BACKGROUND: Downhill and uphill running alter running kinematics, changing force distributions and muscle activities. While changes in the lower limbs have received more attention, research on the back and pelvis remains scarce. Understanding grade-specific changes in the trunk is crucial for developing injury prevention strategies and return-to-activity protocols after back injuries. PURPOSE: This study examined how running on six different gradients (ranging from -15% downhill to +15% uphill) affects back and pelvic posture, as well as muscle activity in trunk and lower limb muscles. METHODS: Twelve healthy recreational runners (6 women, 6 men) participated in a descriptive laboratory study. Kinematic variables (lumbar lordosis, hip drop, pelvic tilt, and trunk inclination) were assessed via 3D motion capture. Surface electromyography recorded muscle activity, normalized to maximum voluntary isometric contraction (MVIC), in selected trunk and lower limb muscles, including the medial gastrocnemius, semitendinosus, biceps femoris, gluteus maximus, gluteus medius, vastus lateralis, rectus abdominis, external oblique, and erector spinae. A linear mixed-effects model with random intercepts was used to compare each gradient to level running. Cohen's d was calculated to quantify effect sizes. RESULTS: Compared to level running lumbar lordosis was increased at -15% (mean difference [MD]: 1.2 ± 4.9 degrees, Cohen's d = 0.73, p-value = 0.007) and decreased at +15% (MD: -1 ± 3.6 degrees, d = 0.39, p = 0.290). Total hip drop decreased at -15% (MD: 14.2 ± 11 mm, d = 1.93, p < 0.001). Muscular activity of certain lower limb muscles was higher at steep downhill and uphill gradients. While the abdominals showed no consistent changes across gradients, activity of erector spinae was reduced at -10% and -5%. CONCLUSION: Downhill running increases lumbar lordosis, potentially elevating the risk of lumbar spine overload. When attempting to prevent and rehabilitate lower back injuries, a progression starting with uphill, followed by level and lastly by downhill running, may be advisable. Gradient-specific training should be considered due to the distinct neuromuscular demands across inclines. LEVEL OF EVIDENCE: 3b.