Some would argue to muscle never uses glucose and only fat, whether under or above aerobic threshold.
From The Bear who's lived at least 50 years on zero-carb:
"Two studies relate to energy source and exercise, both were done with lab rats. One shows by muscle biopsy/analysis that the glycogen in muscle tissue does not vary with exercise, whether anaerobic or anaerobic. The other shows massive gains in endurance over a sufficiently long period of adaptation to a zero carb diet. This study is one of the very few to use a true zero carb diet against a normal rat-chow control in a test."
"Rapid intense effort (anaerobic) uses ATP, which degrades with muscular contraction into ADP. ADP is reconverted into ATP by a mechanism fueled by fatty acids complexed with n-acetyl carnitine. No carbohydrate is involved. Aerobic activity is fueled the same way."
"I am carb free, I exercise anaerobically by lifting weights. I NEVER get sore muscles the next day- or the day after unless I am forcing a severe weight overload, which causes more severe micro damage to the muscles, and some second-day soreness. So there's the 'practical' proof that glycogen does not deplete with exercise."
"You can train for endurance (aerobic) or strength (anaerobic) Each uses a different modification of the basic muscle fibre. Each person has a mix of type 1 and type 2, or fast twitch, slow twitch. If you train as a marathoner, you suppress one and enhance the other- if you train as a sprinter, the reverse happens. It is an either/or situation. You cannot be at the same time both a marathoner and a sprinter- there is no free lunch. ALL muscle fibres use the same fuel, fast and slow both burn only fat. NO muscle fibre uses carbs. (Once more) glycogen is not used to do work, only ATP-ADP reduction is used, that is rebuilt by fat. Glycogen does not produce ATP. Glycogen is not depleted with exercise- this is proven and is in the literature. Opinions to the contrary are just repeating fairy tales from the past. Carb intake reduces strength/speed, and likewise reduces endurance/distance."
"Muscle cells 'run' on ATP-ADP conversion. ADP-ATP re-conversion is done with FFA's. It makes no difference whether the exercise is anaerobic or aerobic, the muscles still work the same way. This is like a car- whether in first gear or in overdrive, it still uses the same fuel. Glucose is not a fuel. Glycogen is not a fuel. Neither can be used to translate ADP back into ATP until converted into FFA first. Mitochondria mediate ADP to ATP conversion which is why there are two 'types' (fast and slow twitch) of muscle cells, fibre bundles with more mitochondria have a different response to the two types of work, aerobic and anaerobic. The mix of types in a given muscle can be altered to some extent by training."
"I am a very active type and back when I was dancing, trust me, any problems with either aerobic or anaerobic exorcise from zero carbs would have been obvious and serious. In fact, I had more energy, more endurance and better control of my body (at my level of skill) than anyone else in the classes I took. I ride a bike hard up the hills and down, and lift weights..."
"Muscle glycogen (or liver glycogen, or ketones) is NEVER used as a 'fuel' for muscles- either in doing aerobic OR in anaerobic work. The glycogen is only there as storage for quick adjustment of blood sugar levels, and in a zero-carb, keto-adapted diet usually does not vary. Please note, carefully: Muscle contraction (i.e.-the standard skeletal 'motor' of the body) is 'fueled' by ATP-ADP conversion. ADP is re-converted to ATP ONLY by a process which uses FFA's. Properly controlled tests have indicated that muscle glycogen is never 'depleted' during exercise."
You can email him directly at the-bear@thebear.org