This is a great time to strengthen your shoulders for summer sports and great posture. Shapely shoulders and a strong upper back allow you to stand tall, lift more and play without injury.

The deltoid muscle gives your shoulder its multidimensional shape: you see different sections of it at the front, top and back of the shoulder. And you use it all the time: when you lift your arms up in front of you, diagonally across your chest, out to the side and behind you.

Though the deltoid is involved in almost every movement of the shoulder, the rear segment of it, as well as the deeper muscles in the back of the shoulder and upper back, tends to be weaker than the front segment. We spend much of our time seated and reaching forward - typing and driving are two prime examples - which cause us to round our shoulders. This bad posture plus the day-to-day lifting can create this weakness in the rear deltoids and upper back.

This rear shoulder fly effectively targets the rear deltoid and upper back – but that's not all. Doing the exercise while you're leaning over a stability ball also challenges your core muscles (your back and abdominals). The stability ball actually creates instability underneath you, which means your core and torso muscles have to work to keep you aligned, making them stronger. If you've never used a stability ball this way before, wedge a weight or block against the front of the ball to keep it from rolling as you lie on it. Once you can balance on it easily, remove the weight or block.

If you're trying this move for the first time, use one- to three-pound (500 gram to 1.5 kilogram) dumbbells. Increase the weight when you can perform two or three sets of repetitions in a slow, controlled manner.

Rear Shoulder Fly
1. Kneel in front of the stability ball, then lean over and rest your chest over the top and your hips against the back of the ball. Keep your knees and toes on the floor about hip-distance apart. Hold a dumbbell in each hand, palms facing the floor.

2. Open your arms out to the sides, keeping your elbows slightly bent (about 45 degrees). To keep your back long and straight, lift the top of your chest off the ball, gently squeezing your shoulder blades together and lifting your abdominals up and away from the ball. Keep your eyes focused on the floor just ahead of you and keep your chin down.

3. Exhale as you lift your arms up and back until your wrists are almost parallel with your shoulders. Remember to squeeze your shoulder blades together toward your spine, without shrugging, as you lift.

4. Pause. Exhale as you slowly return to the starting position. Repeat. Don't relax over the ball until you finish all the recommended repetitions.

Repetitions and sets
Do eight to 12 repetitions, setting your pace according to your breathing. Work up to two to three sets of repetitions, resting for 30 seconds to 1 minute between sets. For a greater challenge, you can do any or all of the following: gradually increase the weight of the dumbbells, train every other day or push your knees off the floor and balance on your toes. To work your shoulders even harder, do the lift one arm at a time - as a bonus, you'll also be building all the muscles in your torso.

Buying the right-size stability ball
When choosing a stability ball, select the size that, when fully inflated, allows you to sit with the tops of your knees parallel to your hips. That generally means a 55 centimetre (21-1/2-inch) ball if you're under five-foot-seven or a 65- centimetre (25-1/2-inch) ball if you're over five-foot-seven. Look for good-quality burst-proof stability balls at fitness-equipment stores or online; prices usually start at about $35.

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