Muscles absorb energy, not force

A few days ago, one of my students showed me an Instagram clip he wanted my thoughts on. It turned out to be a podcast segment where these two hosts explained, in their own self-styled manner, what they believed about flexibility physiology. In that clip, the lead host claimed that passive stretching to boost flexibility was dangerous, insisting it robbed the muscle of its capacity to “absorb force” when you hit that new range of motion. I had to laugh, because these folks hold themselves up as experts on both the physics and physiology of stretching, but that claim has some serious holes, in my opinion. First, muscles don’t absorb force; they absorb energy. Second, simply holding a stretch to boost flexibility still doesn’t mean your muscle somehow loses the knack for absorbing energy.

Why muscles don’t absorb force

I’ve often heard martial arts coaches talk about how muscles “absorb force” when you block a punch or kick. But let’s get precise here: force is a push or pull. Muscles can encounter force, sure, but they can’t literally store that push or pull. They store energy. More accurately, they take on mechanical energy whenever they’re moving under load. Picture yourself dropping into a low stance after a jump, or misplacing your footing when stepping off a curb and having to rebalance. Your body’s in motion, which means you have kinetic energy. When you step into the ground or pivot abruptly, some of that kinetic energy goes into the muscle-tendon unit, stretching it.

In eccentric contractions (where your muscle is still contracting but lengthening at the same time) you’re essentially performing what’s called “negative work.” You’re slowing down the motion and taking on mechanical energy. So, instead of saying “muscles absorb force,” let’s clarify: muscles, along with their connective tissues (tendons and fascia), absorb mechanical energy and sometimes store a bit for later use. Some of that energy becomes heat, and some might get converted into kinetic energy as you release it back into your next movement.

The role of eccentric strength

While muscles can’t absorb force, one thing I’ve learned over the years is that how well you handle a sudden stretch depends heavily on your ability to produce force while lengthening (also known as eccentric strength). I can’t count the number of times I’ve seen a fighter who’s wonderfully flexible but gets hurt because they can’t control a quick, forceful extension. Think about lowering a weight with control, or landing with soft knees from a jump. The muscle is lengthening, but it’s still firing. That’s what prevents the muscle from tearing when the load is high. But raw strength alone isn’t enough. Your brain and nervous system must time everything just right so that when your muscle’s about to get yanked, it contracts appropriately to handle that force.

But that doesn’t mean static passive stretching is dangerous!

When you work on static passive stretching, you gradually lengthen not only the muscle fibres but the connective tissues around them, too. That extra give in the muscle is called “compliance”: the muscle’s ability to bend without breaking. The more easily it bends, the better it can soak up energy before you reach a point where something gives out. Whenever you go into a slow, steady static stretch, there are shifts taking place inside the muscle and its support structures. Down at the fibre level, the sarcomeres (those little power units that let you flex and extend) can adapt by getting a bit longer or possibly even adding more units if you stretch consistently. Meanwhile, tendons, ligaments, and the fascial layers also start to ease up on their resistance.

By improving compliance, static passive stretching cuts down on excessive stiffness. Think about it: if a muscle’s tight as a drum, it’s going to resist any effort to stretch. The tension ramps up quicker, and the window between a safe stretch and injury is way smaller. But if that muscle is more compliant, it’ll handle a heavier load and spread that stress out across the fibres and connective tissues. Spreading it around makes it far less likely you’ll cross the line and wind up with a tear. Another plus is that a more flexible muscle-tendon unit can store more energy, much like a better spring. Picture a spring that’s easier to compress: it’s going to hang on to more elastic energy. Then, when you let go of that force, it can snap back with more power if you’re going for an explosive move, or simply release that tension smoothly if you’re easing out of a stretch.

Injury prevention is about balance

I’d be the first to say you don’t want your muscle-tendon units to become rubber bands with no control. Too much compliance can rob you of stability and might even water down your strength for certain explosive moves. But if you have too little compliance because you’re as tight as a board, you’re asking for trouble. You put a sudden load on a muscle that won’t yield, and the next thing you know, something tears. Finding that sweet spot in between is key.

Static passive stretching is a straightforward, reliable way to gradually boost muscle compliance. Holding a gentle stretch for 15 to 60 seconds allows the muscle-tendon unit to adapt so it’s not as rigid. Over time, you’ll find you can handle deeper stretches, manage higher energy dumps without flinching, and generally lower your injury risk.

