The best runners know that symmetry matters. When you apply force equally on the left and right sides of your body, you become a more efficient—and faster—runner. More importantly, you’re less prone to injury.
Ground Contact Time (GCT) Balance is a metric that can help improve your running symmetry.
In this article, get up to speed on ground contact time balance and see our five tips for improving running symmetry.
To understand ground contact time balance, let’s start with ground contact time, or GCT for short. This is the length of time your foot is in contact with the ground when running. For most runners, a GCT between 200 and 300 milliseconds is normal. Elite distance runners are capable of sub-200ms ground contact times.
The ground contact portion of your running gait is also known as the stance phase. The stance phase starts with the initial foot strike, continues through the ‘load bearing’ midstance, and ends with toe-off.
Running speed relies on the ability to apply force through the ground quickly. The faster you can apply force through the ground, the quicker you will be propelled forward.
Scientists have studied the link between ground contact time and running speed for decades. But until recently, GCT was an inaccessible metric reserved for biomechanists in a laboratory.
Wearable fitness devices have revolutionzed fitness training and how we track our performance. Trackers and accelerometers now offer accurate, real-time biofeedback for runners everywhere.And they work.
A 2020 study was the first to review the effects of wearable device biofeedback on running biomechanics.  The results were positive. Seeing GCT biofeedback reduced runners’ vertical displacement (bouncing) and running time and increased knee flexion–key factors for performance and injury prevention.
That brings us to the next evolution in biofeedback: GCT balance.
What is ground contact time balance?
Ground contact time balance is the measure of how similar your left and right leg ground contact times are. GCT balance is normally displayed as a percentage split. A 50/50 split is the theoretical optimum and indicates an equal GCT for both legs. In reality, GCT balance is rarely 50/50. Anything between 49% and 51% is considered fairly symmetrical. However, if your GCT balance is beyond 49/51 (an imbalance of more than 2%) the asymmetry may affect your performance and put you at risk of injury.
Ground contact time balance is the measure of how similar your left and right leg ground contact times are.
GCT balance is normally displayed as a percentage split. A 50/50 split is the theoretical optimum and indicates an equal GCT for both legs.
In reality, GCT balance is rarely 50/50. Anything between 49% and 51% is considered fairly symmetrical. However, if your GCT balance is beyond 49/51 (an imbalance of more than 2%) the asymmetry may affect your performance and put you at risk of injury.
Some fitness tracker features are little more than gimmicks (e.g., oxygen saturation) . So what makes GCT balance any different?
GCT balance matters because symmetry matters. Elite athletes display high levels of symmetry, and the fastest runners are the most symmetrical. 
Asymmetry, on the other hand, is metabolically and biomechanically inefficient. This is because one side of your body is working harder to compensate for the other.
For efficient forward motion, front-to-back (sagittal) movement is most effective. When an imbalance exists, energy is wasted moving your body in the frontal plane (think hip adduction) or transverse plane (think trunk rotation) to counter the asymmetry.
For example, if one leg is weaker or less flexible, we tend to compensate with excessive arm swing or trunk rotation.
The International Journal of Exercise Science published research last year on the impact of GCT imbalance on running economy. 
Remarkably, the study showed that for each 1% imbalance in GCT, running economy was reduced by almost 4%.
To put that in perspective, for a GCT balance of 49/51 (where one leg is in contact with the ground just 2% longer than the other), the energy required to run at the same speed is 7.4% greater.
For a 70kg runner at 15km/hr, this would increase oxygen consumption by ~4ml.kg.min. That’s a big chunk of your V̇O2 max.
The science is unclear about the extent to which asymmetry causes injury.  Anecdotally, however, any weakness on one side is likely to put extra strain on other muscles and joints, putting you at risk of further injury. 
In short, balancing the time you spend on each foot matters for preventing injury and improving economy and performance.
Use the GCT balance metric to figure out which leg is slower. The leg with the higher GCT balance percentage (>50) is spending more time in contact with the ground. The slower leg is the one to work on.
Test the asymmetry by performing a stretch or exercise on each leg. This will help you identify differences in strength, balance and flexibility.
In many cases, asymmetries stem from past injuries. The pain or weakness of one injured body part can lead to overcompensation in others. Asymmetry becomes a vicious cycle of overcompensation, causing further weakness on the injured side due to underuse.
Use GCT balance biofeedback to correct asymmetry when recovering from injury, or to retrain after adaptation to old injuries.
To improve running symmetry and overcome imbalances in ground contact time, focus on developing muscular strength and power.
Single leg (unilateral) exercises like lunges and calf raises are effective at developing your weaker leg and prevent the dominant leg from taking over.
Inflexibility due to injury (from scar tissue for example) increases the likelihood of asymmetry. Include flexibility training in your schedule to overcome injury related tightness.
Your range of motion should be specific to your activity. You don’t need a gymnast’s flexibility to run long distances. In fact, a certain degree of ‘tightness’ may even enhance performance. 
A recent study showed that runners naturally adapt leg stiffness and ground contact time to optimise running economy.  A stiffer leg spring will return more energy after compression.
Incorporate single leg plyometric exercises into your programme to enhance leg spring stiffness and shorten ground contact time.
Running over monotonous terrain can exacerbate asymmetry. Try running over rough ground and a variety of gradients to enhance proprioception, balance and strength.
If you run on a track, alternate between clockwise and anticlockwise laps to avoid asymmetry due to leaning into the curve.