How long before muscle repair is "secure" after a tummy tuck? 1 week post op.

Congratulations on having undergone the tummy tuck procedure. It will be several months before the muscle plication will be considered "secure".
In general, it would take a significant amount of pressure to “undo" the muscle repair after tummy tuck surgery. This event would likely be associated with significant discomfort. A change in your physical examination, such as increased swelling, bleeding, a new onset "bulge" would also be likely.
 In other words, generally speaking, abdominal wall muscle plication performed during tummy tuck surgery is quite secure; disruption is very unusual.
My best advice: return to your preoperative activity level in a very gradual/careful fashion, under the guidance of your plastic surgeon.  In the longer term, you should be able to return to full activity ( without concern for disruption of the abdominal wall plication), including workouts etc.
Best wishes for an outcome you will be very pleased with long-term.

I my practice patients may begin walking for exercise after two weeks; more strenuous cardio after one month; then after three months they may begin to work the abdominals gradually and low reps.
Theoretically complete healing takes six months to one year.
It would be unusual (though not impossible) to rip the muscle repair by coughing or vomiting.
I hope this helps.