Male labor force participation has been steadily dropping since 1950. This was masked by the entry of women into the labor force between 1950 and 1990. But that process has been saturated for thirty years, whereas male participation keeps failing. That means that absent a major shift, the overall participation rate will continue to decline.

The point is this is a half century trend, much too long to have anything to do with the business cycle. Yes labor force participation today is much lower than the expansions of yesteryear, but that doesn't tell us anything particular about this cycle. Almost assuredly 10-20 years from today labor force participation will be even lower than it is today, even at the peak of the next expansion.

[1] https://fred.stlouisfed.org/series/LNS11300001 [2] https://fred.stlouisfed.org/series/LNS11300002