Why You Should Skip Cycle Syncing
Fitness influencers promote cycle-syncing on social media, but it's not the "hack" some would have you believe.
We typically don’t see substantial changes in the male sex hormone profile until later in life, but the female sex hormone profile is much more dynamic; it fluctuates across each menstrual cycle and it is substantially influenced by pregnancy, the menopause transition, and the introduction of exogenous hormones (i.e., hormonal contraceptives or hormone therapy). “Cycle syncing” is based on the premise that the hormonal profile during one menstrual cycle phase is superior to that of the hormonal profile in another phase for exercise performance or exercise-induced adaptations. Cycle-synced training programs vary; some include two weeks of lifting followed by two weeks of running or cycling, while others include a week of yoga or light stretching. Proponents of cycle-syncing claim that the practice of aligning specific workouts to a particular hormone profile improves performance and training-induced adaptations (i.e. strength and muscle growth) despite the substantial reduction in overall training volume and frequency.
Does Estrogen Signaling Regulate Muscle Mass?
The basis for this claim stems from animal model research. Ovariectomy (surgical removal of the ovaries) is the gold standard model to evaluate the influence of ovarian hormone loss on various biological systems. However, surgical removal of the ovaries does not exclusively affect estrogen; it also depletes progesterone, LH, and FSH, which may influence the interpretation of how estrogen loss, per se, affects physiology. Given the interactions among the ovarian hormones, it is difficult to confidently identify a purely estrogen- or progesterone-specific mechanism using this model. Most importantly, with the exception of women with both ovaries surgically removed, the complete ablation of hormones is dissimilar to the natural human ovarian hormone profile during any lifestage, yet ovariectomy-derived findings are often cited as the foundation for the hypothesis that fluctuations in ovarian hormones may influence skeletal muscle adaptations.
The Menstrual Cycle
On average, the menstrual cycle spans 28 days and ovulation occurs mid-cycle (day 13 or 14). However, there is substantial variability between individuals and, in some cases, within the same individual from cycle to cycle (1). Average cycle lengths are often used to predict the timing of ovulation; however, this practice incorrectly assumes a correlation between cycle length and the timing of ovulation (2)