Your body is a unit, and the muscles that make up your body is a team. One of my most used logic to explain to people the importance and valid placement of compound and isolation movements are the snow dog logic. One snow dog may be able to pull 50lbs uphill for 100 yards, but if you put nine snow dogs together they will pull nine times the weight and for longer distances. Once one of your snow dogs become weaker and can't keep up with the pack, you isolate that snow dog and give it direct focus and bring up its lagging parts; that way, when the snow dogs are reunited, they are stronger as a unit.
The prime mover in your muscles is something called an agonist. The agonist muscle in a particular exercise is found in accordance to the direct function of that particular muscle; what joint that muscle controls. The elbow is controlled by two potential muscles: the biceps and the triceps. The biceps is the agonist muscle in curl and pulling focused movements, because during these movements, the elbows are flexing against gravity. The triceps are responsible for extension and pushing forced movements, because during these movements, the elbows are extending against gravity. In a tricep extension, the triceps are the agonist. In the biceps curl, the biceps are the agonist.
In a single joint focused exercise, there is one particularly focused muscle: the agonist. These exercises are called isolation exercises are intended to bring up a lagging part. For example, if your biceps are causing you to fail during your bent over rows, then adding bicep curls to further strengthen the bicep muscles, as well as increase the amount of motor units activated in your biceps is the logical explanation for continuous progression of your bent over rows.
What happens when you are working more than one point at a time? These movements are called compound movements, and they are the bread and butter of any successful routine with functional strength as an important card on the table. When you perform a compound exercise, multiple joints are activated and this means that different muscles will play different roles.
During a compound movement, you will have a group of prime movers, with one of those prime movers, being the agonist (primary mover). The other prime movers are simply secondary role players that assist with the larger muscle. As I mentioned earlier, your body is a team, and these muscles must learn how to work together in order to accomplish particular actions.
During the bench press, the chest is the agonist, prime mover. Why? Because pushing in-front is a major function of the chest; there are other muscle groups major function activated during the bench press. The triceps are activated during the elbow extension phase of a bench press, and the deltoids; particularly the anterior deltoids, are activated as the scapula flexes.
These other muscles involved in the concentric loading of a bench press are called synergist muscles. A synergist will assist in concentric contraction to another muscle in accomplishing a multiple joint focused completion of movement. For example, without the triceps the chest cannot push in-front, and without the scapula the chest cannot push in-front, so these muscles must coordinate with the chest in order to complete an in-front push: bench press.
The triceps, deltoids, and chest are not the only muscles involved with a bench press. While the chest is the agonist (prime mover), and the triceps and anterior deltoids are synergists to the movement, there are also isometric contractions occurring during this range of motion. These muscles contract isometrically in order to stabilize a particular joint and as a result, they are called stabilizer muscles.
The role of a stabilizer is significantly different from the role of an agonist or synergist. A stabilizer does not usually have a concentric function, which means it does not shorten or move during the exercise, but it stabilizes a specific joint, so the other joints can perform their desired range of motion. In this example, the forearms have to stabilize the wrist during the bench press to keep a right angle between the humerus and ulna/radius. If the wrist would extend, then you would potentially break your wrist and have the weight fall on you and hurt you. The biceps must isometrically contract to keep the bar in neutral positioning during this lift.
There is another muscular role in compound movements that has not been specified yet, and that role is called an antagonist. Since an agonist muscle is a prime mover, then an antagonist is the complete opposite of this. Antagonists oppose the action of the agonist. For example, since the triceps extends the elbow and the biceps flexes the elbow, the biceps is the antagonist in extension exercises and the biceps are the antagonist in flexion exercises. During a bench press, the upper back muscles and retractors of the scapula, as well as the muscles that externally rotate the humerus would serve as antagonists. In order for proper balance at the joint, these muscles will need to be worked equally.
The team of muscles that work together during a compound movement are the agonist (prime mover), synergist (assistant mover), and the stabilizers (muscles that stabilize a particular joint, so the prime movers and assistant prime movers can perform their desired function). These muscles teach better intramuscular coordination and therefore teaches your muscles how to properly work together for real world tasks like pushing a car off the road or pushing an fallen object off your body. If you do not train your muscles how to work together properly, then they will be UN-coordinated and will not learn how to perform specific tasks in which they are required to work as a unit.
For example, if you isolated your quads, posterior chain muscles (glutes, hamstrings) with exercises like hamstring curls and leg extensions, your body will still get stronger. However, your muscles would lack coordination and they would pick up an inability to work together during tasks.
When you see an athlete preparing for a massive vertical leap, they are usually performing power cleans, deadlifts, and squats. These movements are compound movements and recruit all the muscles required for a vertical leap and teaches this proper coordination of muscles. This type of strength is called functional strength, which means that it has potential carryover into the real world.
Another benefit to compound movements over isolation movements is the ability to make continuous linear progression. Each workout, the muscle has to be placed with more stress than it was the previous in order for optimal growth to occur. If you workout with the same weight each week, you're not giving your body a new stimulus to grow from but rather adapt to the stimulus it has already been given and only to perform this desired weight more efficiently.
A person can gain 100lbs on their bench press very quickly and easily, but how many people have ever in a lifetime added 100lbs to their flies? Your flies may "feel better" on your pecs, but that is only because your pecs are the only muscle involved in the loading and the progress will be minimal. Your might find you will gain 20lbs on the lift, but then will find it unable to make further progression. Not to mention, your functional carryover into other lifts will be minimal.
When a person bench presses, their chest, triceps, deltoids, back, trapeziums, and forearms all get a workout. This causes an increased tension to the body as a whole and the result is the body forcing the endocrine system to release "medics". These "medics" I am referring to is an anabolic steroid hormone produced in the testicles called "testosterone". Testosterone is made from cholesterol, and aids in the enhancement and increments of contractile proteins like myosin and actin.
The anabolic hormone "testosterone" that is released, is a result of increased muscular strength, endurance, and hypertrophy. Isolation movements recruit very little if any at all testosterone release. Big compound movements, particularly squats and deadlifts, recruit the most anabolic hormones like testosterone.
Continual linear progression, increased functional strength, more anabolic hormones, and more carryover into other exercises are all reasons so far that compound movements are superior to isolation movements. If your triceps begin to fail on a bench press, then you can begin to add other movements to focus specifically on triceps development, but this will usually take a while. The bigger chances are if your bench press is stalling you are either not eating enough or you have jumped the gun and started too heavy.
First of all, you have to find out if it really is the triceps that is failing. How would you know? Simple. You find out what particular phase of the bench press the triceps have the most involvement in and you would see if that is the phase you are failing. The triceps is responsible for "extension" of the elbow and extension of the elbow occurs at the lockout phase of a bench press. If you can get the bar off your chest, but cannot get it to lockout, then most likely it is your triceps failing you.
The biggest compound exercises are: squats, deadlifts, overhead presses, bench presses, bent over rows, pullups. Those movements all should be staples with any successful compound movement focused routine. Accessory compounds that are alright to have involved are weighted dips, close grip bench presses, power cleans, and front squats.
The moral of the article has hopefully provided you with the knowledge and understanding of compound movements and their importance, as well as an analogy of when isolation movements are necessary.
Personal Trainer- ACE, NASM AFPA; Nutrition Consultant- AFPA
"I can do all things through Christ which strengtheneth me." - Philippians 4:13"
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