Violin tends to be more physically demanding than other instruments for the upper back, shoulders, and arms. If you try holding your arms up without the violin, you’ll see that it is difficult to keep them up there for long periods of time! Like any other physical activities, it takes time to build up the strength to play for longer sessions. However, it is always better to practice for shorter periods of time and more frequent sessions.
Just be careful if you are feeling sore after practicing. This could be a sign that you are too tense and could eventually lead to injuries.