This week we discuss microscopic changes to muscle architecture following eccentric hamstring exercise. Twelve recreationally active participants in their mid-20s performed 9 weeks of eccentric hamstring training via Nordics: 3 session per week, progressively building from 4x6 to 5x8 at the end. After the 9 weeks of training, they had a 3 week detraining period. Eccentric hamstring strength during Nordics, passive fascicle length, and sarcomere length were measured at baseline, end of training, and end of detraining. Serial sarcomere number was calculated using fascicle length and sarcomere length. After the training block, there was significantly increased biceps femoris fascicle length, serial sarcomere number, volume, and knee flexion torque. There was a large effect for all of these metrics. There was a significant regional effect for fascicle length and serial sarcomere number: increased fascicle length at distal region compared to central, but increased serial sarcomeres centrally versus distally. During detraining, fascicle length decreased but remained logner than pre-training lengths. Muscle volume stayed the same and there was a decrease in strength but it was higher than pre-training. Takeaways from this article include evidence for a long-term adaptation of eccentric training being addition of sarcomeres in series which could be protective against hamstring strain injuries, and muscle architectural responses to detraining happen quickly! It would be interesting to investigate if addition of sarcomeres in series is an effect specific to eccentric training, or any specific hamstring muscle training would create this effect.

The abstract can be found here: https://pubmed.ncbi.nlm.nih.gov/39461588/

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