Once you’ve been following a fitness plan for a while, you’ll eventually hit a plateau—where your body adapts to your routine, and you no longer make progress. It sucks, but it’s normal, and it happens to everyone.

The only way to break through the plateau is simply to change things up. This is where lifting heavier weights, adding more reps, or doing both can help you progress.
If you consistently lift heavy weights at a low rep range, you will eventually run into a wall. You would reach a point where you simply can’t add any more weight, and if you push it, you could compromise your form and put yourself at risk of injury.

When you lift lighter weights for more reps, you are still getting stronger, just in a different way, through developing “muscular endurance”

Pushing out more reps is also a challenging workout at a high-intensity level, which burns major calories and has a greater afterburn effect than cardio alone. Side note: there is no such thing as a “light weight” unless warming up – the weight will be relative to the rep range so if I lift 2kg dumb bells and aim for 100 reps of side lateral raises – this is not light, I am keeping my delts under tension for well over 30 seconds, making slow twitch muscle fibres come into play and increasing my muscle endurance while getting a good “burn”.

You need a combination of muscle damage, mechanical tension and metabolic stress (that “burn”) to ultimately build strength. Both heavy-weight and high-rep training check those three boxes.

For long-term progress and to keep things interesting, you can incorporate both heavy-weight, low-rep training and light-weight, high-rep training by switching up the sets and reps on different days or weeks (a technique known as periodization).

Personally I incorporate both, and many lean, healthy and strong females I look up to also change up their routine a lot. When you lift more weight, add more reps, or do both appropriately with good form while keeping effort high, you’re nudging your body toward continually improved fitness and strength.