Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

Investors can expect a strong start to the stock market today, as the Dow Jones Industrial Average (DJINDICES:^DJI) gained 56 points, or 0.4%, in premarket trading. World markets rose in overnight trading, with Europe's index up by almost 1%. Today's main event will be new Federal Reserve Chairwoman Janet Yellen's testimony on Capitol Hill, which is scheduled to begin at 1 a.m EST.

Meanwhile, news is breaking this morning on several stocks that could see heavy trading in today's session, including Boeing (NYSE:BA), Sprint (NYSE:S), and CVS Caremark (NYSE:CVS).

Boeing is struggling to keep up the pace of production of its 787 Dreamliner jets, according to Reuters. The news service this morning quoted several factory employees who described assembly bottlenecks that threaten the 10-per-month production rate that Boeing recently achieved for the aircraft. Any slowdown could crimp the industrial giant's revenue growth, since increased production rates were a key driver behind last year's record results. And Boeing needs to keep those factories humming if it expects to make a dent in its $500 billion worth of backlog. Boeing's stock is unchanged in premarket trading.

Sprint this morning posted surprisingly strong fourth-quarter sales growth: Revenue jumped to $9.1 billion, well ahead of the $8.7 billion that Wall Street expected. That sales boost helped operating loss improve by 22% in the quarter to reach $576 million. Sprint managed a few notable business wins over the last three months, including the addition of 58,000 new wireless subscribers. The company is now focused on rolling out its new high-speed wireless network, with plans to cover more than 100 million Americans by the end of this year. The stock is up 4.2% in premarket trading.

Finally, CVS Caremark today posted fourth-quarter earnings results that included a 4.6% boost in revenue to $32.8 billion. Adjusted profit jumped by 15.8% to $1.12 a share. The pharmacy operator benefited from increased drug prices and from a rise in client traffic. Looking ahead, CEO Larry Merlo said CVS is well positioned to capitalize on the major changes under way in the health care landscape; what the company calls the "retailization of health care." As for the near future, CVS projected earnings of about $4.45 a share in 2014, slightly below analysts' expectations. The stock is up 0.8% in premarket trading.

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