The phrase "challenging trail" is correct and usable in written English.
You could use it in a variety of contexts such as describing a hiking route or referring to an academic program. For example, "The MBA program at Stanford is a challenging trail that will stretch you to your limits."
With the helmet he felt protected enough to ride off-road on a challenging trail, in hindsight perhaps too safe.
The route covers a few miles of the Pennine Way – and it makes a shorter alternative to, or good training for, that longer and more challenging trail.
The most challenging trail is the GR20, taking about 14 days and offering much mountainous terrain as it stretches 200km diagonally across the island from Calenzana in the north to Conca in the south.
Trek the Trail of the Whispering Pines, or the more challenging Trail of the Molten Land or marvel at the cinder cone of Lava Butte (where Nasa carried out astronaut training prior to the moon landing).
For kids between 4 and 12 there are several age-appropriate programs, offering the younger ones instruction in the arena and the older ones more challenging trail rides.
Between the Baths and Devil's Bay, take the slippery and slightly challenging trail leading to the beach.
But there are plenty of challenging trails at Smuggler's Madonna Mountain and plenty of places to get lost off those trails, too.
"We'll see spectacular waterfalls, travel challenging trails and savor the beauty of hundreds of acres of California lilacs.
Consistently, the caspase-3 cleavage activities were significantly activated in A549 cells from animals challenged with TRAIL plus rotenone, meanwhile, this effect was attenuated by NAC.
As far as we know, this is first report showing that VPS34, LC3-II, and CLU are elevated in parallel when PC-12 cells are challenged with TRAIL or nystatin.
Although the lack of age-specific challenge trails for LPAI prevents a reliable evaluation of seasonal differences in susceptibility to AI viruses, we found age-related differences in disease processes for HPAI, indicating faster disease progression (S→I and E→I) in adult than young birds.
