Treadmill trials: is it time to unplug?

Treadmill training offers a number of potential advantages over outdoors running. No matter the weather or how dark outside, you can always train without the need to modify your sessions because of the conditions outside (eg snow, fog, ice, rain). This is particularly true if you live in a rural area with poor street lighting, or for women who have safety concerns while running alone in ‘less salubrious’ areas. Another factor that makes treadmill training appealing is that you can use gradient controls to generate hill/flat sessions – something that many runners can’t perform if they live in a flat/hilly area. The same is true for interval sessions where traffic lights and road junctions make it difficult (or even dangerous!) to execute a set of carefully structured intervals!

Aside from the training flexibility that treadmills offer, they also have a big plus when it comes to injury prevention. This is because with their well-cushioned running decks, modern treadmills can help lower the impact forces experienced by joints and muscles compared to tarmac roads or sidewalks. That matters because research has definitely determined that higher ground impact forces during footstrike are one of the root causes of running-related injuries such as knee, hip and ankle injuries, and stress fractures^(1-3). This injury prevention aspect of treadmill is particularly relevant to novice runners, or those who are carrying extra pounds, and who are more at risk of impact-related running injuries⁽⁴⁾.

Can treadmill replicate outdoors running?

Despite their benefits, some runners struggle with treadmill training sessions. Psychologically, running on the spot without any changing scenery to distract you is harder than running in the great outdoors, which is why many runners anecdotally report feeling bored on the treadmill compared to outside training. Of more concern perhaps is the fear that while delivering cardiovascular benefits, a runner’s gait on the treadmill is slightly different to outdoors running. In the longer term, this could reduce the specificity of training sessions, potentially putting runners at a disadvantage if they train on a treadmill then compete outdoors. But is this fear valid?

In a previous SPB article, Andrew Sheaff discussed this very topic. Andrew looked at research to get a better understanding of the extent to which running on a treadmill is similar to outdoors running – and if there are differences, whether runners are able to make any appropriate adjustments to ensure improved performance. Key to this was a recent collaborative study by Turkish and American researchers examining the differences between the two modes of running locomotion⁽⁵⁾. Thirteen recreationally active runners ran in a number of different conditions:

• Overground and on a treadmill.
• In both of the above, at different gradients - 0%, 1%, and 2%.
• All the above combinations were conducted two different speeds - 2.92 meters/second and 4.58 meters/second, running for 1 minute in all conditions.

During all the trials, the muscle activity of the knee flexors and knee extensors (hamstrings and quads), plantar flexors and dorsiflexors (key muscles controlling foot pointing/flexing motion) and muscles controlling lateral ankle stability was monitored and compared to determine any differences between treadmill and overground running. The key finding was that muscular activity was similar when running overground and on a treadmill across all conditions, but most similar for the thigh muscles at a gradient of 1% and for the ankle stabilizing muscles at a gradient of 2%. Put simply, if you perform training sessions on treadmills with a gradient set between 1%-2%, you can assured that you are still using your running muscles in a similar manner as when running outdoors!

Ditching the motor

Most of you reading this will probably assume that any decent treadmill will be motor-driven and therefore mains-powered. However, over the past few years, a number of non-motorized treadmills have appeared on the market. On a non-motorised treadmill, only the runner drives the belt – ie there’s no motor moving the belt under the runner’s feet. Once seen as the ‘poor man’s treadmill’, non-motorised treadmills are making something of a comeback, especially those with curved running decks, which many runners report helps to produce a more natural ‘feel’ than a motorized treadmill, being closer to normal outdoors running. But is there any research to support this claim?

Compared to a motorized treadmill, where belt speed is controlled by an external motor, a non-motorized treadmill is participant driven, which means the runner using it can pace themselves more naturally by be able to initiate rapid acceleration and deceleration, and vary step-to-step gait, just as would be the case in overground running. And studies looking into the differences between locomotion on a motorized vs. non-motorized treadmill have indeed concluded that a non-motorized treadmill can indeed provide a closer experience (in terms of what the runner perceives) to overground locomotion^(6-8).

Cardiovascular benefits: motorized vs. non-motorized

But what about the physiological differences between using a motorized and a non-motorized treadmill? Do they deliver exactly the same benefits, and if not, how do they differ? For answers, we can turn to research by Aussie scientists, which suggested that non-motorized treadmills are certainly worthy of consideration⁽⁹⁾. In this study, fourteen fit, trained male and female runners completed of five x 6-minute bouts of running at progressively higher speeds (females 9-15kmh, males 10.5-16.5kmh), each separated by six minutes of rest. These sessions were completed on three separate occasions using:

• A conventional motorised treadmill set at 1% incline (which as we have seen provides the closest physiological equivalence to running outdoors on flat terrain).
• A curved non-motorised treadmill (Woodway Curve 3).
• Normal overground running on the flat.

For each of the trials, oxygen consumption (VO2max) and heart rates were monitored during the last 2 minutes of each bout and the results compared.

The key finding was that for any given speed, running on the non-motorised treadmill was significantly more physiologically demanding than running on the motorized deck and also when overground running. Averaged across all the running speeds, the runners’ levels of oxygen consumption was around 22% higher when running on the non-motorised treadmill, while heart rates were around 25% higher for any given speed. Because the belt on a non-motorized treadmill is runner driven and carries some resistance, the effect of higher oxygen consumption for any given speed was more noticeable in lighter runners compared to heavier runners. This is because all other things being equal, lighter runners require less oxygen consumption to maintain any given running speed; therefore, the additional energy needed to overcome the fixed belt resistance was proportionately higher for them compared to heavier runners.