Why do full range of motion exercises not increase strength at all muscle lengths?
When we do biceps curls, the muscle shortens to its shortest length when the weight reaches the shoulder, and lengthens to its longest when the elbow is fully extended.
If we stop the lift on the way down, before the elbow reaches full extension, then the muscle never reaches its longest possible length. Similarly, if we stop the lift on the way up, before the weight reaches the shoulder, then the muscle never reaches its shortest possible length.
So, when we lift through a full range of motion, we work the muscle from its longest possible length throuhg to its shortest possible length. In contrast, when we use a partial range of motion, we *usually* only work the muscle between a moderate length, and its shortest possible length. Of course, some partial exercise (like halting deadlifts) work the other way around, but these exercise variations are rare.
Knowing that strength gains are greatest at the muscle lengths actually used in strength training, many people assume that full range of motion exercises will increase strength similarly at all muscle lengths.
But this is not true.
What happens when we curl a dumbbell?
Consider the dumbbell biceps curl for a moment.
If you use a full range of motion in this exercise, you may recall that the weight is straightforward to get moving, then starts to feel quite hard to move in the middle (the “sticking” region), but then once you pass a certain point, it becomes very easy to finish the lift, and rack the weight on your shoulder.
Why is this?
There are actually a few reasons, but two of them are more important than the others.
- Firstly, the force that our muscles produce to move the dumbbell is equal to the dumbbell weight due to gravity *plus* the force required to overcome its inertia, and accelerate it upwards. Inertia is only applicable when we are trying to change the speed the dumbbell is moving. So the force our muscles produce is made larger at the start while the weight accelerates, and smaller at the end, as it decelerates.
- Secondly, our muscles are working to produce turning forces (“torques”) and in this case our elbow is working as a pivot. This turning force changes depending on the length of the lever between the pivot and the dumbbell. The length of the lever is the horizontal distance between the pivot and the weight. When the dumbbell is at the bottom, lever length is small, so the turning force is also small. But when the dumbbell is halfway through (in the “sticking” region), lever length is maximal, so the turning force is also maximal.
So in a full range of motion dumbbell curl, the biceps have to exert a moderate amount of force to get the dumbbell moving, a high level of force to push through the middle “sticking” region, and only a small force as they ride through the last part of the lift to the shoulder.
In contrast, if you use a partial range of motion variation that only uses the final 45 degrees to the shoulder, you will find that the weight you can lift is much heavier, and the muscle has to work *a lot* harder in that final quarter of the lift, while at short muscle lengths.
Muscles increase strength most at the length that they are challenged most in training. So the full range of motion dumbbell curl will increase strength most in the lower two-thirds, while a partial variation will produce strength gains in the final third.
What is the takeaway?
Even when an exercise works a muscle through its full range of possible lengths, it will not necessarily increase strength similarly at all lengths. The greatest strength gains will occur at the muscle length that is challenged most during the exercise, which is usually the point when the horizontal distance between the pivot and the weight is longest.
And this is why partial range of motion exercises can have an important role to play in a strength training program, even when full range of motion variations are already being used. The full range of motion exercise simply cannot challenge the muscle adequately at all lengths.