Beauchamp JA, Khurram OU, Dewald JPA, Heckman CJPG (2022) A computational approach for generating continuous estimates of motor unit discharge rates and visualizing population discharge characteristics. J Neural Eng. https://doi.org/10.1201/9781315141343 - DOI - PubMed - PMC
    1. Beauchamp JA, Pearcey GEP, Khurram OU et al (2023) A geometric approach to quantifying the neuromodulatory effects of persistent inward currents on individual motor unit discharge patterns. J Neural Eng 20(1):16034. https://doi.org/10.1088/1741-2552/acb1d7 - DOI
    1. Binder MD, Powers RK, Heckman CJ (2020) Nonlinear input-output functions of motoneurons. Physiology (Bethesda) 35(1):31–39. https://doi.org/10.1152/physiol.00026.2019 - DOI - PubMed
    1. Boccia G, Martinez-Valdes E, Negro F, Rainoldi A, Falla D (2019) Motor unit discharge rate and the estimated synaptic input to the vasti muscles is higher in open compared with closed kinetic chain exercise. J Appl Physiol 127(4):950–958. https://doi.org/10.1152/japplphysiol.00310.2019 - DOI - PubMed

LinkOut - more resources