Active vs Passive Insufficiency: How Two-Joint Muscles Change Your Exercise Choice

Active vs passive insufficiency is a simple idea with a big gym payoff. A muscle that crosses two joints can be stretched or shortened by either one, so the position of the second joint changes how well it produces force and how stretched it is during a lift. In recent trials, setups that kept two-joint muscles lengthened (seated leg curls, overhead triceps extensions, standing calf raises, and a reclined rather than upright leg extension) grew those muscles more than the alternatives.

This article is part of our guide to range of motion and stretch-mediated hypertrophy. Here we stick to one thing: muscles that cross two joints, and what that means for picking exercises.

What are biarticular (two-joint) muscles?

Most muscles you train move one joint. A few cross two, and these are the ones that behave oddly.

• Hamstrings cross the hip and the knee (the short head of biceps femoris is the exception, crossing only the knee).
• Rectus femoris, the quad muscle on the front of the thigh, crosses the hip and the knee.
• Triceps long head crosses the shoulder and the elbow.
• Gastrocnemius, the visible calf muscle, crosses the knee and the ankle.
• Biceps also crosses the shoulder and the elbow.

Their one-joint neighbours (vastus lateralis, soleus, the lateral and medial triceps heads) only care about one angle. A two-joint muscle cares about two. Change the hip angle and you change what the knee exercise does to the hamstrings. That is the whole story, and everything below is a version of it.

Active vs passive insufficiency: what the terms mean

Active insufficiency happens when a two-joint muscle is shortened across both joints at the same time. It simply cannot generate much force from that position. Try this: stand tall or lie face down with your hip straight, curl your heel toward your glutes, and squeeze. It often feels weak, crampy and hard to hold. That is a hamstring shortened at both the hip and the knee, running out of useful length.

Passive insufficiency is the opposite. The muscle is stretched across both joints, runs out of length, and limits how far a joint can move. Straighten your knee with your hip flexed and the stretch you feel behind the thigh is the hamstring saying no more.

Neither one is a flaw. They are just how muscles with two jobs work. The useful part for lifters is that you choose the second joint position through the exercise you pick.

Do biarticular muscles grow more when the second joint keeps them lengthened?

This is the question that matters, and recent trials have started to answer it. Most share a design: the same person trains one limb one way and the other limb the other way, and MRI measures muscle volume afterwards. That controls for genetics and effort quite well. The catch is that the participants were mostly untrained, so keep that in mind throughout.

Broader context helps first. A systematic review of range of motion studies found that full range of motion appears to benefit lower body hypertrophy compared with partial range, while the upper body evidence was limited and mixed, and some evidence suggested the response may be muscle-specific (Schoenfeld and Grgic, 2020). Two-joint muscles are one reason it may be muscle-specific.

Hamstrings: why hip position matters in leg curls

In a 12-week trial, 20 healthy adults did seated leg curls with one leg and prone (lying) leg curls with the other, at 70% of 1RM, 10 reps, 5 sets, twice a week. MRI showed larger gains for the whole hamstrings in the seated leg (+14% vs +9%). The difference showed up in the two-joint hamstring muscles (+8-24% vs +4-19%) but not the one-joint short head (+10% vs +9%) (Maeo et al., 2021). Both setups protected equally against damage from a later eccentric bout.

Why? Sitting puts the hip in flexion, which stretches the hamstrings at the hip while the knee does the curl. Lying prone leaves the hip straight, so the muscle starts shorter. The one-joint short head did not care, which fits the idea neatly.

There is a flip side. Hip-extension exercises such as stiff-leg deadlifts load the hamstrings at the hip rather than the knee. A review on hamstring injury prevention reports that biceps femoris long head and semimembranosus activity tend to be higher in hip extension movements, while semitendinosus and biceps femoris short head are more active in knee flexion movements (Bourne et al., 2018). That is muscle activation, not growth, so it is a reason to include both a hinge and a curl, not proof that one is better. For a deeper comparison see our seated vs lying leg curl guide.

Triceps long head: arm position changes the exercise

The triceps long head crosses the shoulder, so lifting the arm overhead stretches it. In a 12-week trial, 21 adults did cable elbow extensions with one arm overhead and the other in a neutral position (hands near the sides). The overhead arm grew more across the board: long head +28.5% vs +19.6%, whole triceps +19.9% vs +13.9%. It happened even though the overhead arm lifted loads about 34-39% lighter (Maeo et al., 2023).

Even the one-joint lateral and medial heads grew more overhead (+14.6% vs +10.5%). The abstract does not settle why, so leave room for uncertainty.

The practical read: if you only have room for one triceps exercise, make it an overhead extension. Pushdowns are fine as a second movement. Our guides on overhead vs neutral triceps training and overhead cable extensions vs pushdowns go deeper.

Calves: why the gastrocnemius needs a straight knee

The gastrocnemius crosses the knee as well as the ankle. Bend the knee 90 degrees, as in a seated calf raise, and the muscle shortens at the knee. That is the textbook example of active insufficiency: the gastrocnemius has less to give at the ankle, so the one-joint soleus is thought to take on more of the work.

