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.

1 week is too early to consider your muscle repair totally healed.  The first stage of wound healing is approximately 6 weeks but I would not stress the muscles with heavy lifting for 3 months after muscle repair.  Of course discuss this in detail with your plastic surgeon..