Acute Lateral Ankle Sprain Impairs Function and Strength Without Altering Muscle or Tendon Stiffness: A Controlled Observational Study

Abstract

Introduction: Acute lateral ankle sprain (LAS) frequently results in persistent functional limitations. Understanding changes in calf muscle and Achilles tendon (AT) stiffness after LAS may shed light on mechanisms underlying impaired function.

Objective: To investigate the effects of acute LAS on the mechanical properties of the calf muscles and the Achilles tendon, ankle function, pain, edema, and strength.

Methods: This controlled observational study was conducted from August 2023 to January 2025. Fourteen participants with acute LAS and 14 healthy controls were evaluated twice, 6 weeks apart. Shear wave elastography (SWE) assessed the stiffness of the triceps surae and AT. Ankle function, pain, and edema were evaluated using the Foot and Ankle Outcome Score, Visual Analog Scale, and figure-of-eight method. Plantar flexion strength was measured via isometric dynamometry.

Results: No significant differences in stiffness were found between or within groups (soleus: p = 0.932; MG: p = 0.760; LG: p = 0.800; AT: p = 0.070), although a time effect (p = 0.005, η² = 0.269) indicated a general increase in AT stiffness over time (MD = -0.72, p = 0.05, d = 2.86). At baseline, the LAS group exhibited reduced ankle function (MD = 3.43, p < 0.001, d = 2.20), increased pain (MD = 1.88, p < 0.001, d = 1.86), and greater edema (MD = -51.27, p < 0.001, d = -3.58). Over time, improvements were noted in function (MD = -37.04, p < 0.001, d = 2.27), pain (MD = 2.66, p < 0.001, d = -1.31), and edema (MD = 1.07, p = 0.014, d = -0.95), but ankle function remained lower in the LAS group at follow-up (MD = -14.17, p < 0.001, d = -1.79). For plantar flexion strength, no group × time interaction was found (p = 0.745), but a group effect indicated lower peak torque in the LAS group (MD = -32.05, p = 0.012, d = -3.82). A time effect (p < 0.001, η² = 0.622) showed increased torque across both groups (MD = -18.74, p < 0.001, d = 3.07).

Conclusion: LAS reduces ankle function and leads to pain and edema but does not induce notable changes in calf muscle or AT stiffness within 6 weeks.

Keywords: Achilles tendon; ankle sprains; elastography; functional performance; muscle strength.

FIGURE 1

STROBE flowchart. STROBE, Strengthening the Reporting of Observational Studies in Epidemiology.

FIGURE 2

Data collection protocol. Participants underwent three assessments: (1) Clinical Assessment, including demographics, injury history, functional outcomes, edema, and pain; (2) Tissue Stiffness Assessment, using shear wave elastography (SWE) on the soleus (SO), medial gastrocnemius (MG), lateral gastrocnemius (LG), and Achilles tendon (AT), where red indicates more stiffness and blue less stiffness; and (3) Strength Assessment, measuring maximal voluntary isometric contraction (MVIC) of the plantar flexors with an isometric dynamometer. Assessments were conducted at baseline and after 6 weeks. (A) Patient positioning for SW Participants lay prone with the foot relaxed and hanging off the table. (B) Anatomical landmarks: MG and LG at 25% of tibial length (TL), SO at 70% TL, and AT in the free tendon region. TL was measured from the medial tibial plateau to the distal medial malleolus. Muscle stiffness was calculated from 30 regions of interest (ROIs) per scan, averaged across three scans. Due to equipment limits, scans used 20 ROIs (C1) and 10 additional ROIs (C2). Achilles tendon stiffness was measured using 9 ROIs per scan (C3), with three scans averaged. To ensure unbiased analysis, color elastograms were removed before ROI selection, preventing qualitative bias in stiffness interpretation.