Lower-extremity biomechanical differences between self-selected in-place running and 180-bpm ultra-slow jogging in middle-aged adults

Ultra-slow jogging (USJ) at a prescribed cadence of 180 steps per minute is increasingly recommended as a low-impact aerobic modality for middle-aged adults, yet its lower-extremity biomechanical profile relative to conventional self-selected-speed running has not been systematically characterized in this population. The purpose of this study was to compare lower-extremity biomechanical differences between self-selected in-place running and 180-bpm ultra-slow jogging in middle-aged adults across whole-cycle and gait-event analytical domains. Twenty-five middle-aged adults (13 men, 12 women; age 50.71 ± 5.48 years; BMI 26.03 ± 4.42 kg/m²) performed two in-place running conditions in a within-subject repeated-measures design: self-selected-speed running (SS-Run) and 180-bpm ultra-slow jogging (USJ-180). Three-dimensional kinematics and ground reaction forces were collected, and joint forces and moments were derived using inverse dynamics. Linear mixed-effects models (LMMs) were fitted to 86 outcomes (56 whole-cycle peaks; 30 gait-event angles), with Benjamini–Hochberg false discovery rate (BH-FDR) adjustment applied separately within each outcome family. After BH-FDR adjustment, 23 of 56 whole-cycle variables and 16 of 30 gait-event outcomes differed significantly between conditions. USJ-180 yielded a shorter cycle time (d = − 0.52) and higher cadence (178.44 vs. 172.35 steps/min, d = 0.46). SS-Run generated greater peak vertical compressive forces at the hip (1.199 vs. 1.123 N/BW), knee (1.328 vs. 1.244 N/BW), and ankle (0.192 vs. 0.164 N/BW; all p-BH < 0.001). USJ-180 exhibited a more flexed landing posture, with greater hip flexion (23.11° vs. 20.93°), knee flexion (44.97° vs. 41.83°), and ankle dorsiflexion (p-BH < 0.001) at foot strike, and greater hip and knee flexion at toe-off (hip: p-BH = 0.003; knee: p-BH = 0.023). USJ-180 also showed a more negative peak hip extension moment and a greater peak hip external rotation moment. In middle-aged adults, in-place self-selected running and 180-bpm ultra-slow jogging impose qualitatively distinct lower-extremity biomechanical demands. SS-Run produces systematically greater multi-axial compressive and propulsive joint loading, whereas USJ-180 elicits a flexed-hip, flexed-knee gait strategy with cadence-driven elevation of eccentric hip extensor demand. USJ-180 should not be regarded as a low-intensity version of conventional running, but as a biomechanically distinct modality requiring tailored exercise prescription in middle-aged populations.