Ultrasound shear wave elastograms longitudinal scaning, M.vastus Lat. & M.gastrocnemius med.

Muscle stiffness was estimated using an Aixplorer Ultimate system (V.12, Supersonic Imagine, Aix-en-Provence, France) equipped with a linear transducer (SL15-4) with a 5 cm footprint. The system was configured for shear wave elastography, MSK preset with 90% gain, depth 2,0 - 2,5 cm, focus zone centered, sampling frequency 1.2 - 2.0 Hz, dynamic range 72 dB, a maximal measurement range of <10 m/s, low persistence, a standard setting between resolution and penetration, and 50% opacity.

Individuals with knee OA were asked to point out and rank their two most painful muscles. The most painful muscle was then scanned using B-mode ultrasound to determine and mark the direction of the muscle fibers, a longitudinal and a transverse scanning location. The locations were chosen on top of the muscle belly, approximately in accordance with the SENIAM recommendations (Hermens et al., 2000). Adhesive foam tape was used to mark and frame the transducer locations to ensure consistency for repeated measurements (Dieterich et al., 2020). To standardize the potential influence of previous activity (Proske and Gandevia, 2012), the prepared participants walked for 5 min at comfortable pace on a treadmill. Data collection started with participants positioned supine or prone on a plinth, depending on their most painful muscle. The ultrasound probe was hosted in a custom-cut foam block, covered with sufficient ultrasound gel, and secured to the leg with hook-and-loop straps. The elastogram was set to a maximal width of 3 cm and a depth of 1.2 - 1.5 cm, positioned just beneath the superficial fascia of the muscle. SWV of the most painful muscle was recorded at four different conditions, (i) baseline, i.e. resting state, (iii) double-leg stance and (iv) single-leg stance. Measurements were taken in longitudinal and in transverse transducer orientation. Trials were recorded as 30-second video clips except single-leg stance (20 seconds) to prevent exacerbation of pain. Dependent on recording duration and depth, each clip included 24-54 elastograms (images). All conditions except baseline were repeated three times. After recording SWV in the most painful muscle, measurements were taken from the second most painful muscle only during the more challenging tasks double-leg and single-leg stance. The same muscles considered as the first and second most painful muscles by a participant of the OA group were measured in a sex- and age matched participant from the old control group and a sex-matched participant from the young control group.