The brachialis muscle: It is the main flexor of your elbow joint. When too tight, it may give you pain at the base of your thumb, in your elbow or at the front of your shoulder.
Because it is covered by the biceps brachii, this muscle is unknown to a lot of people. People who frequently overload this muscle include fitness enthusiasts, guitar players or rock climbers. But anyone can experience trouble in this muscle.
When this muscle is too tight, or has trigger or tender points, it can impair the extension of your elbow. Furthermore, lifting or holding heavy objects with your arm in a flexed position may be painful. Of course, any pulling motion may be impaired or painful too.
Such activities may be
The brachialis muscle gets overloaded and develops trigger points mainly by repetitive stress and overuse. That means if you use the muscle too often or too hard, it may punish you.
So, when doing arm curls with weights, wielding a heavy chainsaw, or lifting furniture, take frequent rests, especially if you are not used to that kind of work.
It is really sustained stress which is problematic: The furniture-moving example being exemplary: You may over-stress the muscle because mostly you carry things with your elbows bent, which activates the muscle permanently.
Although this muscle lies under the biceps brachii, it is pretty accessible as the biceps can be pushed to the side quite easily.
Put your thumb in your elbow crease and move it a couple of times sideways. You should feel a muscle jumping under your thumb.
Travel a little upwards your arm to feel where the muscle runs. That muscle is your brachialis.
For massage, I recommend using the thumb technique.