A joint complex is multiple joints that work together as one and muscle groups are multiple muscles that together perform the same task.
Instead of trying to learn the roles of all the individual joints and muscles in the body, it’s much simpler to break them down into nine main joint complexes and the opposing muscle groups (agonists and antagonists) around those joint complexes.
For example instead of thinking of all the joints through the toes, feet and ankles we can refer to them as the Ankle joint complexes and even more simply the ankles. Taking this through the entire body we have six pairs of joint complexes, Ankles, Knees, Hips, Wrists, Elbows and Shoulders and well as the three joint complexes of the pelvis and spine. The lumbosacrococcygeal joint complex which for ease can be simplified to the Lumbar spine joint complex (Lower back), the Thoracic spine joint complex (middle back) and the Cervical spine joint complex (upper back i.e. neck). Giving us 15 main joint complexes.
Then instead of referring to the individual purpose of each muscle throughout the entire body we can simplify to the groups of muscles around the joint complexes and the actions they perform as a group. For example the muscle group that flexes the knee joint are referred to as the knee flexor group and the muscles that oppose these that bring the knee into extension as the knee extensor group.
There are four states you can create with muscle activation around each joint complex in the body. Before we go further a little more explanation of agonists and antagonists is required. Agonists are the group of muscles that activate to create a joint action, i.e. when you raise your leg in front of you, the hip flexor group active to create hip flexion and are referred to as the agonists. The antagonists are the opposing muscle group that are required to relax and lengthen to allow the joint action to occur. In the case of hip flexion this would be the hip extensors. This is known as the reciprocal stretch reflex, which states when a muscle group activates/tenses (agonists) the opposing muscle group (antagonists) must relax/lengthen.
However, there are four states that can be created around a joint complex.
1. The agonists and antagonists can be relaxed. This would be referred to as a passive stretch. Which may have some usefulness however during a passive stretch there is no protection around the joint complex and it may become compressed. There is also a chance if you move too quickly into a passive stretch you may initiate a myotatic (stretch) reflex. This occurs when the antagonist (the muscle you’re probably trying to lengthen) feels sudden tension (due to lengthening too quickly) and due to a nerve reflex suddenly tightens to stop it lengthening. This is quite useful if you’re walking along and you unexpectedly role your ankle, the reflex will tighten the suddenly stretched muscle and stand you back up before you fall over. However, if it occurs in a passive stretch you will either shorten the muscle you were attempting to stretch or worse potentially tear the muscle.
2. The agonists can be tensed and the antagonists relaxed. This is the reciprocal stretch reflex that I referred to above.
3. The agonists can be relaxed and the antagonists can be tensed. Once you’ve moved actively into a position you can then reverse the action of the muscles by actively trying to come out of the position without actually moving. For example when moving into a lunge as you step your back leg back, the hip extensors are the agonist (active) and the hip flexors are the antagonists (lengthen/relax). Once in position if you attempt to pull your back leg forwards without actually moving, the hip flexors will become active and the extensors can relax. This can be quite useful as once you then relax your hip flexors they will lengthen more, essentially speeding up the effect of the inverse myotatic reflex which states if you hold a stretch long enough the muscles will lengthen.
4. The agonists and antagonists can both be tensed. This is known as bandha. In the case of the back leg in a lunge if you keep the hip extensors active in the lunge while simultaneously activating the flexors by attempting to pull the leg forwards you have active agonists and antagonists. This is the most stable way to be around a joint and is useful in aiding to simultaneously strengthen and lengthen muscles. In the case of the lunge the hip flexors in the back leg would be getting lengthened and strengthened at the same time.