At what point during my workout set will extra carnosine concentrations exert their strongest effects?

Boosting carnosine levels with beta-alanine is effective at all points during your set, whether you’re lifting heavy or doing endurance work, but will exert it’s largest benefits in our energy system called glycolysis. Your body uses three energy systems to perform work: the ATP-PC system(think creatine), which is primarily used during heavy lifting and for sets up into the 5-6 rep range; the glycolytic system(think beta-alanine), which is predominantly used roughly within the 7-15 rep range and up; and the oxidative/fat system, which is used primarily in endurance training. Our energy systems are utilized simultaneously; however, depending on the level of intensity or duration of exercise and fitness levels of the individual, certain energy systems will become more dominant in producing energy needed for that activity. Anybody who trains with weights will primarily use the first two systems and, in both cases, the build-up of hydrogen ions will contribute to fatigue in both systems, especially glycolysis.

The 7-15 rep range is where creatine falls short and beta-alanine is strongest.

Creatine is mostly effective in the ATP-PC system, which relies on stored ATP and re-synthesis using phosphocreatine (PC) for intense, high-energy contractions. Taking creatine will help your explosive strength but it likely won’t help you as much as beta-alanine will in the 7-15 rep range. As anyone trying to build bigger muscles knows, you must train in both low (1-6) and moderate to high (7-15) rep ranges to maximize muscle mass gains. Beta-alanine, by increasing carnosine concentrations, can buffer/fight the H+ build-up that occurs in both these ranges, but is most effective in the moderate to high(7-15) rep ranges. On the contrary, creatine is better suited than beta-alanine when lower reps ranges are used(5-6).

Decreasing cellular fatigue is an additional strength of beta-alanine. A recent study demonstrates that beta-alanine outperformed creatine in decreasing cellular fatigue, giving it yet another advantage over what has been considered the most effective sport supplement of the last decade.