Actually they don't. Severe shoulder, arm, and back pain is a big deal for truckers. I'm a massage therapist. I've worked on guys who were in "level 10" pain. Real agony.
I've seen issues with the muscles: Teres major, Teres Minor, Latissimus Dorsi, Deltoids, Trapezius, Scalenes, Extensors and Flexors of the forearm, Triceps. (not the bicep very often tho), corocobrachialis. Oh and infra and super spinatus and levitor scapula.
Plus the muscles: gluteus max and min (but not med), multifidus, erector spinae/spinalis, quadratus lumborum and psoas major.
The best position would be in a comfortable char with your arms resting but not crossed watching the road attentively with little "attention" quizzes where you had to tap a button when a light came on. And with the machine observing you were in a capable state-- not falling asleep or looking away from the front for over 10 seconds at a time.