A 12-week trial in 14 untrained adults compared standing (knee straight) and seated (knee bent) calf raises, one leg each. Standing raises produced larger muscle volume gains in the lateral gastrocnemius (12.4% vs 1.7%), medial gastrocnemius (9.2% vs 0.6%) and whole triceps surae (5.6% vs 2.1%). The soleus grew similarly either way (2.1% vs 2.9%) (Kinoshita et al., 2023). The seated leg’s gastrocnemius changes were not significant.

The practical read: do your main calf work standing. Seated raises are a supporting act for the soleus. More on the anatomy in straight-leg vs bent-knee calf raises.

Rectus femoris: why leg extensions are not redundant

The rectus femoris is the odd one out. It flexes the hip as well as extending the knee, so it is longest with the hip extended and the knee bent. In a squat, the hip and knee flex together: the muscle shortens at the hip while it lengthens at the knee, so its overall length changes less than a one-joint quad muscle. On a normal upright leg extension, the hip is flexed, which actually shortens the rectus femoris at the hip while the knee does the work.

So the squat versus leg extension trials are not a clean test of muscle length, but they are still useful. A 2026 trial randomized 63 untrained young women to Smith machine squats or leg extensions (3 sets of 8-12, twice a week, 8 weeks). Leg extensions grew the rectus femoris more at all three sites (proximal +11.4% vs +2.0%, middle +12.3% vs +5.7%, distal +17.5% vs +7.9%), whereas squats grew the vastus lateralis more at the distal site (+18.2% vs +11.2%) (Kassiano et al., 2026). An earlier trial with equal volume and intensity also found all three rectus femoris regions grew in the leg extension group, while only the central vastus lateralis grew in the squat group (Zabaleta-Korta et al., 2021).

The cleaner length test is within the leg extension itself. In a 10-week trial, 22 untrained men trained one leg with the hip at 40 degrees of flexion (backrest reclined) and the other at 90 degrees (upright). The reclined leg showed greater rectus femoris growth, with “extreme” Bayesian evidence, while the vastus lateralis grew about the same either way (Larsen et al., 2025). That is exactly what the two-joint idea predicts: lengthen the muscle at the hip, and the two-joint part of the quads responds more.

So is lengthened always better? The caveats

The trials above point the same way, and the authors of several suggest training at long muscle lengths promotes hypertrophy. Still, a few honest limits apply.

• Untrained participants. Most of these trials used untrained adults. Trained lifters may respond differently.
• Short and small. Eight to twelve weeks, with 14 to 63 people. The results are useful but not final.
• One variable at a time. Seated vs prone also changes machine design, stability and range of motion in practice.
• No injury or comfort verdict. None of these papers says deep stretch positions are right for every joint. If a position hurts, it is not worth it.

The cautious takeaway is to lean toward lengthened setups for two-joint muscles and keep them in the mix, not to throw out the others. Our long muscle length training guide covers how far to take this.

How to pick exercises for two-joint muscles

A simple order of operations:

1. Name the muscle’s two joints. Hamstrings: hip and knee. Triceps long head: shoulder and elbow. Gastrocnemius: knee and ankle.
2. Choose the setup that keeps the muscle long. Seated leg curl. Overhead extension. Standing calf raise. Leg extension with the backrest reclined for the rectus femoris.
3. Use a full, pain-free range. The stretch is the point, so do not bounce out of it.
4. Keep one shortened or neutral option as a second movement if you like it: lying curls, pushdowns, seated calf raises.
5. Check fatigue and recovery. A deep stretch under load can be sore at first, so ramp volume up gradually.

Here is how that looks in a plain table.

Muscle	Longer setup	Shorter setup	Notes
Hamstrings	Seated leg curl	Prone leg curl	Seated grew whole hamstrings more in one 12-week trial
Triceps long head	Overhead extension	Neutral pushdown	Overhead grew more even with lighter loads
Gastrocnemius	Standing calf raise	Seated calf raise	Seated is mainly soleus work
Rectus femoris	Reclined leg extension	Upright leg extension	Leg extensions in general grew rectus femoris more than squats; squats still won for distal vastus lateralis and squat strength

If you want to put this into a week of training, our leg day plan already includes the leg curl, extension and calf work above. For tracking weekly sets per muscle, use the training volume calculator.

When to see a doctor

Muscle soreness from a new stretched position is normal. Sharp pain, a pop, bruising, swelling, numbness or pain that lingers for days is not, and hamstrings in particular can strain. Stop and see a doctor or physiotherapist if that happens. This article is general education and cannot diagnose anything.

The bottom line

Active insufficiency shortens a two-joint muscle across both joints, and passive insufficiency stretches it across both. For lifters, the lesson is practical: pick the setup that keeps the muscle long. In the studies we covered, seated leg curls, overhead triceps extensions, standing calf raises and reclined leg extensions gave the larger gains for the targeted two-joint muscle, mostly in untrained people. That is a strong lean, not a law. Use a pain-free range, keep a mix of exercises, and give each change several weeks before you judge it.