What Muscles to Strengthen
As you embark on your strength-training program, turn your attention at the outset to the part of your body that probably needs it most — your hips. It doesn't come naturally to people to exercise muscles they rarely think about, so this part of your body is most likely to be neglected. Here is a list of muscle groups to work on:
The glutes — the gluteus maximus (buttocks), medius (pelvis), and minimus (hips)
Hamstrings, quadriceps, and calf muscles
Pectorals (minor and major) and rhomboids (the two sets of muscles balance each other)
Biceps, triceps, and deltoids
If you don't want to use weights or other exercises to strengthen your quadriceps (thigh muscles), there is an alternative. Running and riding hills is a great way to get stronger quads, and there is a side benefit to doing so. Strong quads help stabilize the knees.
These are all outside your core, but they are very important to your strengthening program. One of the best exercises you can do is the leg press. For that, you do need a machine. The leg press strengthens your quadriceps, hamstrings, and gluteus maximus — all important to your balance and stability.
That list of muscles to train might make it seem as though you will be spending hours getting stronger. Don't forget that many of the exercises train muscles in groups. You can actually get the job done with four or five exercises in a relatively short time span.

