Trail: The characteristic soft, uneven surface and rolling terrain challenge the muscles of the lower body more than a firm, flat run - so they have to work harder to maintain balance.
Road running: Due to the even surface, road running isn't quite as effective as trail, although it still boosts strength and endurance in all the major muscles of the lower body.
T: Would be more demanding if you could maintain the same pace. A study of elite orienteers (who run only on rough terrain) found their anaerobic performance to be lower than that of road runners.
RR: Elite roadrunners display among the highest aerobic capacity levels of any athletes - and the benefits are also reaped at lower levels.
T: For most of the UK population, rails, forests, hills and fells are less easy to find than roads and pavements. So, unless you live in the country, it's an exercise you have to plan for.
RR: You can run any time, anywhere. You're not restricted by daylight hours or by the geography of where you live - and the weather doesn't really matter, either.
T: Exercising in natural surroundings is particularly beneficial. The so-called "biophilia effect" relates to our innate desire to be connected to nature and its ability to lift us psychologically.
RR: Not quite as mood-enhancing as the countryside, but will still reduce anxiety - an Australian study found that it was as effective at lifting mood as meditation.
T: Running on a constantly changing terrain hones "proprioception", our sense of balance and body awareness, and develops strength and responsiveness in muscles and joints.
RR: Entails repetitive movement in a single direction, so is less effective at building core stability and balance required on less predictable surfaces.