Back to All Stories

Want to Build Serious Strength? This Feature Ensures Your Intensity Never Dips

Smart Flex challenges your body in a way that cannot be duplicated by free weights.

man performing a biceps curl with variable resistance strength training Smart Flex feature, weight is changing based on range of motion and mechanical advantage of the exercise

Leveling up the weight you lift can be a slow and arduous process, but Tonal’s Smart Flex feature gives you the boost you need to safely reach new strength gains faster without compromising your form.

“Smart Flex dynamically adjusts the digital weight throughout each rep based on whether you are at a mechanical disadvantage during a movement,” says Jenna Moore, a certified strength and conditioning specialist and Fitness Programming Specialist at Tonal. “It challenges your body in a way that cannot be replicated with traditional free weights.” 

Here’s everything you need to know about the unique Smart Flex feature and how it can help you reach new strength gains, faster and safer. 

What Is Smart Flex?

Smart Flex is a variable resistance training system unique to Tonal where the resistance adjusts in real-time, so you are training at full capacity throughout every phase of every rep. To do this, Tonal calculates the strength curve, or points of difficulty, throughout every movement and Smart Flex adjusts the digital weight to optimize the resistance. 

“The strength curve captures the maximum force your muscles can produce at each point of an exercise or movement,” explains Lauren Benson, PhD, Senior Manager of Research at Tonal.  “But that maximum force is not always the same. It depends on your body position and changes through the entire range of motion.” In other words, your muscles are stronger at certain parts of the movement, and weaker at others. Smart Flex senses these changes and scales appropriately. 

For example, when lifting a weight up in a biceps curl, the movement is easiest at the beginning, gets harder as you get closer to 90 degrees, and then gets easier again as you go past 90 degrees. Think about how you may sometimes get stuck in the middle through the center of the biceps curl. This is due to a longer moment arm (the weight is farther away in space from the body) at 90 degrees and requires more force from the muscle to overcome that disadvantage. On free weights, you can’t ditch weight to help you push through a rough patch and then dial it up on the parts of your lift where you’re feeling strong. But Smart Flex increases the weight at the beginning and end and lightens it in the center so you can challenge the weaker parts of the movement so you can easily progress to a heavier weight. 

Image explaining variable resistance strength training saying "Smart Flex: Automatically loads the strongest phase of any exercise and seamlessly removes it during the weakest phase of every rep."

The most impressive part? The strength curve for every single move is unique and Tonal has over 300 of them, individualized to your movement pattern. 

Can You Replicate Smart Flex at the Gym?  

The simple answer is no. The weight you lift using free weights is limited by your strength at the weakest part of the movement, explains Benson. With traditional free weights, you cannot control for external factors such as gravity and momentum or when you have more or less leverage over lifting the weight.

In fact, only a specific kind of weight machine called a variable cam increases or decreases the load during a rep. While research shows training with variable cam resistance machines can improve peak muscle force, strength, power, and hypertrophy, you’ll be hard-pressed to find this equipment in gyms anymore (their popularity peaked in the 1980s and 90s and waned since then). Smart Flex, however, uses the same principle of variable cam machines with more sophisticated technology so you can capitalize on these benefits and improve your performance.

How is Smart Flex Programmed For Me? 

Tonal’s expert programming team integrates Smart Flex when you need it most to build strength safely over time. For beginners, or when you’re trying out a new exercise for the first time, Smart Flex promotes healthier movement patterns by providing some overload without maxing out. And for advanced lifters, the feature adds weight to the portions of the exercises that are traditionally easier so you continue to be challenged through the whole rep and build strength. Think of it as a stepping stone between the current weight you’re lifting and preparing you for the next challenge. 

Smart Flex feature turning on on Tonal for variable resistance strength training

Josh Clay, a certified strength and conditioning specialist and Fitness Programming Specialist at Tonal, programs Smart Flex particularly for higher volume sets. For example, your first set of an exercise may feel pretty manageable, but as you perform additional sets, that same load may start to feel considerably more intense. Instead of lowering the weight, Smart Flex tunes into when to load and unload the exercise to actively drive adaptation through volume while under fatigue. 

“We always say ‘intensity drives adaptation,’” notes Clay. “To that extent, optimizing intensity is always at the forefront of our minds. By prescribing Smart Flex, we can maintain intensity across all reps and sets to ensure athletes maintain a high level of effort while still maintaining movement quality.” 

How Can I Activate Smart Flex on Tonal? 

You can add Smart Flex to any exercise on Tonal by swiping left on your digital weight dial. Select the option from the list of dynamic weight modes, and it will kick in on the second or third rep of your set. The first couple reps senses your range of motion and strength curve and adjusts the weight based on this data. 

While you can toggle Smart Flex on at any time on your own, it is already built into many of the programs and workouts on Tonal such as Push-Pull Strength and Tony’s 20 in 20 Ignite so the work is intuitively done for you. All you have to do is show up.