Background/Objectives: The aim of this study was to compare tibial acceleration and acceleration-based indicators associated with tibial loading during running under full body weight (BW) conditions and with body weight support (80%, 60%, and 40% BW) on a microgravity treadmill, and to examine the effect of treadmill surface by comparison with a conventional treadmill. Methods: Twenty-six healthy, physically active adults (age 18–40 years) completed running trials at a constant speed of 8 km/h. Tibial acceleration was recorded using inertial measurement units. Results: Cadence decreased significantly with increasing levels of body weight support, while no differences were observed between a conventional treadmill running and microgravity treadmill running at 100% BW. Axial acceleration parameters demonstrated a consistent reduction in acceleration magnitude associated with progressive unloading. The magnitude of axial acceleration (MAA), as well as peak positive (PPAA) and negative (PNAA) axial accelerations, decreased significantly, with the largest reductions observed at 60% and 40% BW. Temporal parameters showed a different pattern: time to peak positive axial acceleration increased with unloading, indicating delayed peak loading. The duration of positive axial acceleration exhibited a non-linear response, increasing at moderate unloading levels (80% and 60% BW) and decreasing at 40% BW. In contrast, the duration of negative axial acceleration did not change significantly, although overall contact time increased with unloading. Conclusions: The findings of this study indicate that progressive body weight unloading during running on a microgravity treadmill leads to a significant reduction in impact-related tibial acceleration indicators associated with mechanical loading exposure, particularly in parameters describing the amplitude and dynamics of axial accelerations. Running at 100% BW on both treadmill systems demonstrated overall biomechanical similarity; however, several side-specific differences in tibial acceleration parameters suggest that treadmill surface and system characteristics may exert a subtle influence on local acceleration measurements. Moderate unloading (~80% BW) may provide an optimal balance between load reduction and preservation of natural running mechanics.

Inter-limb differences may be observed after anterior cruciate ligament reconstruction (ACLR), even after return to professional sport. This study examined whether anterior–posterior (AP) and medial–lateral (ML) ground reaction force impulses provide information complementary to vertical metrics during bilateral and unilateral jump-landing tasks in professional football players after ACLR, and whether inter-limb asymmetry profiles vary according to task type and movement phase. Twelve male professional football players assessed 12–40 months after primary ACL reconstruction were initially enrolled. One player was excluded before analysis because of insufficient signal quality, leaving 11 players in the final analytical sample. They completed bilateral and unilateral countermovement jumps, drop jumps, and a multi-rebound jump on three-dimensional force platforms. Vertical (V), AP, and ML impulses were analyzed during braking and propulsion together with jump height, reactive strength index, contact time, power, and stiffness. No operated-versus-non-operated difference remained significant after correction for multiple comparisons. Under the prespecified exploratory criteria, cross-direction discordance was observed in 73% of players and occurred more frequently during unilateral than bilateral assessments. AP or ML was the direction of greatest asymmetry in 89% of pooled athlete–task–phase observations; these repeated observations were interpreted descriptively. Vertical asymmetry was not consistently associated with AP or ML asymmetry, and comparable jump performance could coexist with directional force differences. Unilateral tasks and multiplanar analysis therefore provided complementary information, but the clinical relevance of these findings requires validation in larger controlled studies.

Background/Objectives: Isokinetic testing is widely used after anterior cruciate ligament reconstruction (ACLR), but isolated knee strength may not reflect function during braking, re-acceleration, and stabilization. This study examined relationships between isokinetic strength and vertical and multidirectional performance in professional football players after ACLR and compared asymmetry across functional domains. Methods: Eleven male professional players, 12–40 months post-ACLR and back in unrestricted training and competition, completed isokinetic concentric knee extensor and flexor strength testing, single-leg countermovement and drop jumps, and unilateral and bilateral forward and lateral jumps on 3D force-plates. Limb differences, strength–performance associations, bilateral contributions, and domain-specific asymmetry were assessed. Results: No operated–non-operated limb difference remained significant after Holm correction. Knee extensor strength was positively associated with drop-jump reactive strength index (ρ = 0.612), countermovement-jump height (ρ = 0.491), anterior–posterior braking impulse during forward hop-in (ρ = 0.382), and propulsive impulse during forward hop-out (ρ = 0.442); however, none of the eight prespecified strength–performance correlations was statistically significant either before or after Holm adjustment. Median asymmetry differed across domains, reaching 20.9% for temporal control and stabilization and 14.7% for directional braking, compared with 9.9% for strength and 6.2% for vertical performance. Fifty percent of players had a median multidirectional-task asymmetry ≥20%. The operated limb contributed 49.9–53.0% of the total output during bilateral tasks. Conclusions: The absence of significant group-level differences in isokinetic strength and vertical jump outcomes did not exclude asymmetries in directional braking and stabilization. Isokinetic, vertical, and multidirectional assessments provide complementary, non-interchangeable information after ACLR. Multidirectional testing may therefore provide additional information on movement strategies that is not captured by strength and vertical jump tests alone.