Here's When You Should Use Isolation Exercises

Here’s what you need to know about isolation exercises in a nutshell,

  • Compound exercises do not properly work ALL muscles.
  • With a little bit of anatomy knowledge, you can decide when isolation is necessary.
  • Length-Tension Relationship helps determine which isolation exercises are best.

First, let’s define isolation and compound exercises.

Isolation refers to movements that train one muscle. Compound movements train multiple muscles.

“I keep being told that a beginner should stick to big compound movements. Is that right?”

Yes and no. Compound exercises are best for muscle building, and that is true for a beginner, intermediate, or advanced lifter. However, compound exercises have holes in them.

Wait wait. Before you get your pitchforks and get ready to burn me alive, hear me out.

I look at it like this. If a muscle is not placed under tension through a “full” range of motion during the compound exercise, then that muscle needs an isolation movement. Simple right?

Here are a couple of examples.

The biceps have one primary function, flexing the elbow. Every time you flex your elbow, your biceps are doing the work. Therefore, every pulling exercise that you do (including every type of row or pull-up) involves the biceps.

So, do you NEED an isolation exercise? If you’re doing as many pulling exercises as you should be in the gym, then no.

The calves are another good example. The calves are responsible for extending the ankle. The squat, and many other lower body push exercises, do involve some ankle extension. But the range of motion that the ankle must go through is VERY short.

Do you need to isolate the calves? Yep!

“Ok, so that’s WHEN I should use isolation, but how do I know WHICH isolation exercises to use?”

With a physiology concept called the Length-Tension Relationship (LTR). LTR refers to the length of a muscle, and how much tension it can produce at a particular length.

If a muscle is too long (stretched), or if it’s too short (contracted) the muscle cannot produce very much tension. Tension is the king of muscle building, and without enough tension  placed on the muscles, they won’t grow.

“How do I know if a muscle is too short or long?”

There’s this other concept called active and passive insufficiency. Together, active and passive insufficiency make up the Length-Tension Relationship.

A muscle enters passive insufficiency under two circumstances.

  1. When the muscle crosses two joints.
  2. When the muscle is lengthening at both joints.

If you feel an extreme stretch in the muscle, it’s too long. Active insufficiency occurs when,

  1. The muscle crosses two joints.
  2. The muscle contracts at both joints.

If you feel a muscle contract REALLY hard, almost like it’s about to cramp, it’s too short.

A muscle trying to lengthen or contract at two joints at the same time is like trying to write an essay and watch Netflix. Sure, you CAN do both, but you can’t do either particularly well.

*If you’re particularly interested in the science of the Length-Tension Relationship, read this next part. If not, feel free to skip*

The length-tension relationship is the result of a concept called sliding filament theory. The sliding filament theory states that for muscle movement to occur, myosin (a protein) cross bridges connect with actin (another protein) and drag the actin toward the center of the sarcomere, which is the contractile element of a muscle fiber. The pulling of the actin across the myosin causes a shortening of the sarcomere and generates force. When the myosin reaches its limit, it detaches from the actin site and returns to its original position. It then attaches to another actin site and continues to pull.

When a muscle is in active or passive insufficiency, it means that not enough myosin are able to connect with actin. As a result, not very much force is produced.

*Science lesson over. Back to your regularly scheduled programming*

“So, what does LTR have to do with isolation movements?”

Certain muscles don’t generate much force during compound movements because of their length-tension relationship. Also, certain muscles don’t have their functions served by the compound movements (like the calves and the long head of the triceps).

Consider two things when choosing when and which isolation movements you should do.

  1. Is the muscle I want to train being worked by this movement? (If you’re not sure if a muscle is being trained by a particular movement, check out my BODY PART GUIDES. They’re dope I swear.)
  2. Does the muscle enter active or passive insufficiency during the exercise?

Number two is how you can find out which isolation exercises are good.

So, because I’m such a big ol’ sweetheart, I’ve given you five classic examples of LTR at work. There are also videos to accompany each.

Example 1: Quadriceps

The quads have four heads, hence the name QUADriceps. Three of the heads only act on the knee (the quads extend the knee). However, one of the heads, the rectus femoris (RF), has an added function because it also crosses the hip joint. That added function is flexing of the hip (hip flexion refers to bending over at the hip or bringing your upper thigh toward your torso).

Like I said, a muscle can’t properly contract or lengthen at two joints simultaneously. So the RF can’t flex the hip while it is extending the knee. So to train the RF you need to have your hips extended while extending the knee.

I give you, the Standing Leg Extension,

Example 2: Hamstrings

The hamstrings have four heads. Three of those heads (long head, semimembranosus, and semitendinosus) are responsible for knee flexion (bending the knee) and hip extension (thrusting your hips forward). The short head of the hamstrings doesn’t have the hip extension function and only flexes the knee.

For optimal hamstring growth, use both seated and standing/ lying leg curls. Because your hips in extension, the lying/standing leg curl will train the semitendinosus, semimembranosus, and long head. Because your hips are in flexion, the seated curl will preferentially recruit the short head of the hamstrings.

Example 3: Biceps

(This example goes toward WHICH isolation exercise you should choose)

The biceps have a long head and a short head. Both heads have pretty much all the same functions, and so we can’t recruit one head over the other.

But, because the biceps cross two joints, passive and active insufficiency is still applicable here. The biceps are responsible for shoulder flexion (raising your arms directly out in front of you) and elbow flexion (what happens at the elbow when you do a curling movement).

The biceps enters active insufficiency when you flex both the elbows and the shoulders at the same time. The biceps enters passive insufficiency when the elbows and shoulders are both extended.

Here’s an example of both,

Stay away from those^^. Do curls like this instead,


Example 4: Triceps

The triceps is the one muscle that most people seem to know about when it comes to LTR and how it affects the movements you should choose. However, most of these folks don’t understand why you should do overhead extensions to target the long head of the triceps.

Only the long head of the triceps acts on the shoulder joint (the long head extends the shoulder joint). The other heads (lateral and medial) only operate on the elbow. Because of that fact, the long head gets better recruited when the shoulder joint is in flexion.

But, there’s a little more nuance to the long head than it’s given credit. First, if the shoulders are too flexed then the long head will enter passive insufficiency, which you don’t want. Also, if the shoulders are too extended, the long head will enter active insufficiency.

Last but not least, the shoulders must be externally rotated for the long head to do its job of extending the elbows.

So avoid these,

And do these,

Example 5: Calves

There are two calve machines in EVERY gym, with good reason. There are two parts to the calves, the soleus ( the part that travels down to the ankle), and the gastrocnemius ( the part that makes up the muscle just below your knee). Both the soleus and the gastrocnemius extend the ankle joint (what happens when you lift your heel up and push your toes down).

However, the gastrocnemius also flexes the knee. So, if you want to target the gastrocnemius, you need to extend your ankles while your knees are in extension. Kind of like this,

If you want to work the soleus, which, I don’t know why you would, do a seated calve raise.

To finish this bad boy up, here’s what you should do about isolation,

  • Look at the compound movements you’re doing (or will be doing), and see which muscles are being trained.
  • If a muscle is not being trained by those compound movements, find a proper isolation exercise.
  • To find a good isolation exercise, look at the LTR of the muscle as described in this article.