To prevent pain, need balanced pull of muscles, as joints rotate.
Without balance, joint axes ("instant center of motion") will move incorrectly causing stress and wear on joints, producing misalignment and pain.
1. Force couples: Muscles pull in different directions, must be balanced to rotate joints around an axis correctly or will cause pain.
E.g. upper and lower trap during upward rotation. If lower trap is weak, scapula will stay downwardly tipped, produce impingement at the shoulder.
E.g. TFL with glut maximus during hip abduction (also anterior and posterior gluteus medius). Weight bearing, would cause hip rotation if unbalanced, cause knee and ankle misalignment.
E.g. During elevation of the arm, if posterior shld cuff muscles are too weak, will get anterior subluxation of The humeral head by the deltoid and pect major.
2. Muscle length: Muscles that are too long or too short will cause abnormal motion, pain.
Muscles habitually kept in a stretched position grow more sarcomeres, cannot strongly contract in shortened range, sometimes called stretch weakness. The muscles become longer.
Muscles habitually kept in a shortened position lose sarcomeres cannot be lengthened normally. The muscles become shorter.
Too short: E.g. Short pects and intercostals prevent thoracic extension, shoulder retraction
E.g. Short hip flexors cause anterior tilt of pelvis when in standing
Unilateral short hip flexors cause the pelvis to rotate forward at the end of stance each step, rotating the lumbar spine
E.g. Short lumbar extensors cause excessive lumbar extension (lordosis)
Too long: E.g. Long thoracic extensors causes kyphosis
E.g. Long abdominals causes lumbar lordosis
E.g. Long hip abductors on one side, long adductors on the other side cause the pelvis to be positioned to the side of the long abductors, causes lumbar
E.g. Long upper trapezeii cause shoulders to be too low, produce TOS (thoracic outlet syndrome)
E.g. If the pect minor is too tight, lower trapezius too long, the scapula will stay
downwardly tipped, causing impingement at the shoulder during reaching.
General rule: Need to lengthen shortened muscles first to permit normal excursion. If strengthen first, may compound the problem. (can do simultaneously)
E.g. if have tight pects and subscapularis, strengthening middle trap, infraspinatus and teres minor may just produce less shoulder motion overall.
General rule: To shorten a posturally lengthened muscle, exercise it in its SHORTENED RANGE, against MINIMAL RESISTANCE.
E.g. if have forward, downwardly tipped scapulae, strengthen stretched lower trapezius by doing depression with the shoulder in full flexion against a wall or prone.
3. Muscle recruitment - The CNS can incorrectly learn to use one muscle rather than another, causing weakness in the underused muscle, tightness in the over used muscle.
E.g. If use hamstrings for hip extension, not gluteus maximus, get weakness of glut, tightness of hamstrings
E.g. If use lateral hamstrings, not medial ones for knee flexion, get laterally rotated knee
E.g. If use lumbar extensors instead of hip extensors at end of stance to push off, get lumbar pain
E.g. If don't push off well with gastrocs, get pain in ant tibialis from constant contraction,
ischemia (shin splints)
4. Relative flexibility - "Stretchability" or flexibility is associated with muscle 1) tightness 2) looseness and with CNS recruitment.
Either excess alters motion, causing pain-producing movements. This pain is
generally in the segment that gets too much motion.
General rule: proximal muscles should stabilize segments so most of the motion occurs at the distal joint (or get pain syndrome).
E.g. Hip should bend, not lumbar spine during bending/lifting.
E.g. If abdominals are more flexible than rectus femoris, as knee is flexed, rectus will pull pelvis into anterior tilt, cause lumbar pain.
E.g. If bending "at the waist" occurs at the lumbar spine instead of at the hip, get lumbar pain. (common in tight hamstrings).