Even those who forced a smile during an unpleasant task - seemed to have lower stress levels, - according to a Web Md article.

本句三个意群，我用连字符分开。第一个意群是主语those后接限制性定语从句，限制性定语从句必须和先行词一气呵成。第二个意群是谓语加宾语，seemed和不定式构成合成谓语，及物动词和宾语属于同一个意群。第三个意群是介词短语作状语，介词和介词宾语属于同一个意群。