So a muscle close to the joint would be considered a shunt muscle?

No, the muscle crosses the joint. Look at the image of the brachioradialis muscle I added. I have labeled its origin and insertion.

First, a little review of how we determine which attachment of a muscle is its origin and which is its insertion:

The origin and insertion of a muscle is determined by two factors, which I will list in a sort of descending order of importance.

1. A muscle, when it attaches to bones, pulls on those bones. Usually one of those bones is less movable than the other one. The origin and insertion of a muscle, then, is determined by considering the affect of the muscle's contraction on the bones it attaches to. So, the amount a bone moves when the muscle contracts determines whether that bone is the origin or insertion.

2. Sometimes, however, the relative location of the attachment to the midline or center of body is also considered. For certain muscles that have various origins and insertions, some of which are not even on bones, this can be difficult. But most muscles can be easily determined.

Each skeletal muscle is usually fastened to an (mostly) immovable or fixed part on one end and to a moveable part on the other end.

The origin is determined by the bone that is least movable by the contracting muscle that is attaching to it. But it is also considered to be the "proximal" attachment that is closest to the midline or center of the body.

The insertion is determined by the part that is most movable and most acted on by the muscle's contraction. However, it is often considered the part of the muscle that fastens farthest from the midline or center of the body, called the distal attachment.

Movement is the most important consideration. As a general rule a muscle's insertion is pulled toward its origin.

Ok, now, as you can see, the brachioradialis has its origin on the humerus (distal end) and its insertion all the way at the end of the radius bone at the wrist (the distal end of the radius). The humerus cannot be moved by the brachioradialis since its other end is anchored at the shoulder joint plus the humerus is the part that is proximal, or in other words, closer to the midline of the body. So the humerus is the origin.

The radius attachment is the insertion. It is the bone that is moved by contraction of the brachioradialis and it is the part that is most distal. So we've met all our criteria for determining the origin and the insertion.

Now, what is the general criteria for considering the brachioradialis a shunt muscle?

When the distance of the insertion is greater than the distance of the origin, the muscle is considered a shunt muscle. When the origin is farther from the joint axis than insertion, the muscle is a spurt muscle.

The distance from where?

…the distance of the origin and insertion of a muscle to the joint axis of rotation determines whether a muscle acts as a spurt or shunt muscle.

The axis of rotation is the joint center so basically its the distance of the attachments to the elbow joint which determine whether the brachioradialis is a spurt or shunt muscle. As you can see, its insertion on the end of the radius is much further from the joint than its origin on the humerus.

Look at the elbow as a simple lever system. As I stated in the article, when a muscle acts on that lever, the forces can be resolved into two components (technically three since there is a "spin" or twising component on the bone). The swing component is the rotary component. Third is the perpendicular component that is the the part that causes the bone to rotate around the joint axis. They call this the "swing" component. The other component is a transarticular or parallel component. This can either be a stabilizing component if it acts to pull the bone toward the joint, or a dislocating or translational force, if it acts to pull the bone away from the joint. Now, with most muscles in the body, the transarticular component changes depending on the angle of the joint. When the angle between the line of the muscle force and the bone is less than 90 degrees the transarticular component tends to pull the bone away from the joint. When the angle is greater than 90 degrees it pulls the bone toward the joint.

See the images below. I made my own crude little illustrations to try to show this. This is supposed to be an elbow joint flexing. The thick blue arrow lines are the line of muscle force. The dotted red lines are the components. See in the one below we have the elbow flexed so that the long axis of the radius bone is greater than 90 degrees to the line of force. So in general the parallel component tends to pull the bone toward the joint, as you can see from the red dotted line.

Now, see in the next one we have the angle of the bone and the muscle force less than 90 degrees. The parallel force becomes more a dislocating one because the bone is pulled away from the joint.

Shunt muscles are special in that their insertion location gives them the ability to always be a stabilizing component. They pull the bone toward the joint even as the angle changes. The brachioradialis is an example of this and it is used in this PDF which is probably a much better explanation: Spurt and Shunt muscles.

I need to sit and re-read it but the diagrams, etc help a lot.

So I am assuming that say I was doing Pull-ups which is primary a back exercise, but because I am using my biceps, then in that case the triceps are the antagonistic muscle group, right?

Maybe it works better with isolation exercises because the whole point of compound lifts is to utilize all the muscles in which case the "antagonistic" muscles are actually stabilizers (example: back muscles in the bench press).

When the distance of the insertion is greater than the distance of the origin, the muscle is considered a shunt muscle. when the origin is farther from the joint axis than insertion, the muscle is a spurt muscle.

So a muscle close to the joint would be considered a shunt muscle?