Biceps femoris long-head motor unit discharge rates and recruitment thresholds vary across 9 weeks of eccentric training

Increases in strength are mainly due to changes in how the nervous system activates muscles, while increases in muscle size are mainly due to the total amount of training performed within a repetition range of 4 to 30 reps.

After nine weeks of Nordic hamstring exercises, recreationally active men showed a 17% increase in knee flexor strength, with noticeable improvements starting at three weeks, indicating a gradual improvement in muscle force production.

Performing Nordic hamstring exercises for three weeks leads to a 6% increase in the rate at which nerve signals activate the biceps femoris muscle, but this increase stops after nine weeks, suggesting that initial strength improvements are driven by nervous system changes that become less influential over time.

Performing Nordic hamstring exercises over several weeks leads to a measurable increase in the amount of force needed to activate muscle motor units, with greater increases seen after longer training periods.

After nine weeks of Nordic hamstring exercises, the force required to stop muscle fibers from firing increases by 12%, but no change is seen after three weeks, suggesting that the nervous system's control of muscle activation adapts slowly over time.

After nine weeks of Nordic hamstring training, the nervous system activates muscle fibers at higher force levels, and the firing rate of those fibers returns to normal, suggesting that muscle structure changes become more important than nervous system changes for gaining strength.