You can use an article for more than just content. Check to see:
The first two sources consistently offer "cited by" information, the others offers it sometimes.
If you have an article citation (or title) and want to get to the full-text, consider using Google Scholar Advanced Search to search by article title and then using the Check Simmons Full-Text link to get to the full article.
It pays to learn how to execute articulated ("smart") searches in GS, especially for systematic reviews and meta-analyses. (In addition to a MeSH-enhanced PubMed/Medline Search.
insubject:"heart rate variability" intitle:("systematic review" | meta-analysis)
or the somewhat more permissive relaxed smart search:
insubject:"heart rate variability" intext:("review" | meta-analysis)
leveraging the power of the scope qualifiers "insubject", "intitle" and "intext" when coupled with appropriate Boolean operators.
It also pays to remember that GS is an "opportunistic" search engine, as it will try to data-mine any resources that could be of relevance rather than honing to the more narrow constraints of a formal PUBMED search (often providing riches not otherwise easily uncovered, so that its claimed lesser precision is not at all necessarily a disadvantage, as some of the discovered resources (like dissertations, commissioned monographs, peer-reviewed CMEs, etc.) could themselves - as I have often found - contain bibliographical references to invaluable materials not located through PUBMED, that could greatly enrich the quality of any paper using its technology.