Without strength training, up to 25% of your weight loss can come from lost muscle rather than fat. So if you have lost a lot of weight, it is not surprising that you will have lost some muscle as well.
While you can maintain existing muscle while losing weight through a program of ST, it is difficult to add muscle while running a calorie deficit, as the body tends to burn protein for energy rather than creating new muscle tissue.
Most bodybuilders deal with this through alternating cycles of 'cutting' (running a calorie deficit to lose fat) and 'building' (a calorie surplus to add muscle).
As for strength training, lift heavy - aim at a weight heavy enough to fatigue your muscles in 12 reps or less (preferrably even 4-8 reps).
The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.
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