Weekly versus monthly testosterone administration on fast and slow skeletal muscle fibers in older adult males

Context: In older adults, loss of mobility due to sarcopenia is exacerbated in men with low serum T. T replacement therapy is known to increase muscle mass and strength, but the effect of weekly (WK) vs monthly (MO) administration on specific fiber types is unknown. Objective: To determine the efficacy of WKvs MOT replacement on the size and functional capacity of individual fast and slow skeletal muscle fiber types. Design, Setting, and Patients: Subjects were randomized into a 5-month, double-blind, placebo-controlled trial. All subjects (ages, 61-71 y) were community-dwelling men who hadT levels < 500 ng/dL Intervention: Subjects were dosed weekly for 5 months, receiving continuous T(WK, n = 5; 100 mg T enanthate, im injection), monthly cycled T (MO, n = 7; alternating months of Tand placebo), or placebo (n = 7). Muscle biopsies of the vastus lateralis were obtained before and after treatment. Main Outcome Measures: Main outcomes for individual slow and fast fibers included fiber diameter, peak force (P₀), rate of tension development, maximal shortening velocity, peak power, and Ca²⁺ sensitivity. Results: Both treatments increased fiber diameter and peak power, with WK treatment 5-fold more effective than MO in increasing type I fiber P₀. WK effects on fiber diameter and force were 1.5-fold higher in slow fibers compared to fast fibers. In fasttype II fibers, diameter and P₀ increased similarly between treatments. The increased power was entirely due to increased fiber size and force. Conclusions: In conclusion, T replacement effects were fiber-type dependent, restricted to increases in cell size, P₀, and peak power, and dependent on the paradigm selected (WKvs MO).