Finding the “Goldilocks Zone”

A certain degree of muscle, tendon, and connective tissues stiffness helps you move efficiently. I see it like using a well-tuned spring: you want enough give to store energy and bounce back, but not so much that you lose power or stability. If your tendon or muscle barely stretches, you won’t store and release much elastic energy. Your body might also get hammered by energy spikes. On the other hand, if it’s too easy to stretch everything, you might sink into your movements and lose that recoil. That can also create unstable joint positions, forcing your muscles to pick up the slack in awkward ways. The bottom line is you need enough flexibility to avoid harmful overstretches, but also enough stiffness to snap back with some pop. Building this “just right” zone isn’t as mysterious as you might think.

How do we take all this talk of energy and make it work in the real world? There are a few training pointers that have guided my approach over the years. First, train eccentric strength. Exercises like Nordic hamstring curls have shown a great deal of promise against hamstring injuries. You can turn any exercise into an eccentric-focused one by doing slow negatives; when you lower weights, control them rather than just dropping them. It's a great way to build flexibility, too. I teach my students to build up to eccentric phases lasting 10-15 seconds in exercises like split walks: start in a standing position and slowly walk or slide your feet apart until you're as deep into a split as you can go, but you have to take at least 15 seconds to get there. Slowing down your descent is incredibly challenging but it can feel like stretching on steroids!

Second, use a mix of stretching techniques. We know static passive stretching is great for increasing long-term flexibility. Conventional wisdom dictates you should do it after training or in separate sessions to avoid losing explosive power during your workout. But this depends on your goals, the specific static passive stretching methods you use, and your body's response to those methods. For example, a powerlifter might experience a drop in muscle force output in the deadlift if they do a 2-minute passive hold for their glutes immediately prior, as this will cause reduced alpha motor neuron excitability in those muscles.

However, a karate fighter might experience increased performance (higher, faster, stronger kicks) by doing a passive hold of equal duration for, say, the adductors right before a fight. The adductors are antagonists during most kicks. Stretching them for two minutes will acutely relax them, so they will exert less opposing force during the agonists' contraction. The net effect is a potential improvement in the agonists' efficiency or force, especially if the antagonists are especially tight. Following up such a strategic use of static passive stretching with targeted dynamic active stretching of the agonists will help to further prepare the muscles and tendons for the impending demands of the workout or athletic event.

Third, focus on full range of motion (ROM) strength. If you have increased your flexibility, don't leave those new angles weak. Strengthen the entire path, so you can absorb energy and generate forces wherever you end up. PNF stretching is very effective for this purpose because you can hold strong isometric tension at every angle along a joint's ROM (isometric contractions build strength). For example, let's say you can do a full side split already, but it takes you a solid 5 minutes for your muscles to relax enough for you to get there. You can try an isometric stretching approach I came up with in the late 1990s, for people who struggle to close the gap between their cold and warm flexibility (the thermal reserve), which I call "laying the tracks.”

Laying the tracks

• Start in a standing side split with your legs about 90-100° apart.
• Tense your inner thighs by squeezing or pulling your feet into the floor.
• Build up the tension over 5 seconds until you reach 60-80% of your maximum effort (i.e., 6-8 out 10, with 10 being the absolute hardest you can tense those muscles); once you reach the target intensity, hold steady tension for 10 seconds.
• Relax and immediately walk your feet apart another 2-3 inches (approximately equal to a 10° increase in ROM).
• Tense your muscles again as described in the steps above.
• Repeat this process of tensing, relaxing, and increasing the stretch by 2-3 inches all the way until you are doing it in the full split.

You may find you can tense with less strength the further along your ROM you go; this is normal, and the strength of your end-range contractions will increase over time. Increase the duration and intensity of your contractions over weeks and months. If you're feeling particularly ambitious, you could aim for 1-minute holds at 100% maximum effort at each "stop" along the way.

I called this approach "laying the tracks" because each time you hold a contraction, you’re laying another "rail tie," letting you move farther down the track toward better flexibility: ROM that is more stable and readily accessible, such that you can drop into the splits without a warm-up or waiting for your muscles to relax. Or in simpler terms, you're riding the PNF train to Cold ROM City!

Remember, muscles don't literally "absorb force." They absorb energy. Force is just a push or pull; energy is what gets stored, released, or dissipated. Being flexible enough to move through all your sport-specific ranges is great, and offers some degree of protection. But if you can't back it up with strength (especially eccentric strength) and proper coordination, you're still at risk. High passive stiffness can spell disaster if you push past your limited range of motion; ultra-low stiffness might leave you unstable and sap your power. You've got to find that sweet spot. Eccentric strength and quick neuromuscular reactions are essential for greater protection against sudden, nasty strains. The best approach is balanced: some static passive stretching, some dynamic active stretching, consistent eccentric work, and sport-specific drills to keep you well-prepared for whatever demands you face.

Yours in flexibility,

Dan

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