STOCK-BASED COMPENSATION

At September 25, 2019, options to purchase 2,302,225 shares of common stock were outstanding, including 1,746,164 vested and 556,061 unvested. Unvested options vest over time; however, upon a change in control, the board may accelerate vesting. At September 25, 2019, 1,381,010 premium options, options granted above the stock price at date of grant, remained outstanding. A summary of stock option activity as of September 25, 2019 and changes during the thirty-nine weeks ended September 25, 2019 is as follows:



	Shares		Weighted-Average Exercise Price
Outstanding - December 26, 2018	2,102,404		$	7.68
Grants	320,536		11.47
Exercised	(6,709	)	11.94
Forfeited, cancelled or expired	(114,006	)	13.35
Outstanding - September 25, 2019	2,302,225		$	7.91
Vested and expected to vest at September 25, 2019	2,289,725		$	7.90
Exercisable at September 25, 2019	1,746,164		$	6.81

The fair value of each stock option was estimated on the grant date using an exercise price of the closing stock price on the day prior to date of grant and the Black‑Scholes option-pricing model with the following weighted average assumptions:



	Thirty-Nine Weeks Ended September 25, 2019		Thirty-Nine Weeks Ended September 26, 2018
Expected volatility	28.7	%	28.4	%
Risk-free interest rate	2.3	%	2.9	%
Expected term (years)	6.25		6.25
Expected dividends	—		—

At September 25, 2019, the Company had total unrecognized compensation expense of $1.9 million related to unvested stock options, which it expects to recognize over a weighted-average period of 3.17 years.

A summary of restricted share activity as of September 25, 2019 and changes during the thirty-nine weeks ended September 25, 2019 is as follows:



	Shares		Weighted-Average Fair Value
Unvested shares at December 26, 2018	490,700		$	10.91
Granted	285,996		$	11.47
Released	(142,861	)	$	10.61
Forfeited, cancelled, or expired	(53,882	)	$	11.81
Unvested shares at September 25, 2019	579,953		$	11.17

Unvested shares at September 25, 2019, included 455,230 unvested restricted shares, 36,058 unvested performance stock units and 88,665 unvested restricted units.

At September 25, 2019, the Company had unrecognized compensation expense of $4.8 million related to unvested restricted shares, which it expects to recognize over a weighted-average period of 3.00 years, unrecognized compensation expense of $0.1 million related to performance stock units, which it expects to recognize over a weighted-average period of 3.62 years and unrecognized compensation expense of $0.9 million related to unvested restricted units, which it expects to recognize over a weighted-average period of 2.76 years.

Total stock-based compensation expense was $0.7 million and $1.8 million for the thirteen and thirty-nine weeks ended September 25, 2019, respectively, and $0.4 million and $1.5 million for the thirteen and thirty-nine weeks ended September 26, 2018, respectively.