The short answer: Most of the time, running on a treadmill is the better option from a performance, biomechanics, and injury-prevention perspective.
Now, let's dive into this topic a little deeper!
Indoor tracks
This is the worst option if the track is shorter than an outdoor track. Running in tight circles leads to an imbalance in how the inside and outside legs are stressed during the run. This means that you are more likely to get overuse injuries in the overstressed tissues and you are more likely to develop side-to-side strength differences.
This study found that female runners developed side-to-side hip strength differences after running on an indoor track. They specifically looked at hip abductor strength differences, which is the 3rd most used muscle in long distance running and it’s important to have equal strength in these muscles to avoid hip and knee injuries!
This study found side-to-side strength differences in ankle muscle strength in runners who ran on an indoor track. In this study, runners developed more strength in their inverters (inside ankle muscles) on their inside limb side, while the outside limb developed greater strength in the evertors (outside ankle muscles). Just like with the hip strength study, having side-to-side strength differences in your ankles will lead to greater stability with some aspects of ankle movement, but instability in others. From an injury standpoint, if you already have “weak ankles” or chronic ankle instability, the extra stress of running on an indoor track might lead to injury vs. strength gains in the ankle muscles just discussed due to the ankle tissues being too weak to handle the extra stress. If tissues are stressed appropriately and given enough time to recover, they get stronger. If tissues are stressed beyond their limits and/or do not have enough time to recover, this leads to injury.
This study looked at track and field injuries and found higher injury rates during the indoor season compared to the outdoor season. Running on an indoor track might not be the only reason for the higher injury rate, but it is likely a contributor.
The main take away: If your run is longer than 20-30 minutes, then hit the treadmill. If you need to do a run that is longer than 20-30 minutes on an indoor track, switch directions every 30 minutes so that your tissues are stressed equally on both sides.
Running on a treadmill
First, I think it is unfortunate that the treadmill has such a bad reputation in the running community. Being viewed as a “dreadmill” seems to lead to runners believing that this is the worst option for running and that it will be an emotionally unpleasant experience. I have also noticed the runners who think that the treadmill is a terrible option will often opt to run on surfaces that lead to higher risks of injury and lower performance gains. The treadmill is a fantastic fitness tool, especially for those of us who deal with difficult weather and running surface conditions for 5+ months out of the year. I have spent countless hours running on the treadmill during the tough Minnesota winters and it has helped me continue to improve my running economy, running speed, and general fitness throughout the year. But that’s just my experience. Let’s dive into what the research says.
This systematic review combined the data of 33 studies that included 494 total runners that looked at the differences between treadmill running and overground running. The study concluded, “Overall, the findings of this review indicate that the biomechanics of MT (motorized treadmill) running are largely comparable to overground running, with most outcomes not being significantly different, and some outcomes being significantly different but of trivial magnitude.”
It is often believed that runners don’t have to work as hard when they run on the treadmill because the movement of the belt does some of the work for you. As of right now, the evidence for this is unclear with some studies saying that muscle activity is the same, others report more muscle activity, and others report less muscle activity. The differences were often concluded to be due to motor power differences between treadmills.
There are some minor differences in ankle angle, knee flexion, hip flexion, and stride frequency and length when comparing treadmill running to overground running. When taking differences like this in isolation, they are likely too small to be of practical relevance. For example, the study states that “ the ~4° lower peak hip flexion during MT running is smaller than the standard error of measurement with manual marker placement." This means that the degree of change is smaller than what is allowed simply from differences in marker placement on the subjects' bodies.
Another great point made by the researchers was that although multiple small differences are seen between overground and treadmill running, this is likely due to differences in surface stiffness. These biomechanical changes are normal and effective because it means that runners are adapting to the different surfaces they are running on. For example, if you run on soft trails, your running biomechanics will likely be slightly different than running on hard concrete. The change is not bad but rather a sign that your body is effectively adapting to the surface you are running on.
One last point to consider with biomechanical differences betwen treadmill and overground running is familiarity with running on the treadmill. Consistently across the studies, the runners who were more familiar with running on a treadmill had more consistencies between their treadmill and overground running form. The studies defined "familiarity" periods from just 30 seconds to 9 minutes! With that said, everyone is different and it might take you a little longer to get used to running on the treadmill.
