Background <p>Muscle architecture, including thickness, pennation angle, and fascicle length, underpins function and adaptation, but its relationship with aerobic capacity in young adults remains unclear.</p> Objective <p>To compare lower extremity muscle architecture and aerobic capacity between regularly exercising and non-exercising young adults, and to examine associations between architectural parameters and maximal oxygen uptake(VO₂max).</p> Methods <p>Fifty-two healthy participants (18–35 years) were included:26 in the exercise group (EG) and 26 in the non-exercise group(NEG). The EG had been performing supervised exercise at the university fitness center for 6–18 months, at least three times per week, whereas the NEG had not engaged in any structured exercise for at least six months. Ultrasound imaging was used to assess muscle thickness, pennation angle, and fascicle length of the rectus femoris(RF), vastus intermedius(VI), vastus medialis(VM), vastus lateralis(VL), tibialis anterior(TA), hamstring, gastrocnemius medialis(GM), and gastrocnemius lateralis(GL). VO₂max, as an indicator of aerobic capacity, was assessed using open-circuit spirometry during a treadmill test.</p> Results <p>VO₂max was significantly higher in the EG(46.95 ± 6.23 mL·kg⁻¹·min⁻¹) than in the NEG(37.29 ± 5.07; <i>p</i> &lt; 0.001). Pennation angles of the VI (<i>p</i> = 0.002), VL (<i>p</i> = 0.003), and TA (<i>p</i> = 0.023) were greater in the EG. VO₂max correlated positively with muscle thickness (RF, VI, VL, TA), VI pennation angle, and TA fascicle length (<i>p</i> &lt; 0.05).</p> Conclusions <p>Regular exercise appears to promote architectural adaptations in lower-extremity muscles that may support improved aerobic capacity. Muscle architecture may therefore represent a meaningful physiological marker of training status in young adults.</p>