With all the bad weather we’ve been having recently, some of you have probably preferred the relative comfort of an indoor treadmill for your training. However, there are some differences between treadmill running and outdoor running that you may not have considered.
The physical energy requirements of running at a given speed on a treadmill is less than it is outdoors. This means that from a physiology perspective, it is easier to run on a treadmill. Why? Well, a couple of reasons. The movement of the treadmill belt under your feet actually assists your leg turnover, making it easier to run faster. The air resistance is also reduced when running indoors, and you don’t have to deal with adverse weather conditions.

For the purposes of running the half marathon, most of you will want to at least partially replicate the demands of outdoor running while on the treadmill. To do this, you can simply increase the gradient. Research has shown that treadmill running at a gradient of 1% raises the energetic cost of treadmill running, making it equal to that of outdoor running at the same speed*.
Of course, you can never totally reproduce outdoor running while indoors. Most treadmills, for example, cannot replicate running on a decline. You also run in a very linear pattern on a treadmill, thus activating fewer muscles than you would if you were weaving along a path or running on alternating surfaces.
There is no right or wrong in this. But, as you are ultimately preparing to run the half marathon outside rather than training purely for fitness, you will get greater benefit from running outside for at least some of your weekly training runs.
All the best with your training!

*Reference: Jones et al. J Sports Sci. 1996 Aug;14(4):321-7.

T.E.A.M (Temporary Emergency Accommodation Midlands), with support from Brendan Marshall
BSc, PhD (Sports Science and Health)