Compound exercises rank far ahead of isolation exercises. They involve movement or action at more than one joint with isolation at only a single joint. Many believe they work better only because you can use more weight. This ignores much more important reasons. These reasons actually make them the only sane choice for lifting heavy weights. The evidence that settles the compound vs. isolation debate appears quite overwhelming.
- They work muscles at their ideal lengths.
Muscles evolved to produce force when needed the most. For most movements, this means when your leverage is worst. This concept may go against our initial impressions without a further explanation.
Tension is the main stimulus for more size and strength. Tension, or the squeeze you feel when a muscle contracts, develops best at middle lengths. This is about halfway for a normal movement. If you were to squat as low as possible and then stand up, your muscles would act strongest at the midpoint.
The parts that form sites for contraction must be close enough to form connections that create tension. Fewer connections form when these parts lie too closely or too far apart. When too close, they overcrowd like a ball of yarn. When too stretched, they fail to reach each other.
Leverage is worst when the resistance resides furthest away from the active joint. Imagine going up a flight of stairs. If you go up one small step at a time, you feel strong. You feel strong because your leverage is ideal, since the knees and hips do not have to bend much. The muscles in this position though bunch up and form less connections.
As leverage grows worse, your body needs more tension. This occurs if you take larger steps. You feel less strong but the muscles work harder. This leads to more tension and better results.
Compound movements reduce the range of motion possible which prevents you from reaching portions where less tension would form.
This argument goes further when analyzing muscles that attach to more than one joint.
- Multi-joint muscles can generate more tension.
Multi-joint muscles attach to multiple joints. They change length depending upon the positions of all their joints. Many major muscles throughout the body act on more than one joint.
The body will attempt to lengthen at one joint before shortening in another joint. This keeps you at that middle length that works best for tension. When you take a step, your hip flexes while your knee extends. When you pull something, your shoulder extends while your elbows flexes. When you push something, your elbow extends while your shoulder flexes. This causes an econcentric contraction, where you have both concentric and eccentric contractions of the same muscle due to action at more than one joint.
When you perform a curl or a triceps extension, your muscles fail to keep the ideal medium length since they move at only one joint. Compounds keep the multi-joint muscles generating ideal tension, stimulating more strength and size.
Multi-joint muscles have to be prevented from acting at all of their joints when movement is desired only at one. Preventing the motion at other joints brings in even more muscles into play.
- Multi-joint muscles act best on the furthest joints.
Muscles generate more force on these joints because they have better leverage .
Many train the hamstrings with leg curl machines, yet the hamstrings work better for hip extension such as during a squat. The hamstrings exists further away from the hip than the knee. Knee flexion occurs much less strongly and only for repositioning of the leg. Heavy leg curls can harm your knees and squats work better anyway.
Each muscle or muscle group has a dominant function due to this long moment arm, or distance between the muscle and the primary joint. These always correspond with compound movements. For example, the hamstrings are dominant at the hip for a squat. The biceps are dominant at the elbow for a row. The long head of the triceps, perhaps surprisingly, works mostly to extend the shoulder on the row.
- They avoid poor joint angles.
Some force at the endpoints for any exercise gets wasted. This happens because some of the force works toward stabilizing or destabilizing the joint by pulling the bone into or away from the joint. Understanding this process involves some physics.
When the forearm extends fully, such as at the bottom of a barbell curl, the tendon of the biceps inserts into a bone on the forearm called the radius at a low angle. Most of the force by the biceps directs into the elbow rather than moving the elbow. This helps with stability but fails to produce movement. This seems sensible when we extend our arms and push on an object with our legs, such as when pushing a car, but this position wastes energy if aiming to move at the elbow.
If the elbow extends at an angle greater than 90° of flexion, tension produced by the biceps tends to pull the radius away and dislocate the joint. This would occur at the top of a curl when your forearm touches your biceps. Just like a stabilizing force, it wastes energy that should instead help with the movement.
Muscle force converts most efficiently at 90° to the long axis of the bone, which again represents the muscle at a medium length.