We covered running form and biomechanics, so now let’s dive into if there is a difference in how hard your body works when you run on the treadmill vs. overground. Many runners believe that they need to run at a 1% incline on the treadmill to simulate the same amount of difficulty as running outside. Again, the belt movement doesn’t give you any sort of advantage so from that standpoint, running at an incline would make the run more difficult than running outside. What doesn’t happen while running on a treadmill is air resistance and this does seem to make a difference when running at faster paces. This study looked at oxygen utilization at a variety of speeds and inclines. The study demonstrated that running between an 8:03 and 9:12 pace outside lead to using the same amount of oxygen as running the same paces on a treadmill at 0% and 1%. Paces at 7:09 and faster outside used similar oxygen as running at 1% on the treadmill. This means that if you are running at an 8-minute pace or slower, it likely will not make a difference if you keep the treadmill at 0%, whereas paces of 7:09 and faster on the treadmill are best to be done at 1% incline in order to best simulate the amount of energy used when running outside. This study looked at blood glucose (calorie) utilization and blood lactate concentration when running overground vs. on the treadmill at a 10:44 minute/mile pace. The study found no differences between the two conditions, which signifies that the intensity of the two conditions is similar.
Since the treadmill leads to a lot of control of your movement due to the lack of variability in the surface and environmental conditions, some runners actually experience more muscle fatigue on the treadmill compared to running outside. This study found that running on the treadmill lead to more quadriceps fatigue, which makes sense if the lack of variability in movement leads to specific muscles experiencing more repetitive stress than usual. In general, runners also report higher rates of perceived exertion (RPE) when running on the treadmill vs. outside. So, even if the running pace is the same as outside, runners internally feel like they are running a lot faster than they really are when running on the treadmill.
Specifically regarding injury rehab and injury prevention, the study states that the treadmill puts less stress on bones and the plantar fascia, making it a great option for rehabbing stress fractures and plantar fasciitis. But, if you are experiencing calf or Achilles concerns, the treadmill might be a worse option since it makes these tissues work harder than overground running.
The main take aways:
Your running form is likely slightly different when running on the treadmill, but the changes are likely insignificant and simply due to changes in surface stiffness.
If you are running slower than a 8:03 pace, you don't need to put the treadmill at 1%, but speeds faster than this should be done at 1% incline.
If you are experiencing an Achilles or calf injury, it might be best to avoid running on the treadmill.
Running outside
There are a lot of things to consider when deciding whether or not you should run outside in the snow or cold.
Here are some quick points to consider:
For those overcoming Covid or who have asthma, the cold air tends to be really irritating, so it is better to run inside.
Running on snow takes a lot more energy than running on flat and paved paths. This means that running 1 mile on snow is not the same as running 1 mile on clear paths. I recommend changing your runs to total time vs. going by miles to account for this difference.
Running on snow and ice is biomechanically not very efficient because your limbs are moving all over the place. If your goal is to improve your running economy, then running on ice and snow will interfere with this goal because your form is less than ideal.
Running on snow and ice leads to your muscles working harder to support your joints, which often leads to extra soreness for runners. This can make your recovery time longer or potentially lead to injury if the muscles are over-stressed over an extended period of time.
Dealing with snow and ice can be mentally and emotionally draining for people because of fear of falling and having to constantly focus on foot placement. Running indoors can help to avoid this frustration and anxiety.
Snow and ice can make it really tough to do speed work. The tough footing makes push-off more difficult, which means you have to work harder to hit faster speeds. If you are doing a speed sessions, it's better to do these indoors so that the goal of the run is achieved, which is hitting your upper limits.
If you are more focused on socializing with your amazing running friends than anything else during your winter runs, then running outside is the best option! It's tough to do a group run on an indoor track and obviously is impossible to do on a treadmill. Social runners who try to do more solo indoor running during the winter tend to enjoy their runs less or skip runs altogether.
In general, if the conditions lead to you feeling destroyed after your run due to the cold air or running on unclear paths, then head indoors. But, if you have appropriate winter gear, you don't care about the conditions of the surface you are running on, and you aren't worried about hitting a specific pace, then I recommend getting outdoors! The fresh air and sunshine tend to lead to runners feeling way more amazing than running on a treadmill or indoor track!