Complex sentence: At one point in 2008, he began experiencing numbness in his pitching forearm, which lingered for rest of his career, and transformed into weakness, intermittently shifting from his arm to his hands, neck, and facial muscles.
Split into simpler sentences: At one point in 2008, he experienced numbness in his pitching forearm. The numbness also transformed and persisted into weakness, intermittently shifting from his arm to his hands, neck, and facial muscles.