Bootstrap radio button “checked” flag

Assuming you want a default button checked.

<div class="row">
    <h1>Radio Group #2</h1>
    <label for="year" class="control-label input-group">Year</label>
    <div class="btn-group" data-toggle="buttons">
        <label class="btn btn-default">
            <input type="radio" name="year" value="2011">2011
        </label>
        <label class="btn btn-default">
            <input type="radio" name="year" value="2012">2012
        </label>
        <label class="btn btn-default active">
            <input type="radio" name="year" value="2013" checked="">2013
        </label>
    </div>
  </div>

Add the active class to the button (label tag) you want defaulted and checked="" to its input tag so it gets submitted in the form by default.