Soleus muscle architecture tied to walking speed recovery after ankle fracture surgery in cross-sectional study

KUALA LUMPUR, 13 OCTOBER 2026 —

*Corresponding author: Hirotaka Mutsuzaki, Center for Medical Science, Ibaraki Prefectural University of Health Sciences, Ami-machi, Inashiki-gun, Ibaraki, Japan.

Reduced ankle plantar flexion strength has been implicated; however, the role of soleus muscle architecture in walking speed reserve (WSR), an indicator of mobility in daily life, remains unclear. This study aimed to investigate the association between ultrasound-derived architectural properties of the soleus and WSR in patients after ankle fracture surgery. This cross-sectional observational study included patients who underwent open reduction and internal fixation of ankle fractures and participated in a standardized postoperative rehabilitation program.

Three months after surgery, soleus muscle architecture, including muscle thickness (MT), pennation angle, shear modulus, and echo intensity (EI), was measured using musculoskeletal ultrasonography. Ankle range of motion, plantar flexion strength, WSR, and pain during walking were also assessed. Multiple regression analyses were conducted to examine factors associated with WSR. Forty-five participants (mean age, 47.0 ± 21.4 years) were included.

Increased fat-corrected EI (β = −0.336, 95% CI, −0.572 to −0.100; p = 0.006), decreased physiological MT (β = 0.529, 95% CI, 0.293 to 0.764; p < 0.001), and ankle plantar flexion strength (β = 0.429, 95% CI, 0.173 to 0.686; p = 0.002) were independently associated with decreased WSR. The multiple regression model had an adjusted R 2 of 0.525.

Physiological MT and corrected EI of the soleus were independently associated with WSR after ankle fracture surgery. These findings suggest that the structural properties of the soleus are associated with WSR in the early postoperative period. Because of the cross-sectional design and assessment at a single postoperative time point, causal relationships cannot be determined.

Ankle fracture (AF) can cause long-term gait dysfunction, 1 , 2 which may persist at 2-year follow-up even after open reduction and internal fixation (ORIF) 2 , thus limiting patients’ activities of daily living and quality of life.

Reduced ankle dorsiflexion range of motion (ROM) and decreased plantar flexion strength are associated with impaired walking performance 3 , 4 , highlighting the functional importance of the ankle plantar flexor muscles for propulsion and endurance during walking. 5 , 6 However, the muscle architecture factors that influence walking function remain insufficiently understood.

Muscle structural characteristics that influence muscle strength include muscle thickness (MT), 7 echo intensity (EI), 7 , 8 pennation angle 9 , and shear modulus. 10 In this study, we focused on the soleus, which has the largest physiological cross-sectional area among the ankle plantar flexor muscles 11 and functions as an antigravity muscle with a pennate structure.

12 The soleus muscle function is more affected by walking ability than by age 13 ; therefore, unloading or immobilization after AF surgery may induce quantitative and qualitative changes in MT, EI, pennation angle, and shear modulus of the soleus. Walking speed reserve (WSR), the difference between maximal and self-selected walking speed, 14 is an important walking function indicator.

14 - 16 WSR reflects the capacity to increase speed when necessary and is associated with fall risk, 14 walking endurance, 15 and mortality. 16 Unlike self-selected walking speed, WSR reflects the additional walking capacity beyond usual gait and may therefore capture functional limitations that are not apparent during comfortable walking. After AF surgery, WSR may provide additional information on residual walking limitations that are not fully captured by self-selected walking speed.

Maximal walking speed requires greater muscular output than self-selected walking speed; thus, impairments in soleus muscle strength and quality may limit the ability to generate additional force needed for speed modulation, thereby affecting WSR. However, the relationships between soleus muscle characteristics and WSR remain unclear. This study aimed to evaluate both the quantitative and qualitative soleus muscle architecture using ultrasound imaging and to investigate its association with WSR in patients after AF surgery.

We hypothesized that MT, adjusted for pennation angle, and EI, adjusted for subcutaneous tissue thickness, are related to WSR. Clarifying these relationships could provide valuable insights into the factors affecting walking function after AF surgery and inform the development of targeted rehabilitation programs aimed at improving walking ability in these patients. This cross-sectional observational study was conducted between July 2022 and June 2025.

Patients with unimalleolar, bimalleolar, or trimalleolar ankle fractures who underwent ORIF and participated in postoperative rehabilitation at our institution were included. All patients underwent postoperative rehabilitation according to the rehabilitation protocol of our institution. The fixation method and implants were selected by the attending orthopedic surgeon according to the fracture characteristics. The postoperative rehabilitation programme included ankle range-of-motion exercises, progressive muscle strengthening, gait training, balance exercises, and functional activities.

The rehabilitation programme was progressed according to the patient's postoperative status, fracture healing, weight-bearing restrictions, and functional recovery. Although the general rehabilitation protocol was standardized at our institution, the timing and intensity of individual interventions could vary according to each patient's clinical condition. The assessment was performed 3 months after surgery.

Those who had multiple fractures, open fractures, postoperative complications such as deep infection and deep vein thrombosis, and a history of neurological or orthopedic diseases, refused to undergo postoperative measurements, or were visiting another hospital, were excluded. The participants’ age, sex, height, weight, body mass index, number of fractures, and Lauge–Hansen classification data were collected from their medical records.

The number of fractures was used as an indicator of fracture complexity and was categorized as unimalleolar (1), bimalleolar (2), or trimalleolar (3). All ultrasound examinations were conducted by a physiotherapist with 9 years of experience in musculoskeletal ultrasound using a 2- to 10-MHz linear transducer (Supersonic Imaging, Aix-en-Provence, France) to assess the soleus muscle architecture.

The same physiotherapist assessed the MT, EI, and pennation angle using B-mode and the shear modulus using shear wave elastography.

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