The strength curve and why it changes your loading

A strength curve is just a picture of how much force you can produce at each point in a movement. In a squat you are weakest near the bottom, where your knees and hips are most bent, and you get stronger as you stand up. In a biceps curl you are weakest at the very start and around the middle, and stronger at the top. Every lift has its own shape.

Two things drive this. First, the length of the muscle: a muscle produces its greatest force at certain lengths and less when very stretched or very shortened. Second, leverage: as your joints move, the mechanical advantage of your muscles changes, so the same muscle force turns into more or less useful lifting force depending on the angle.

You do not need the physics to feel it. You already know that a bench press is hardest off the chest and a deadlift is hardest off the floor. Those are the low points of your strength curves for those lifts.

Here is the problem a barbell cannot solve. The weight on the bar is constant from the bottom of the lift to the top. Your strength is not. So the heaviest weight you can lift is set by your single weakest position, because if you cannot get past that point, the rep fails no matter how strong the rest of the movement is.

That means your strong positions never get fully challenged with a straight barbell. Once you grind past the sticking point of a heavy squat, the rest of the way up feels almost easy, not because you are done working, but because the fixed load is now well below what those stronger positions could handle. The muscles in that range are undertrained relative to their potential.

Bands and chains exist to fix this mismatch. They are called accommodating or variable resistance, because the load they add changes through the range of the movement.

As you stand up out of a squat with chains draped on the bar, more links lift off the floor, so the weight in your hands rises as you move into your stronger range. Bands do the same by stretching: the further you move up, the more they pull down. In both cases the resistance climbs as your strength climbs, so the lift stays genuinely hard across more of the movement instead of only at the bottom.

This is not just a training-hall theory. A meta-analysis comparing variable resistance to plain fixed weight found that adding bands or chains improved maximal strength, and for trained lifters the variable-resistance approach produced significantly greater strength gains than constant resistance. It is a real tool, best used to accent your strong range rather than replace normal heavy lifting.

Machines take the same idea and build it into the hardware. A well-designed machine uses a shaped cam or lever, its resistance profile, to change how heavy the movement feels at different points, so the peak resistance can line up with where you are naturally strong, or deliberately load a range a barbell struggles to reach.

This is why the same muscle can feel completely different on two machines, or between a machine and a free weight. One might load the stretched position hard, another the shortened position. Neither is simply better. They challenge different parts of your strength curve, which is a good reason to use a mix rather than argue about machines versus free weights.

You do not need to overhaul anything. A few adjustments let the strength curve work for you instead of quietly capping you.

1. Know where each lift is hardest. That low point is your weakest position and the thing setting your limit. It is also where targeted work pays off most.
2. Train the weak position directly. Pause reps and partial-range work in the sticking point build strength exactly where a fixed bar keeps failing you.
3. Add bands or chains to accent the strong range. Use them on your main lifts every so often so your stronger positions finally get loaded near their potential.
4. Mix in machines with different profiles. Choose ones that load ranges your barbell work leaves easy, especially the stretched or shortened positions.
5. Keep heavy free-weight lifting central. Variable resistance and machines are accents. The plain heavy lifts still build the bulk of your strength.