(Lido Vizzutti | The New York Times) A mountain biker rides a section of the Whitefish Trails west of Whitefish, Mont., north of Missoula, Oct. 3, 2019. The study suggests that e-mountain bikes might enable many riders, including newcomers, to maintain or gain fitness while transiting otherwise daunting hills, snowy trails, spins with spouses and daily commutes.