Flexibility vs. Mobility: Understanding Resistance Torque in Your Muscles
Many people use the words flexibility and mobility interchangeably, but they are not the same. Understanding the difference explains why some people still have pain doing mobility exercises, while others are extremely flexible but feel weak or unstable at the edges of their range.
Let’s first define these:
Flexibility = Passive Range of Motion
Mobility = Active range of Motion
Flexibility + Strength = Mobility
Passive Stretching = using outside force to increase Range of Motion (ROM), i.e. using your hand to pull and lift your leg
Active Stretching = same as Mobility. Movement or ROM performed entirely by muscles without outside force, i.e. lifting your leg without your hands or hinging into a deadlift
Torque and Resistance
Torque is simply force at a joint. When you lift your leg, your hip joint hinges, and the force needed to create that movement is called torque.
Passive Resistance Torque
Passive resistance torque is the force that resists being moved when the muscle is relaxed. It comes from muscle tissue, tendon, fascia, joint capsule, and your nervous system’s stretch tolerance. High passive resistance torque makes a muscle feel stiff. Stretching lowers passive resistance torque, which is why regular stretching increases flexibility.
Active Resistance Torque
Active torque is produced by muscles that are actively generating force. This can happen while a muscle is shortening, staying the same length, or lengthening.
Active resistance torque specifically occurs when a muscle being stretched is also actively contracting — an eccentric contraction. For example, slowly lowering your torso in a deadlift requires your hamstrings to lengthen under tension, producing active resistance torque.
Resistance torque is the total “pushback” or ‘resistance’ of tissue when you try to move it into a stretch. Strong and tight muscles have high resistance torque and benefit from stretching the most.
Flexibility vs. Mobility
Flexibility refers to passive range of motion. It is mostly about lowering passive resistance torque so your joints and muscles can move farther. An example would be using your hand to lift your leg to lengthen your hamstring passively.
Mobility refers to usable, active range of motion under control, often involving eccentric or isometric contractions at long muscle lengths. True mobility requires both : the stretched muscle must tolerate lengthening with low passive torque resistance , and the contracting muscle must produce enough force to move and control the joint with strength or high active torque capacity at end range. In this example, listing your leg without using your hand is mobility.
This is why mobility feels harder than stretching — your nervous system and muscles are coordinating strength at the edge of the range.
Compensations
In a deadlift, If your hamstrings are stiff and you try to hinge, your body often compensates by rounding the low back. This is because your passive resistance torque is limiting hip flexion and hamstring length, so the spine “borrows” the range from the back instead. Until passive resistance torque decreases and active control improves, compensation will continue. Again, you lower passive resistance torque through Stretching!
Opposing Muscles & Mobility
Mobility also involves the interaction between opposing muscle groups. For example, when you bend your elbow, your biceps are actively shortening to create torque, while your triceps are being lengthened. The triceps contribute passive resistance torque, opposing the motion, while the biceps generate active torque to move the joint. True mobility requires both: the stretched muscle (tricep) must allow the joint to move by keeping resistance low, and the contracting muscle (bicep) must generate enough active torque to control the motion through the full range. Strengthening the active muscle at long lengths improves control (like a negative of slowly lowering dumbells in bicep curls) while stretching the opposing muscle (triceps) reduces passive resistance, together increasing usable ROM.
Negatives and Eccentric Strength
Mobility is closely tied to eccentric strength, which is the muscle’s ability to produce force while lengthening. For example, when performing a controlled “negative” at the gym, lowering into a deadlift, your hamstrings lengthen under tension, generating active resistance torque.
Strength training with controlled negatives improves end-range strength and control, while stretching the opposing muscles reduces passive torque resistance, together enhancing usable range of motion safely and effectively.
Calisthenics - Hanging Leg Raises
Another example of true mobility is a hanging straight leg raise. Your hip flexors and quads contract to lift the leg, generating active torque, while your hamstrings and glutes do not resist being lengthened, providing passive resistance torque. Slowly lowering the legs back down turns the hip flexors into an eccentric contraction, or negative, producing active resistance torque and building control at the end range. This combination shows true hip mobility and improves usable range of motion safely and effectively.
Who Should focus on Stretch vs. Strengthening?
If you are Stiff and strong: Your ROM is limited by passive resistance torque → focus on stretching and mobility work to increase flexibility.
If you are Flexible but weak: Limited by active torque capacity and true mobility → focus on controlled strength work at end ranges and eccentric contraction to build usable mobility.
The Takeaway
True mobility is not just about flexibility. It is the combination of a joint’s passive range of motion and your ability to control it with active strength. Stretching reduces resistance torque, while end-range strength builds active torque capacity. Both are needed for safe, functional Mobility and usable range of motion. Ultimately, understanding how to isolate muscles, along with understanding good posture, alignment as a whole is essential. I teach all of these techniques and practices inside my membership.
