Abstract: Fattorini, L, Pittiglio, G, Federico, B, Pallicca, A, Bernardi, M, and Rodio, A. Workload comparison between hiking and indoor physical activity. J Strength Cond Res 26(10): 2883–2889, 2012—Walking is a physical activity able to maintain and improve aerobic fitness. This activity can easily be performed in all seasons both outdoors and indoors, but when it is performed in its natural environment, the use of specific equipment is required. In particular, it has been demonstrated that the use of trekking boots (TBs) induces a larger workload than those used indoors. Because an adequate fitness level is needed to practice hiking in safety, it is useful to know the energy demand of such an activity. This research aims at defining the metabolic engagement of hiking on natural paths with specific equipment at several speeds and comparing this with indoor ones (on a treadmill). This can thence be used to define the load that better reflects the one required to walk on natural paths. The walking energy cost (joules per kilogram per meter) at several speeds (0.28, 0.56, 0.84, 1.11, and 1.39 m·s−1)—on level natural terrain while wearing suitable footwear (TBs) and on a treadmill at various raising slopes (0, 1, 2, 3, 4%) while wearing running shoes—was measured in 14 healthy young men (age 23.9 ± 2.9 years, stature 1.75 ± 0.04 m, and body mass 72.9 ± 6.3 kg). A physiological evaluation of all the subjects was performed before energy cost measurements. The results showed that outdoors, the oxygen uptake was consistently less than the ventilatory threshold at all speeds tested and that a 3% slope on the treadmill best reflects the outdoor walking energy expenditure. These findings will prove useful to plan proper training for hiking activity or mixed (outdoors and indoors) training program.