The interest in using curved treadmills is leading to some great questions about not only the differences between land running and the equipment, but specifically about the curved shape and running mechanics. The truth is that for every benefit to using a treadmill, there are a few drawbacks, as with any technology. If you use a curved treadmill, be mindful of the reasons you do so, as it’s not a replacement for tempo running—it’s a training option that can complement a running program if used correctly.
Countless athletes, both elite and recreational, are now jumping on the curved running bandwagon and this could either be a problem or an important step forward. Curved treadmills are great for those wanting to ditch the pavement or the snow, but they do have shortcomings. Running on a curved treadmill has some benefits that rehabilitation therapists may want to explore, but it also has limitations that must be addressed.
What Is a Curved Treadmill?
A curved treadmill is a non-motorized, concave device that allows users to walk and run on it with each foot strike propelling the belt behind them. The system is designed to exploit the vertical and horizontal contours, pulling the belt down and back from the walking or running stride, thus removing the need for an electric motor—a benefit for those who want to pace themselves naturally. There are a few systems on the market today, as WOODWAY, TrueForm, Technogym, and Assault have curved non-motorized systems available. All of them promise the same thing: a comfortable running experience that exploits a curvilinear leg stroke to hopefully improve the workout.
Image 1. The design of the curve is meant to mimic the natural stride pattern, thus reducing the load on the joints and possibly increasing the demand to the posterior chain. The research is still early, and no EMG studies are available to prove that the recruitment is significant enough to create a performance benefit.
Treadmills have been around for years as an exercise option, and in use 100 years ago in the home. It’s important to know the differences between a motorized and non-motorized treadmill, and between the flat and curved options. The Buyer’s Guide to Sports Training Treadmills covers the instrumentation for measurement needs, but it doesn’t mention curved treadmills much because they aren’t a research device. There have been some studies done to explore how a curved treadmill could be used in testing athletes, but they are more about reliability in conditioning tests than ground reaction forces or running mechanics.
How Does a Curved Treadmill Work?
The mechanical secret of curved treadmills is a mix of gravity, friction, and exploration of the forces during running. During the downstroke backwards, the foot and weight of the body literally pull the tread down and back, and this happens because of the curved shape of the equipment. The point of contact is significantly ahead of the center of mass; thus, the experience of support is different than with other non-motorized treadmill options, or running on the ground.
The differences, while not highly visible to the naked eye, are enough to help slightly deload the body with vertical ground reaction forces. Running on the curve may help some athletes or fitness users reduce the stress on their legs from vertical ground reaction forces. However, because curved treadmills are designed uniquely, measuring forces on them is a difficult process because force plates are usually large and obviously flat.
The key to a good curved treadmill is the ball bearings or ability to reduce horizontal friction of the early part of foot strike to reduce artificial changes to the firing pattern. Ideally, the the less friction there is earlier during foot strike, the more the muscle firing pattern will improve, but a tradeoff exists because a braking phase is a necessary evil for propulsion. Technically, at top speed you use very little horizontal force contribution, as most of the speed is generated by redirecting vertical force, but submaximally the hip uses horizontal forces in running locomotion. The debate about what is more important for speed has cooled off, as recent research clearly points out that a balance of all three forces—lateral, horizontal, and vertical—is necessary for sprinting success. Using a curved treadmill is very similar to overground running, but it’s far from interchangeable.
Logic and Reasoning of Curved Treadmill Function
Curved treadmills are great for running at a constant speed, but lousy for acceleration. With sports being accelerative in nature, only a few treadmills can provide a true benefit to short sprinting. The reason is simple: The nature of acceleration is pushing behind you while leaning, and the body’s nervous system is marvelous in ensuring you don’t fall on your face.
A few acceleration treadmills exist, but most of them are incline options that use support bars and one of them— the HiTrainer—uses a chest support. There are a few non-motorized flat treadmills, but they use a tether, making them artificially biased, as the athlete can literally use the cord to change their movement strategy. Sometimes this is a great option with light sled use, as minimizing air time reduces overstriding during acceleration. It also reinforces pushing behind instead of up unnecessarily, beyond what is needed.
So, what are the reasons to use a curved treadmill? I do see that going green is nice for the environment. It also has fewer moving parts to repair, and anyone that has worked at a fitness or commercial gym knows the pain of dealing with a repair service and angry customers. In addition to helping the environment and its reliability, the curved treadmill uses the force of every step, thus creating a more natural experience, as constant speed belts in motorized options reduce movement variability with some runners.
Video 1. Athletes can modify their running technique only so much on a curved treadmill. This sprinter is running on the Technogym product.
