A question I often get is "do you have to be strong to build muscle?" The simple answer is "no" and here's why...
There's not many that will argue against the fact that a curvy, lean, muscular body is a desired appearance as a result of physical activity. In fact, this is the number one goal of most people who begin and adhere to any fitness program out there. Building muscle doesn't just happen during any sort of physical activity, there is an actual method to the process...and this is where most people fall short or get confused.
There are 2 directions I see most people go when trying to get lean and put on good muscle, they go the extreme "cardio" route or they go the "lift super heavy" route. While these aren't bad tools to use on your way to reaching your goals, neither should be your primary method for achieving that "toned" look (if that's what your after).
Cardio can help shed fat. Fat is usually what is hiding our muscle. So eliminating cardio all together isn't a good decision. Cardio has it's place, but instead of steady state cardio (all the time), incorporate a method that is slightly more effective...I refer to it as "conditioning" or "metabolic conditioning" Metabolic Conditioning typically will use a type(s) of resistance as part of it's protocol. For example, swing a heavy kettlebell for as many reps as possible in 3-5 minutes. Use battle ropes for 30s work/rest intervals. Using some sort of resistance to increase your heart rate will help preserve muscle as well as burn fat in the recovery stages. Typically when using a resistance for "conditioning" you will use something lighter than normal (considering the intensity). So, as you can see, you do not have to be very "strong" in this instance to improve your body composition.
*Conditioning is best done as a "finisher" to a typical resistance training workout, adding it in during the last 10 minutes or so.
Lifting "maximal" or close to maximal loads (super heavy) can and will get you stronger, so if that's your goal, then by all means push those heavy weights. It also comes with a high risk, high reward tag attached. Lifting close to your "max" is great for increasing the neural functions of your body. It teaches us how to control and handle objects/loads that we normally wouldn't handle very well in everyday circumstances...but it also can be the cause of severe injury, which is why it's high risk/reward. As a result of lifting so close to our max loads, very few reps are done overall, the body can only handle so much of this maximal effort before breakdown. Since your total volume (overall reps) are much lower, your muscles don't receive the stimulus they need to "grow", only the stimulus they need to get "stronger"...resulting in much more strength than muscle size (which won't really change the way you look physically).
A better option is to work on sets involving mid-range reps/weights that you can fully control and puts the muscle under tension for longer periods of time. So a normal set of "strength" would be 5x5, and a normal set of "hypertrophy" (mid-range reps/weight) would be 3-4 sets x (8-12 reps). This allows you to control your tempo and isolate the targeted area much more effectively than you would with 5x5 or 8x3. This is not to say you can't get "stronger" with hypertrophy sets, as you can always slightly progress in the weights you use, as long as you reach the necessary rep range with your controlled tempo. Putting an isolated and controlled focus on the targeted muscle helps that muscle to grow, and growing a muscle gives it better shape, and better shape gives you a body that appears lean and "toned." So the main idea is to not lift "heavy" but lift with focus and with a weight that isolates/challenges the targeted muscle. This is most effectively done by doing at least 3-4 different exercises for the same muscle, with set/rep ranges of 3-4 sets and 8-12 reps.
- Stready-State Cardio will burn fat, but also muscle...if your goal is to have lean muscle and look "toned", this is not the method you want to use exclusively.
- Use "Conditioning" instead. Using some sort of resistance to increase your heart rate will help preserve muscle as well as burn fat (mostly in the recovery stages).
- Lifting "super heavy" exclusively will help get you stronger, but very rarely help build a lean, toned body. It's more a neural improvement than a muscle one.
- Hypertrophy is the best method to build muscle and look lean. Sets of 3-4 in the range of 8-12+ reps. Always control the tempo of the weight and make sure you can isolate the targeted muscle.