The simple answer is that most patients needing a tummy tuck have a degree of muscle separation (diastasis) that should be repaired, but there are some that do not. It is usually related to pregnancy, so weight loss patients without prior pregnancy are less likely to need it. Sometimes it is only the lower muscles that separate, and if there is a C-section then they might repaired as part of the routine closure. Overall I do a muscle repair in about 90% of patients but if it is not needed then the recovery is quicker.
Patients that have had children and wish to have a Tummy tuck will have the muscles tightened and the excess skin removed. If a patient is only wanting to have the skin excised and not the muscles repaired this would be referred to as a Mini Tummy Tuck. Visiting with a Board Certified Plastic Surgeon that can examine and advise you on the procedure and what would give you the result and outcome you are looking for. Good luck.