It’s safe to say that, if you want to work on starts and first step quickness, curved treadmills are bad speed tools. Upright running seems fine, but every coach that uses a treadmill needs to see the big picture and ask themselves what they expect to happen differently than running on the ground, specifically the grass or track. Is it to help teach? Is it to help condition the neuromuscular system differently?
Treadmill running is about conditioning, not top speed development. While technically, any training that an athlete does contributes to their success, there are differences between a maximal all-out sprint and a curved treadmill maximal sprint.
What Does the Science Say on Curved Treadmills?
I will back up my logic and reasoning with additional peer-reviewed science and my own data collection. It seems the research on treadmills and sprinting is not a priority, so I will share what is available and what I know from measuring the variables that matter.
In my investigation, the studies showed the value of a curved treadmill, but didn’t really address the differences between ground-based running much, if at all. A few studies noted the challenges with non-motorized running and how they are generally not comparable. Overall, the studies seem to dodge the simple question: What is different between running on a curved treadmill and on the flat earth?
Overall, I wanted more range on the total number of studies, and some of the depth of the research was solid. A few of the studies were fascinating and important to those with specific health needs. The best example I found was on Parkinson’s, a neurological disorder affecting millions. My own grandfather struggled with this heinous disease, so I was excited to see that walking on a curved treadmill had potential benefits for improving walking for those with Parkinson’s.
Study 1: Reliability of the WOODWAY Curve Non-Motorized Treadmill for Assessing Anaerobic Performance
I was confused about the purpose of this study, as it seemed to be stuck in a comparison between a Wingate test and a curved treadmill, while attempting to show how the treadmill is useful for athletes. The researchers used recreational subjects who performed 30-second “sprints” at 6 meters per second at peak velocity. It was a relief that the researchers, in their conclusion, were very aware that the speed was not a representation of true sprinting. The researchers did not validate the data accuracy of the treadmill or show how sports could benefit from the equipment besides for fatigue monitoring. It is worth noting that the primary goal of the article was to find another option besides conventional Wingate testing.
Study 2: A Self-Paced Intermittent Protocol on a Non-Motorised Treadmill: A Reliable Alternative to Assessing Team-Sport Running Performance
This interesting study researched 10 (mixed) team sport athletes in their 20s to see if the treadmill’s non-motorized component could be useful for reliability testing of self-paced running. The researchers investigated two questions: how the treadmill could fare in a simulation protocol for reality, and how quickly the athletes could familiarize themselves with the curved shape.
The results were exactly what a sport scientist would ask for, but I was concerned that the maximal sprint was in the 6 meters per second range again. After a while, I wondered if any true maximal velocity work was done in the research. A good takeaway is that the curved treadmill allowed for self-pacing, which supports the idea that it’s generally more natural than paced options like motorized treadmills.
Study 3: Non-motorized Treadmill Running Is Associated with Higher Cardiometabolic Demands Compared with Overground and Motorized Treadmill Running.
This study on cardiometabolic demands of a curved treadmill is one of the stronger studies. It used isometric pulls and countermovement jumps to evaluate athlete power—not bad for an “endurance” investigation. I expected no less from Australians, who tend to do a better job adding power complements to endurance studies that are physiological in nature.
While this doesn’t prove that a curved treadmill (specifically the WOODWAY Curve) isn’t useful for testing conditioning, it does show that we need to look at all of the treadmill systems differently. It was fascinating how the study looked at athlete body mass to see the relationships of the belt friction and athlete build. We will eventually need to address how this research data will benefit prescription in training.
Study 4: The physiological and perceptual demands of running on a curved non-motorized treadmill: Implications for self-paced training
This article on curved treadmills from Runner’s World gave a mixed message to those who didn’t read the full study. Most would consider a system that adds more demand on the body to be a positive benefit, but I see it as a mixed bag.
What is great is that those in fitness who want a more demanding workout can see the research study as proof that they get a better workout by burning more calories. If they want to elevate their heart rate with a higher demand from the equipment, that is fine, but I am suspicious about adding less-efficient running as a resistance modality. True, the 25-30% jump in physiological demand is interesting, but how that metabolic load improves speed or performance remains a mystery to me. Adding a weight vest or running on an 1% grade is a theoretical overload, and I have no confidence that this would make athletes faster even in endurance sport, let alone sprinting.
Study 5: The Effect of a Curved Non-Motorized Treadmill on Running Gait Length, Imbalance and Stride Angle
My favorite of the five studies shared the effects of running on the ground after spending a few bouts on the TrueForm, and the results were subtle but still notable. The researchers made a little bit of a leap of faith when saying the device was responsible for improving running economy through a decrease in contact time, but it’s the only study I know that looked at curved running to test if it made a difference in running on the ground later. If the TrueForm can do this at slower speeds, the next question is how much influence it has on sprinting. A reduction in contact time with lower-level recreational athletes running is not the same as maximal velocity sprinting with an elite.
