In many cases, companies have used debt to repurchase their own stock, flattering their bottom-line financial performance. While not all buybacks are financed by debt, Lapthorne did note a correlation between net repurchases and the change in corporate indebtedness.

"U.S. corporate balance sheets are a major risk going forward," he says. "U.S. corporates are massively overspending."

To be fair, servicing this debt load isn't as onerous as it might appear, because of low interest rates. And despite the recent steepening of corporations' yield curve, companies have continued to extend duration, which offers them more certainty about what their interest payments will be over the long term.

"For corporate credit, there's very little concern about short-term coverage from the market," analysts at Bespoke Investment Group wrote. "We note that maturities continue to creep up slowly; despite higher spread costs, corporates are generally borrowing further out the curve and 'locking' low rates."

But over the long haul, the performance of stock markets will be primarily driven by earnings increases—and the level of corporate indebtedness implies that any latitude to boost earnings per share by shrinking the denominator is limited.

Corporate profits in the U.S., meanwhile, have declined for five consecutive quarters. As Bloomberg's Matthew Boesler reminds us, the combination of near-record corporate debt-to-GDP, record low return on equity, ever-higher labor costs, and subdued pricing power doesn't paint an inspiring picture for growth.

U.S. profitability is on a "cyclical downtrend," Lapthorne concludes.

Bloomberg News