Sciatica is a medical condition characterised by pain going down the leg from the lower back.[1] This pain may go down the back, outside, or front of the leg.[3] Onset is often sudden following activities like heavy lifting, though gradual onset may also occur.[5] Typically, symptoms are only on one side of the body.[3] Certain causes, however, may result in pain on both sides.[3] Lower back pain is sometimes but not always present.[3] Weakness or numbness may occur in various parts of the affected leg and foot.[3]

Myofascial trigger points, also known as trigger points, are described as hyperirritable spots in the fascia surrounding skeletal muscle. The trigger point model states that unexplained pain frequently radiates from these points of local tenderness to broader areas, sometimes distant from the trigger point itself.  They are associated with palpable nodules in taut bands of muscle fibers.

The iliopsoas refers to the joined psoas and the iliacus muscles. The two muscles are separate in the abdomen, but usually merge in the thigh. As such, they are usually given the common name iliopsoas. The iliopsoas muscle joins to the femur at the lesser trochanter, and acts as the strongest flexor of the hip.

​The iliopsoas muscle is supplied by the lumbar spinal nerves L1-3 (psoas) and parts of the femoral nerve (iliacus).

​Rounded shoulders are an unnatural posture characterized by an exaggerated curvature of the upper back, often a forward positioning of the head where the shoulder girdle is protracted, and a thoracic kyphosis.

This is a nice little video by DNews on the effect massage has on the body.
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The erector spinae is not just one muscle, but a bundle of muscles and tendons which run more or less vertically and are paired left to right. These muscles lie in the groove to the side of the vertebral column and extend throughout the lumbar, thoracic, and cervical regions 
If the erector spinae muscle is too tight or contains tender or trigger points it may limit certain movements or make them highly unpleasant.​

This is an excellent short video from Pain Australia 

​http://www.painaustralia.org.au/

Tennis elbow or lateral epicondylitis is a condition in which the outer part of the elbow becomes sore and tender at the lateral epicondyle. The forearm muscles and tendons become damaged from repetitive overuse. This leads to pain and tenderness on the outside of the elbow. Any activity, including playing tennis, that involves repetitive use of the extensor muscles of the forearm can cause acute or chronic tendonitis of the tendinous insertion of these muscles at the lateral epicondyle of the elbow. The condition is common in carpenters and laborers who swing a hammer or other tool with the forearm, and is similar to golfer's elbow, which affects the medial epicondyle on the inside of the elbow. Continuing activity after onset of the condition and avoiding mandatory rest may lead to permanent onset of pain and may only be treatable via surgery.