These studies all had excellent insights into curved treadmills, but they were not very exciting for understanding the kinetics and kinematics of the device compared to regular running. What was most interesting to me was the study on the effects of using curved running and stride parameters immediately afterward.
My Investigation of Curved Treadmill Running
The above studies showed progress in understanding the interaction of foot strike and the curved product, but they did not really connect to maximum speed or get into solid direct measures of function. Therefore, I had to do the very simple science of comparing estimated kinetic and high precision kinematic evolution of the device. I looked at four systems, all having various friction levels of the treads and slightly different designs of the curve, including radius and slope. Simply put, each system was similar enough to summarize the findings on the differences between running on the ground and on a curved treadmill. WOODWAY did have a slightly lower friction point, so the belt speed was a little faster, but the design recruited the lower limbs in a more demanding manner metabolically.
The kinematics of running at 9-10 meters per second (calculated) were like fast intensive tempo running, but the pressures were different and the electromyography was also different. The differences were big enough to be seen visually compared to running 22- to 25-second 200m reps. I did not test all-out sprinting, as the athletes didn’t feel comfortable hitting a maximal effort and I purposely wanted it as fast as tolerable.
Some athletes felt more resistance on the less-expensive models, while other athletes felt that they were slipping early in touchdown. As the speed increased, either the treadmills became more demanding on hip extension or the athlete cut off their stride to improve front-side positioning of their running mechanics. I didn’t have a sample size statistically powered enough to see a conclusive group trend, as everyone tended to respond differently.
Vertical oscillation of the center of mass and leg stiffness were significantly lower, and athletes had more gradual rises in pressure compared to ground running. The grab velocity was very high with the better equipment (WOODWAY) and I didn’t have a basketball player tall enough to see whether the curve radius was appropriate for the jogging speeds seen in rehabilitation programs. Overall, the response was enough that such small differences would show up metabolically locally and globally.
Not all muscles worked harder, as some lower leg muscles below the knee seemed to decrease their activity, but again, this was a small sample size. I didn’t look at rotation velocities or rear foot motion in great detail, but any coach with a Dartfish video app can film from behind and from the side to triangulate how much difference in foot strike is occurring. We didn’t do more than three trials, so it’s hard to see if the familiarization with the equipment “saturates” after a few weeks or not.
The main takeaway of the curved treadmill is the fact that the athlete is running in place and theoretically able to receive instruction and perhaps work on technique, whether rhythm or something similar. I am not aware of technical development dependent on this approach, nor do I have any experience using the equipment for extended periods of time. In my opinion, the curved treadmill has some possibilities with stride change, but so far, there has not been much concrete evidence of stride changes coupled with improvements in speed demonstrated in the applied setting.
A Summary of Pros and Cons
Overall, curved treadmills are useful for getting a workout in, but I don’t know whether they are appropriate for rehabilitation or elite training. The differences in kinetics and kinematics remind me of resisted sleds, as the changes or possible negative motor skill influences might be mitigated with the development of power, so it’s hard to say that running on it in small doses is a problem. For the average Joe, conditioning is not something I would worry about. Anything that can deliver a safe and effective way to challenge the body to me is good progress.
Pros for Curved Treadmill Running
The cost of a curved treadmill makes it an interesting option for those who want to get a great workout done in less time, and provide a specific resisted running option for the masses. Curved treadmills allow for self-pacing and possible anaerobic testing alternatives to Wingate assessments.
Cons for Curved Treadmill Running
Based on the limitations of curved running, the equipment is a tempo replacement or alternative running option if an athlete needs a different modality than conventional running. Curved treadmills don’t provide the vertical force oscillation necessary to help with replicating maximal speed development, and can’t provide acceleration postures necessary for short sprinting.
You will have to decide if curved treadmills are a good choice for your situation. I am convinced that they have great value for the general fitness population, and are an interesting option for recreational runners, a creative interval option for performance running, and a possible benefit to sprinters in some circumstances. It will take a few mores studies for the science to catch up to the technology, but curved treadmills are popular and their use is growing.
Before You Run on a Treadmill Again
I am not against treadmills. I use them in the winter if I am traveling up north or when the weather is not cooperative, but only when walking with a weighted vest on a very small incline. I am more than aware of the popularity of treadmills, especially curved ones, with sprinters and runners. What I recommend is matching your needs with the right training approach, not just jumping on a curved treadmill and hoping for the best.
The current pros and cons are very embryonic, and over the next 10 years expect more research to potentially expand the list of benefits and limitations of the equipment. Don’t be scared to run on a curved treadmill—they are fine for fitness and great for getting a workout done in less time, but they are different than running on the ground.