I've watched people doing stretching exercises with fitness trainers. How does this help anybody lose weight?

Sedentary individuals are prone to low back pain (LBP) that makes regular exercise difficult, if not impossible. A person who cannot exercise cannot lose weight.

A frequent cause of LBP is poor posture due to tight hamstrings, hip flexors and calf muscles. Stretching these muscles on a regular basis will help improve posture, eliminate LBP and prevent future "flare-ups" of LBP. Stretching should be done at least 3 times per week (but could be done daily) after a thorough warm-up (a low intensity aerobic exercise) of at least 3-5 minutes. The warm-up increases muscle temperature, making the muscles more pliant for stretching. The stretch should be held to the point of mild tension or tightness, never to the point of pain.

There are many methods of stretching; all involve contracting the muscle opposite the muscle that is to be stretched. I personally prefer Active Isolated Stretching (AIS) and use it with all my clients. AIS does not require a warm-up (it is both a stretch and a warm-up). AIS differs from other stretching methods in that the stretch is held for only 2 seconds with multiple repetitions of the stretch. A 2 second stretch prevents the muscle from sensing that it is being forced into a stretch. A muscle sensing a forced stretch will contract rather than elongate which is the exact opposite of what we are trying to accomplish.