Fatigue Induced by Walking Uphill and Downhill Similarly Disrupts Postural Balance but Their Disruptive Factors Differ

Background/Objectives: Walking-induced fatigue disrupts postural balance, but the differentiated effects of walking uphill and downhill remain unclear. The aim was to compare the impact of two walking sequences, either uphill (+10%) or downhill (−20%), with an identical number of steps (7000 steps) on a treadmill at 5.5 km·h−1 on postural balance. Methods: Nineteen healthy young participants performed the two walking sequence sessions (56 and 57 min) eight days apart. Eyes-closed bipedal postural balance (in three randomized conditions: an unmanipulated condition, a tendon vibration manipulation condition—TV—and a galvanic vestibular stimulation manipulation condition—GVS), maximal voluntary contraction and central activation ratio were assessed before (PRE), immediately after (POST) and 20 min after (POST20) each walking sequence. Results: Walking uphill and walking downhill sequences generated similar muscular and central fatigue in the POST and POST20 conditions. In the unmanipulated postural condition, postural balance was disrupted after both walking sequences in the POST condition, with no difference between walking downhill and walking uphill. In the manipulated postural conditions, postural balance was modified by the walking sequences. It was disrupted in the presence of GVS in the POST condition with no difference between walking downhill and uphill, whereas it was not disrupted in the presence of TV and was even improved after walking uphill. Conclusions: Although the postural alteration was broadly similar between the two walking sequences, their disruptive factors would differ at the muscular, metabolic and sensory levels.