I'm about to return to the gym after a break of a year. I have weak knees and want to build them up, but I'm worried about making them worse. What exercises do you recommend?

Many people feel apprehensive about exercising with weak knees, and inappropriate exercise can exacerbate knee problems. But with the right exercise and techniques you can improve your knee condition. You should address this from three areas.

First, establish how your body wants to stand (for example, do you roll in on your ankles or stand knock- kneed?) because this can affect the impact your knees sustain as you move. Stand barefoot, hip distance apart. Now look down and draw an imaginary line through the midline of your kneecap down to your toes. With good alignment, this imaginary line should extend down over your second toe. If your knees or ankles tend to roll in, this line may be more over your big toe, which places more strain on knees, indicating possible fallen arches. Help strengthen the foot arch by rolling a pencil on the floor backwards and forwards with your bare toes. Do this 10-20 times with each foot, three to four times a week.

Second, focus on how you execute each exercise. Try to replicate this good-knee imaginary alignment on all exercises involving a weighted bend at the knee, such as lunges, step aerobics and squats. Make sure you can always see your toes and try to distribute more of your weight into the back leg in a lunge and in your heels as you perform a squat. If your knees roll in excessively, do not take cardio-step classes, and avoid flexing the knee beyond 90 degrees.

Third, strengthen inner-thigh muscles. Sit in a chair, place a small weight on the foot, rotate the foot out slightly from the hip, then straighten the leg at the knee and lower. Repeat 12 to 16 times, three times a week.

· Joanna Hall is a fitness expert. Send your exercise questions to: Weekend, 119 Farringdon Road, London EC1R 3ER (weekend@theguardian.com).