Most people begin an exercise program with excitement. Often they have lofty goals and great expectations but that may end up being the very reason they don't stick with it. They tend to overdo instead of taking the slow but sure route and this can lead to burnout and injuries which slows progress even more.
If you're just beginning a fitness routine, some soreness can be expected. However, back pain and knee injuries are common complaints among those who are not accustomed to working out. Some exercises that, at one time, were common have been found to be harmful for some people. Exercises that involve bouncy, jerky motions or swinging momentum can tear muscles, ligaments or tendons, and should be avoided.
Any exercise that involves deep knee bends places strain on the ligaments and cartilage in the knee joint and should be avoided. Double leg lifts while lying flat on the back puts strain on the lower back ligaments and muscles. Straight leg sit ups, standing toe touches, windmills, feet over the head back stretches, waist circles or any hyperextension of the back can stress the discs and damage the spine, leading to debilitating back pain.
Choosing appropriate exercises for all the muscle groups and performing them in proper form will help prevent injury and move you steadily toward your goals. It's exciting to finally decide to make some changes for better health and it's easy to fall into the idea that "more is better" or "no pain, no gain" but patience and commitment are your friends. Take it slow and easy in the beginning to prevent sprains and torn ligaments or muscles. A good warm up of 3 to 5 minutes before any exercise routine will increase blood flow to muscles and also help prevent injury.