Contralateral Thigh Ultrasound After Hip Fracture: Reserve, Rehabilitation Exposure, or Limb Function?

We read with great interest the study by Masuda et al. [1], who reported, in a prospective cohort of older adults undergoing hip-fracture surgery, that ultrasound-measured anterior thigh muscle thickness on the nonaffected side, but not on the affected side, was independently associated with lower Barthel Index scores at discharge. This clinically relevant finding suggests that bedside ultrasonography may have a role in early functional risk stratification. We would like to offer several comments to further contextualize these results and to strengthen their clinical applicability. First, the association may partly reflect premorbid reserve and early rehabilitation exposure rather than contralateral muscle size alone. The model adjusted for walking-aid use, cognition, grip strength, nutritional intake, and surgical delay, but did not fully capture prefracture activities of daily living, instrumental activities of daily living, baseline Barthel Index, or early postoperative functional recovery. This distinction is important because prospective evidence suggests that both prefracture functional status and Barthel Index on postoperative day 5 independently predict later instrumental activities of daily living after hip fracture [2]. A recent systematic review also found that earlier mobilization after hip-fracture surgery was associated with better patient-reported outcomes, including mobility and daily activity measures, although definitions of early mobilization and reporting of weight-bearing status varied considerably [3]. Future analyses should therefore incorporate prefracture function, early postoperative Barthel Index, time to first mobilization, and rehabilitation exposure, or model functional gain relative to baseline rather than discharge function alone. Second, the nonaffected limb may not be functionally unaffected. Contralateral limb performance after hip fracture is shaped by fracture pattern, fixation strategy, weight-bearing prescription, pain control, and adjacent-joint symptoms. These factors determine whether preserved muscle morphology can be translated into standing, transfers, and gait. Evidence that activity-related postoperative pain and knee extensor strength are associated with recovery of independent walking after hip-fracture surgery further supports the need to assess functional performance, not morphology alone [4]. Without information on weight-bearing status, analgesic adequacy, lower-limb pain, or contralateral knee and hip pathology, the observed association may partly reflect unmeasured constraints on lower-limb use rather than systemic muscle reserve alone. Third, the non-monotonic pattern across quartiles also limits clinical actionability. In the multivariable model, the lowest and third quartiles, but not the second quartile, were significantly associated with lower Barthel Index scores at discharge compared with the highest quartile. Although this finding may indicate a true nonlinear association, it may also reflect limited statistical power, residual confounding, or the loss of information caused by categorizing a continuous ultrasound measure. Methodological guidance has cautioned against categorizing continuous predictors in regression models because doing so can reduce statistical power, obscure dose–response patterns, and generate unstable thresholds [5]. Future analyses could therefore model muscle thickness as a continuous variable, for example with restricted cubic splines, while also reporting internally validated thresholds that clinicians can interpret. A prediction approach that combines ultrasound findings with knee extensor strength, sit-to-stand performance, or early gait measures may ultimately be more useful for anticipating functional recovery. In summary, this study supports ultrasound-derived nonaffected-side thigh thickness as a promising early marker after hip fracture. Its translational value would be strengthened by future work that incorporates premorbid function, early rehabilitation exposure, weight-bearing strategy, pain, and lower-limb performance. Such work may help clarify whether reduced nonaffected-side thickness mainly reflects frailty, represents a modifiable rehabilitation target, or both. Jiaqi Cheng: writing – original draft. Yujia Lai: writing – review. All the authors have read and approved the final version of the manuscript. The authors have nothing to report. The authors have nothing to report. The authors have nothing to report. The authors have nothing to report. The authors declare no conflicts of interest. Data sharing not applicable to this article as no datasets were generated or analysed during the current study.