In the sketches below, I will attempt to demonstrate the proportions of the arms in various poses when the subject has her arms in front of her. Once you have a basic idea of the pose you are trying to achieve you will first need to set out the points between the shoulders the elbow and the wrist. See here.
In this first example, the subject is sitting. She is of a slim build and leaning slightly to her right. Her right arm is resting just above her knee slightly raising her elbow. Notice the shape of her arm. Her forearm appears shorter but it is easy to make out the shape of the extensor and flexor muscles that give the forearm its shape. Her upper arm is relaxed but the shape of her bicep is still visible. The left arm is hanging loosely over her left leg and it is easy to see the shape of the forearm as it rests gently against her inner thigh. Her upper arm and forearm are of equal length and it is easy to see the shape of the elbow.
Subject seated on the floor
In this sketch, the subject is sitting on the floor. She is leaning back on her left arm causing her body to twist forward. Her right arm appears straight and rotates in toward her body. It is easy to see the triceps and bicep muscles as they gently curve from under the shoulder to the elbow. Notice how the back of her elbow is visible forming a little bump. There is also a small indent to the front at the joint. The top of her forearm appears almost flat while the flexor muscles are visible on the base of the forearm forming a gentle curve from the wrist to the inside of the elbow joint. Notice also how her upper arm and forearm appear to be of almost equal length at three and a half spaces each.
Arms. Frontal pose. Subject seated, Pose 2.
In this sketch, the subject is in a similar pose. She has her left leg raised and her right leg tucked slightly under her bottom. Again, she is leaning back on her left arm and her body turns away. She has her right hand placed on her thigh and her arm bends back slightly making the forearm appear shorter. Her shoulder is elevated making it is easy to see the shape of the triceps and bicep on her upper arm. Her forearm rotates slightly, tightening the extensor and flexor muscles and making them more visible. Note again the small bump of the elbow on the back of the arm and the crease at the front of the arm where the elbow is bent.
Full frontal seated nude.
This sketch that is a full frontal seated nude, the subject is sitting upright on the floor. She is facing forwards and is holding her shins. She is leaning slightly to her right causing her right shoulder to appear lower than the left. Note the proportion of her upper arms. They are both of equal length despite the right shoulder being slightly lower. You can also see the curve over her bicep and triceps muscles as the triceps curve into the back of the elbow. Again, there is a visible dip at the elbow where the triceps and extensor muscles meet the elbow joint. Her forearms are slightly different in length. Her left forearm appears slightly longer as she is gripping her shin a little lower than on her right and her left hand is tipped forward more creating more length in the wrist.
In this sketch, the subject is squatting with her body turned away. Her arm is straight and the heel of her palm is resting on her knee. As she is of a slim build, she has very little definition on her upper arm, however, notice how her arm tapers toward the elbow. In addition, notice the shape of the elbow joint, see how it joins the forearm. The forearm again has little shape but you can just see the slight curve of the extensor and flexor muscles. Note again the proportion of the upper arm and forearm.