Unable to engage in many activities, ... (Para. 8)
Meaning: Although he was unable to engage in many activities, ...
Here the adjectival phrase "Unable to ..." is a clause showing concession. In English adjectives or adjectival phrases are often used, especially at the beginning of a sentence, to show reason, concession, etc. More examples are:
Glad to accept, the boy nodded his agreement. 那男孩乐意接受, 点头表示同意。
Anxious for a quick decision, the chairman called for a vote. 主席急于快速作出决定, 要求投票表决。
Rather nervous, the man opened the letter. 那个人很紧张地拆开了信。