Compound exercises require less force at the endpoints and often avoid them entirely.
- They work more efficiently overall.
They stimulate more than just the muscles.
More muscle working at once gets your heart rate up. Using lots of muscle achieves the best cardio. You will never raise your heart rate much with even a brutal set of reverse wrist curls.
They stimulate hormones due to more total stress on the body that comes with heavier weights. Steroids show the power of improving your hormone levels. This increases overall strength and size further.
They teach balance and coordination by working stabilizing muscles and training common movement patterns.
A program of pure isolation would require countless exercises to train the whole body. Compound movements also stimulate deep muscles often ignored when focusing on small superficial muscles. The muscles of the feet work harder with a squat than a leg extension.
- They are functional.
Compounds load all the muscles worked during natural patterns of movement. Isolation could cause strength gains out of proportion which can allow imbalances. If a body part gets injured, you can rely on other muscles to compensate during recovery for a compound action.
Muscles attempt to adapt to imposed demands. Isolation may change your muscles in a way that make your muscles less strong in positions used during everyday life and sports. This argument may be overblown but remains possible.
From a functional point of view, single-joint movements change positions when unloaded or dealing with very light loads. Think of adjusting your wrist while writing or typing. Why would it make sense to load the muscles used in these actions heavily?
- They allow synergy.
Muscles contract harder when working with other muscles and help each in subtle ways.
Cross your arm toward the middle of your chest. Notice that this will extend the elbow slightly. The chest indirectly assists with elbow extension during any pushing movement.
The big muscles therefore can assist the small muscles in performing their functions, which improves performance during compounds.
- Closed chain exercises are usually compound.
Open chain exercises have the hands or feet moving freely through space. A leg extension on a machine or a dumbbell curl is an open chain exercise.
Although not always, closed chain exercises tend to associate with compound movements. Open chain exercises usually associate with single-joint movements. Closed chain exercises may challenge the core muscles more so since they move our bodies through space.
- They encourage the indirect effect.
The indirect effect is a secondary growth effect from an exercise that builds muscles not directly worked.
Some legends state that squats, the king of all exercises, can even build up the chest which would seem to make no sense.
This may occur due to more direct work than believed or perhaps from hormonal effects, but either way it occurs best through the big compound exercises.
- They have a better strength curve.
Poor exercises feel heaviest when the muscles work weakly. The dumbbell fly for the chest has the longest moment arm at the stretched bottom position instead of in the middle. Many isolation exercises work the endpoints harder than the ideal middle range.
- They emphasize compression over shear.
Compression presses the ends of our bones together. Specific bones handle large compressive forces. The tibia and femur of the lower body function strongly against compressive forces.
Shear consists of two equal and opposite forces acting parallel to each other. This tends to displace an object by causing molecules to slip past each other, as opposed to smashing together during compression. Think of shear as the force applied when you snap a pencil using your hands.
Bones and other connective tissue function strongest in compression and weakest in shear. Single-joint exercises maximize shearing over compressive forces which harms our joints. Multi-joint movements compress the bones bearing the heavy loads.
- Co-contraction supports the joints.
When you squat, the hamstrings of the back thigh and the quadriceps of the front thigh support the knee since they contract for each side. This occurs during a push, since the biceps support the elbow during shoulder flexion along with the triceps for elbow extension. This occurs during a pull, as the long head of the triceps supports the elbow during shoulder extension along with the biceps for elbow flexion.
On an isolation exercise, you only contract on one side of the joint. This allows less support in addition to the whole host of other problems mentioned.
Choose Compound Only in Compound vs. Isolation
Isolation is impossible. All movements are compound, just differing to the degree. Isolating a single joint motion is a possible but poor decision though.
By choosing compound, you generate more tension and place safer forces on the joints, in addition to the whole host of other benefits.
Although compound wins in the compound vs. isolation debate, this does not automatically make them safe. The overhead press is a good example. I suggest focusing on only three compound movements.