Figure 10.

Time course of muscle growth in response to a regular resistance training program (exercise of Fig. 8, every 3 days). (a) The total muscle load F is kept constant, so the force per titin f effectively diminishes as the CSA increases. (b) The force per titin f is maintained constant (20 pN, as discussed before), which effectively implies that the total muscle load F increases in proportion with CSA (vertical axis). Several curves for different values of the energy barrier ΔG₀ are labeled on the plot. As might be expected, muscle CSA changes are faster and greater in magnitude if the energy barrier ΔG₀ is smaller (i.e., TK opens faster during exercise, and signals to a greater extent). We overlay the predictions of our model with measurements of fractional changes in muscle CSA over an 8-week period, measured by De Freitas et al. (49) (red crosses, same values in both plots). An initial force per titin of 20 pN matches well with real data, showing an ∼1% growth per week. To see this figure in color, go online.