Use this as a guide to know what the muscles do and the effects of working them.
Rectus Capitus Lateralis: this muscle helps bend the top of the neck down (flexion of the first two cervical vertebrae). If your horse makes continuous stretching movements with his head and carries his head low to one side, consider treating the stress point and adjusting either the atlas or axis.
Brachiocephalicus: This muscle wears many hats; it helps extend (raise up) the shoulder, head and neck, assists in moving the head and neck to the side, and also serves to move the foreleg. If your horse has difficulty walking a straight line and is off in circles, treat the stress point here and look for a subluxation at the seventh cervical.
Multifidus Cervicus: besides flexing the neck to one side, this muscle rotates the head to the opposite side. IF your horse has a problem here, you’ll notice he resists turning his neck to the opposite side of the muscle. Look for a lower cervical subluxation
Rhomboidius: These are actually two muscles, one named after the neck and the other for the top of the mid-back. These muscle help draw the shoulder blade upward (dorsally and forward (cranially). If there is a problem here, you will notice your horse flinch when you touch his shoulders, exhibit difficulties with coordination and display a non-specific loss of power. A lower cervical subluxation may be present
Trapezius: This muscle is also named for the neck and mid-back. The problems here a are very similar to those of the rhomboids, namely, generalized tightening of the shoulders (spasms), loss of power, and decreased neck flexibility. A lower cervical subluxation may also be present here.
Suprasinatus: This muscle extends (opens) the shoulder joint and helps prevent dislocation of the shoulder. If there is pain here, your will bind his knees in response to firm pressure over this muscle. Again, a lower cervical subluxation can be noticed.
Infraspinatus: this muscle also helps to support the shoulder to prevent dislocation; it also abducts the foreleg and rotates the foreleg outward. Problems with this muscle are similar to those of the supraspinatus.
Serratus Thorascis: this muscle helps support the trunk when the leg is planted on the ground; it also moves the scapula back towards the tail (caudally). When this muscle is stressed, you might notice the saddle slipping off to one side. This often coincides with mid-to-lower cervical subluxation.
Triceps (upper end): this part of the triceps flexes the shoulder joint. Trouble here may result in shortened foreleg stride, as well as looking lame at extended trot. If this is the case, look for a lower cervical or upper thoracic subluxation.
Triceps (lower end): this muscle extends and locks the elbow joint. If this muscle was affected, you might see a shortened stride, as well as your horse avoiding the lead when jumping. A lower cervical or upper thoracic subluxation may be partially to blame.
Pectoralis: this muscle and its divisions draw the front leg backwards as well as advancing the leg. A problem here may result in a short extension of the forelimb; your horse may refuse to take a proper lead and/or show discomfort while tightening the girth. Look for a lower cervical and an upper thoracic subluxation
Longissimus Dorsi: This is a powerful extensor on the back and also assist in the lateral flexion of the spine. When injured; this muscle is often involved in pack pain, loss of coordinated power while in motion and the site of the “cold back.” Jumpers often stress this muscle due to jarring upon landing. Nerves from various vertebral levels supply this muscle area, including nerves from the cervical, thoracic and lumbar regions. Therefore, check all spinal levels if this muscle is stressed.
Longissimus Costrarum: this muscle assists in lateral flexion of the trunk. When stressed, the longissimus costarum will restrict lateral bending of the trunk. Look for thoracic and lumbar subluxations.
Gluteus Medius: aside from extending the hip, the gluteus medius moves the hind limb outward. This muscle is frequently the area associated with back pain, a shortened stride, and restricting hip extension. It is also closely involved with the actions of the longissimus dorsi. This muscle will dip or sag upon deep palpation. A subluxation between sixth lumbar and sacrum can affect this muscle
Biceps Femoris: this muscle is used to extend the hip, stifle, hock and rear leg and is also for rear leg propulsion. Stress at this muscle can lead to shorted forward movement. Subluxations often occurring at the lower lumbar region.
Gastrocnemius: this muscle extends the hock and flexes the stifle. Stress at this muscle can make the horse uncomfortable while standing, or may not allow his stifle to straighten. Lower lumbar subluxations can be found here when this muscle is stressed.
Semitendinosus: the Semitendinosus extends the hip and hock joints, and assists in flexing the stifle as well as rotating the lower leg inward. Stress here can result in shorted forward movement and resisting the stifle to be straightened. Lower lumbar and sacral subluxations may be present.
Semimembranosus: The action of this muscle is to extend the hip joint as well as adduct the lower leg. A problem with this muscle can be seen as a shortened forward stride, resisting lateral movement, and refusals to straighten the stifle. Upon firm palpation of this muscle, the lower leg may react by tucking in. Look for lower lumbar and upper sacral subluxations.
Tensor Fascia Lata: this muscle flexes the hip joint and extends the stifle. IF this muscle is stressed, the horse may throw his rear limb outward on the forward stride, as well as restricting lateral movements. Lower lumbar and upper sacral subluxations may cause the above problems.
Illiacus: this muscle flexes the hip joint and rotates the thigh outward. When affected, this muscle may cause the hind leg(s) to buckle, can cause stumbling on circles and appears as lower back pain. Lower lumbar subluxations may be involved.
Glutes Accessories: aside from assisting the gluteus muscle in extending the hip, this muscle rotates the thigh outward. Stress here may cause back pain, restricted hip movement and a shortened forward stride. Lower lumbar and sacral subluxations may be involved.
External Oblique: this muscle flexes the trunk. A problem here may be associated with “tying up,” which is a term used to describe a form of azoturia. With azoturia, excessive amounts of nitrogen waste products are found in the urine caused by the excessive breakdown of muscle tissue. Kidney failure may result if this condition is not properly treated. Look for lower lumbar subluxations.