Load Curves Explained
How Resistance Changes Through a Range of Motion
A “load curve” describes how challenging a movement becomes across its range of motion, not just how heavy the weight is. In strength training, equipment and exercise setup can make resistance feel easier in one portion of the rep and harder in another. Variable-resistance designs (cams, lever arms, pulleys) exist specifically to change resistance across the range of motion in an attempt to better match the exercise’s strength curve.
This guide explains load curves in plain language, so you can understand why two setups using the “same weight” can produce very different training stress.
What This Guide Covers
- What a load curve is (and how it relates to “resistance curve”)
- What a strength curve is, and why it varies by exercise
- The three classic strength-curve shapes
- How machines (cams/levers) and bands change resistance across the rep
- How to use load-curve logic when thinking about hip-focused training
Strength Curve vs Load Curve
How to read this chart
Use this chart to compare two things across the same range of motion: your strength (torque) curve, how much torque you can produce at each joint angle, which changes because both muscle force and moment arm change during joint rotation, and the load (resistance) curve, how the setup changes resistance demand through the rep.
Variable-resistance devices (like cams or lever systems) exist specifically to alter resistance across the range of motion in an attempt to match these strength changes. When the curves don’t line up, the perceived difficulty shifts to the parts of the rep where demand is high relative to your capability.
Load curve (resistance curve)
A load curve describes how much resistance the exercise setup demands at each point in the range of motion. Many resistance training machines use mechanisms (lever arms, cams, pulleys) intended to alter resistance throughout the ROM in an attempt to match the exercise’s strength curve.
Why the distinction matters:
- A strength curve is about the human (capability).
- A load curve is about the setup (demand).
When these curves mismatch, a movement can feel disproportionately hard in one segment and under-challenging in another.
Strength curve
A strength curve describes how your ability to produce force (or joint torque) changes across the range of motion of an exercise. In practical strength-training literature, strength curves are commonly described as ascending, descending, or bell-shaped.
The Three Classic Recistance Curve Shapes
A common way to classify strength curves is into three broad patterns.
Ascending resistance curve
You can generally produce more force (or lift more) as you move toward the end of the concentric range of motion. ACSM’s personal training resource uses the squat as an example of an ascending strength curve.
Descending resistance curve
You are strongest earlier in the concentric range of motion and comparatively weaker later.
Bell-shaped resistance curve
The exercise is hardest in the middle portion of the range of motion and weaker near the start and end. ACSM’s resource uses elbow flexion as an example of a bell-shaped curve.
Important reality check:
Even within the same exercise category, the “curve” is not identical for everyone. ACSM notes that variations in limb length, tendon attachment points, and body size make it difficult for any single machine design to match every individual perfectly.
How Equipment Changes the Load Curve
Different tools create different resistance patterns.
Constant external resistance (free weights and many machines)
In many “traditional” setups, the external resistance does not change (e.g., the barbell weight stays the same, the selected weight stack stays the same). ACSM notes that even when external resistance is constant, the force exerted by the muscles is not constant, it varies with mechanical advantage and muscle length at different points in the movement.
What that means: constant external load ≠ constant difficulty.
Variable-resistance machines (cams / lever arms / pulleys)
Variable-resistance devices operate through lever arms, cams, or pulley arrangements with the purpose of altering resistance throughout the ROM in an attempt to match changes in strength across the movement.
A practical way to think about it:
- A cam or lever system can change the effective lever arm through the rep.
- Changing lever arm changes how difficult the movement is at different points.
Weight-stack machines with cams can change the resistive torque through the range of motion and are designed to better match how human strength changes across joint angles.
Elastic resistance (bands)
Elastic bands typically increase resistance as the band stretches. ACSM notes that this “constantly increasing” resistance pattern aligns best with an ascending strength curve and can be a drawback if the exercise does not match that pattern.
The practical implication is simple: band setup matters, because where the band is most stretched will be where resistance is highest.
Why recistance Curves Matter for Hip-Focused Training
This page isn’t a hip thrust technique guide, but load curves are one reason hip-focused training can feel different depending on the setup.
The “hard part” of the rep is not fixed
If the resistance demand is highest in the middle of the range of motion, you may feel strongest or most challenged there. If the resistance demand rises later, you may feel the lockout portion become the limiter. Variable-resistance designs exist specifically to shift where in the range the movement is most challenging.
Load curves influence repeatability
One advantage of machine-based training in general is that the setup tends to be more repeatable from rep to rep. Whether a given machine’s resistance curve is the “best” match depends on the goal and the individual, but the concept is the same: resistance profiles can be engineered.
Load curves are a decision tool, not a marketing claim
A “better” load curve is not universal. It depends on:
- the movement pattern
- the person’s limb lengths and joint structure
- whether the goal is strength, hypertrophy, or control
how consistently the setup can be repeated session to session
That’s why understanding load curves is useful: it gives you a framework to evaluate setups without relying on hype.
Continue Learning
Weight is a number.
Resistance profiles determine where the rep is hardest.