What is the most likely cause of shoulder flexion weakness and scapular winging in a patient who chopped wood?

The most likely cause of shoulder flexion weakness and scapular winging in the scenario provided is due to compression of the long thoracic nerve. The long thoracic nerve innervates the serratus anterior muscle, which plays a crucial role in stabilizing the scapula against the thoracic wall and is responsible for protraction of the scapula during arm movements, including flexion.

When the serratus anterior is compromised, as seen in long thoracic nerve injury, the scapula can become prominent or "winged" as it no longer properly stabilizes against the ribcage. This winging becomes particularly noticeable when pushing against resistance or raising the arm. Additionally, if the serratus anterior is weak, it can contribute to overall shoulder dysfunction and can manifest as weakness in shoulder flexion, as the normal scapulohumeral rhythm is disrupted.

In this context, activities like chopping wood can involve repetitive overhead motions or pushing actions that may place stress on the shoulder girdle and contribute to nerve compression or injury, particularly affecting the long thoracic nerve.