"From a calorie-burning point of view a constant 5km/h walk on a treadmill is not going to burn as many calories as a constant 5km/h walk outside in most circumstances because of the difference in conditions between the two environments," says Step Bite Step exercise physiologist Kim Allsopp.

"However, the opposite may occur if you slow down for any reason, or lose concentration. Whereas once you set the treadmill at a certain speed it does push you and you are going to walk at that speed or find yourself stepping off the back of it," Allsopp says.

Treadmills offer a push-off force that assists forward movement.

In the outdoors, runners and walkers lose those forward motion forces and have to work harder.

But Allsopp says the question of whether one option is better than the other can only be answered by the individual, their preferences and what they want to achieve.

"People who are purposely training for a competitive reason are better off to mix it up and go outdoors too," he says.

Allsopp says older people, in particular, benefit from using a treadmill because of the consistency and predictability.

"For anyone who doesn't feel safe walking outdoors, then the treadmill is a great alternative to doing nothing," Allsopp says.

The convenience and safety of treadmills is not only handy for the elderly, but also mums with babies who are looking to exercise without leaving the house. For those with hay fever allergies, walking or running indoors also restricts your exposure to seasonal pollens.

The treadmill is also good for those who can exercise only in the later hours. However, it's a more isolating exercise, and those who prefer working out in a social context may prefer the outdoors.

Allsopp says treadmills are also good for providing data feedback for training purposes, although GPS devices are able to match that information during outdoor runs.

Treadmill technology is improving all the time, and they are a lower impact exercise for those with injuries.

"Outdoors, even when you think you're on flat ground, the ground is never completely flat," he says.

"This means your foot, leg and core muscles are constantly making small adjustments to contend with the varying surfaces.

"These small muscle movements are important for your co-ordination, balance and ability to do everyday things."

Allsopp says the biomechanics - such as the runner's gait - are not varied by the treadmill as long as the able-bodied users do not use the handrails.

"The rails are at a fixed height which my be too high or low for you and will cause an altered gait for a large portion of treadmill users," he warns.

An arm swing when using the treadmill is necessary for balance, walking efficacy, co-ordination, fitness and posture, Allsopp says.