Human hedge funds are as bullish as their robot competitors -- raising alarm bells at JPMorgan Chase & Co.

As U.S. stocks flirt with all-time highs, evidence is building that fast-money investors have stretched positions with echoes of the January melt-up.

Hedge funds pursuing long-short equity strategies have record exposure to the S&P 500 Index, judging by the portion of their returns attributable to the benchmark, a measure known as beta.

Meanwhile, returns posted by risk-parity and statistical-arbitrage funds -- two types of quantitative investors -- are ever-more linked to U.S. shares, and near levels notched on the eve of the February correction, according to JPMorgan.

It all raises the danger that the next sell-off will amplified by quants and discretionary hedge funds beating a retreat.

“Our analysis suggests that institutional investor positions on U.S. equities are rather elevated and not far from last January’s levels,” JPMorgan strategists including Nikolaos Panigirtzoglou wrote in a note on Friday.

The fire-power of the investing cohort suggests caution ahead. Discretionary long-short equity managers alone oversee about $900 billion, according to estimates from the New York-based bank.

Risk-parity managers, meanwhile, who slice and dice exposures across assets to juggle volatility, are estimated to hold anything between $120 billion to $500 billion.

The other quantitative faction JPMorgan notes for their high exposures: relative value multi-strategy funds, which aim to arbitrage statistically significant relationships between various instruments.

Not all metrics are signalling extreme sentiment.

Some hedge-fund books tracked by prime brokers capture weaker risk appetite. Clients on Morgan Stanley’s platform have reduced net leverage to 49 percent, down from a peak of 60 percent in March.

And JPMorgan points to another source of comfort when tracking the quantitative cohort: Trend-followers’ equity exposures are relatively muted at around two-thirds of their January peak. Meanwhile, the mom and pop crowd are playing defense.

“August and September to-date have seen modest outflows from equity funds and inflows into bond funds, suggesting that retail investors retain a rather cautious stance,” the strategists wrote.

Still, with short interest on American stocks on the decline while volatility traders bank on subdued price swings, signs of U.S. equity-market 'vulnerability” are building, JPMorgan concludes.