In a very fast paced weight training session, you may burn 600 calories per hour. At that pace, about half of your caloric needs are being met by glycogen. Even a high fat (non-ketogenic) dieter can’t get more than about 360 calories an hour from fat burning, so at least 240 calories (60 grams) of their fuel would come from muscle glycogen. Low carb dieters also have comparably less muscle glycogen.
A high carb dieter may have 500 grams of muscle glycogen and burn through about 75 grams of it in that intense hour, so that is about 15% of your muscle glycogen stores, and if you could theoretically train just 25% of your muscle mass and maintain that 600 calorie per hour level, then those trained muscles would be about 60% depleted of muscle glycogen in an hour.
If you up the intensity to 900 calories per hour, since beta oxidation of fat can’t go higher, now you are burning about 150 grams of muscle glycogen in an hour. All calorie burning above about 600 calories an hour has to come from glycogen or ketones because beta oxidation of fat is basically topped out at around 300-350 cals/hour for a standardized 160 pound man. So an hour of running is going to deplete muscle glycogen by about 30% (150 of 500 grams).
If glycogen did not get depleted, then you wouldn’t ever see people lose more than 1.5-2.0 pounds in the first week of a diet, but people exceed this all of the time. On a 1000 calorie deficit, whether from exercise or restriction, your muscle glycogen levels will be a fraction of normal levels within a week, and your muscles can lose 3-5 pounds of glycogen+water from muscles.