Objective To investigate the diagnostic value of skeletal muscle and diaphragm muscle in the extremities， as assessed by bedside ultrasound， for ICU?acquired weakness in elderly patients. Methods A total of 47 elderly patients with severe illness， admitted to the intensive care unit （ICU） of the Second People's Hospital of Yuhang District from March 2023 to April 2024， were included in this study. The average age of the patients was 74.51 years. Medical Research Council （MRC） scores were assessed twice within a 24?hour interval. Among them， when MRC score < 48， 20 cases were classified into the ICU?AW group， while the remaining 27 cases formed the non?ICU?AW group with MRC score ≥ 48. Ultrasound measurements for lower extremity skeletal muscle thickness and diaphragm parameters were conducted on the first day， third day， and seventh day after ICU admission. The rate of shrinkage in upper and lower extremity skeletal muscle thickness at different time points was calculated. Differences in ultrasound parameters between the two groups were compared， and ROC curve analysis was performed to evaluate the diagnostic value of upper and lower extremity skeletal muscle as well as diaphragm parameters for ICU?AW. Results Significant differences were observed in gender， age， BMI， APACHEⅡ score， SOFA score， number of mechanical ventilation cases， and number of sepsis cases between the two groups （P < 0.05）. Compared to the non?ICU?AW group， the ICU?AW group exhibited higher rates of atrophy in quadriceps femoris （RF） and total quadriceps femoris （QF） on day 3 and a lower degree of diaphragm displacement （DE） （P < 0.05）. On day 7， the ICU?AW group showed higher rates of atrophy in quadriceps femoris ［rectus femoris （RF）， intermedius femoris （VI）， and total quadriceps femoris （QF）］ along with lower diaphragm parameters ［diaphragm displacement degree （DE） and diaphragm thickness fraction（DTF）］ （P < 0.05）. According to ROC curve analysis， the critical value of ICU?AW predicted by ΔRF?MLT atrophy rate on day 7 was 9.9%， AUC was 0.743， sensitivity was 75.0% and specificity was 77.8%. On day 7， the critical value of ICU?AW was 5.69%， AUC was 0.828， sensitivity was 80.0%， and specificity was 77.8%. On day 7， the critical value of ICU?AW was 16.96%， AUC was 0.835， sensitivity was 60.0%， and specificity was 88.9%. On day 7， DE predicted that the critical value of ICU?AW was 1.67 cm， AUC was 0.818， sensitivity was 75.0%， and specificity was 85.2%. On day 7， DTF predicted that the critical value of ICU?AW was 33.96%， AUC was 0.889， sensitivity was 80.0%， and specificity was 85.2%. On day 7， the AUC of ICU?AW was 0.976， the sensitivity was 80.0%， and the specificity was 100%. Conclusion The quadriceps atrophy rate and diaphragm parameters measured by bedside ultrasound can identify and diagnose ICU?AW as early as possible.