I don’t want them them to be too strong, just a little stronger, firmer and more sporty.

ok, here’s the story:

There are two types of skeletal muscle: fast-twitch and slow twitch. There are actually a couple of different types of fast-twitch, but that doesn’t really matter for this discussion.

Slow-twitch muscle fiber is small and slow, but is great for endurance. Endurance athletes (marathon runners, tri-athletes…) have highly developed slow-twitch muscle fiber. You can develop this type of muscle by walking, running (generally over 400 meters), or weight-training with weight that you can perform 15 reps or more.

Fast-twitch muscle fiber is the only muscle fiber that grows in size significantly, is fast, explosive, but fatigues much faster than slow-twitch. This type of muscle can be developed by sprinting, plyometrics, weight training with a weight that you can perform less than 15 reps with (4-12 is typically the best). Walking, long distance running, weight-training with weight that you can do 15 reps or more with, will do very little or nothing for developing strength and speed. Remember: SPEED IS KING.

Most sports rely on fast-twitch muscle fiber and you want to gain some strength and you question suggest that you play sports. So, I suggest something along the lines of:

1>weight-training once/week with a weight that allows you to perform 8-12 reps.
2>Performing “long” sprints (400 meters) once or twice per week.
3>Performing “short sprints (50-100 meters) once or twice per week.
4>Running stadiums once per week.

The three best exercises for your legs are squats, leg press, leg extensions.

1>performing a three-quarter squat will recruit more muscle fiber in your legs (quadriceps) than any other exercise, BUT it does very little for your gluteus maximus. Performing a “full” squat (going down far enough so that your leg passes below 90-degrees) works your quads just about as well and is one of the best exercises for your glutes. There was a recent study done by an NFL strength coach relation knee injuries for 3/4 squats to deep, below parallel squats. He found the deep squats reduced every type of knee injury. If some one tells you that squats are bad for your knees, stop listening to that person – it is a myth perpetuated by people who do not know how to squat.
2>Leg press works the quads just as well as a 3/4 squat, but does almost nothing for your glutes
3>Leg extensions used to be the favorite exercise of doctors and physical therapists, but not any longer. Leg extensions are bad for you knees – stay away from them. One of the many problems is that the quadriceps (muscle on the front of you leg) and your hamstrings (muscle on the back of your upper leg) were designed to work together. Leg extensions eliminate the hamstrings – causing instability in the knee and possible patella (knee cap) misalignment problems.

DON’T FORGET TO TRAIN YOUR HAMSTRINGS
Athletes who do not weight-train often have hamstring and knee problems because their quads are developed, but their hamstrings are not. This causes a muscle imbalance that can cause patella misalignment, patella tracking problems, and hamstrings are not strong enough to deal with the stress placed on the leg by the highly-developed quads.

Leg curls, stiff legged deadlifts, and glute/ham raises are great for your hamstrings.

It is impossible to really know what volume of work will work best for you because everyone adapts/recovers from workouts at a different rate. Age, gender, diet, sleep…all effect how your body will respond. When weight-training, never sacrifice “form” for “weight.”