Hamstring strains are one of the most common injuries in team sports, and they lead to substantial amounts of lost playing and training time. They are also very prone to recurrence. Once an athlete has suffered one hamstring strain, they are much more likely to be injured again.
Consequently, strength coaches are often tasked with reducing the number of hamstring strains that their athletes incur.
The Nordic curl is a commonly-used exercise for preventing hamstring strains, and recent analysis suggests that it is very effective.
However, it is not clear exactly how the Nordic curl produces its beneficial effects.
As an eccentric exercise, it increases fascicle lengths, and short biceps femoris fascicles are a risk factor for hamstring strains. Changes in fascicle length could therefore be a key mechanism.
However, some conventional (eccentric-concentric) exercises can cause similar (or perhaps slightly smaller) increases in fascicle length. Yet, to date, these exercises have not been identified as having injury-prevention potential.
So why do some conventional (eccentric-concentric) exercises produce similar (or perhaps slightly smaller) changes in fascicle length to the eccentric-only Nordic hamstring curl?
It is often assumed that only eccentric-only training can increase fascicle length.
In reality, both eccentric loading and training at long muscle lengths can independently increase fascicle length.
Indeed, eccentric-only training at long muscle lengths produces even greater increases in fascicle length than eccentric-only training at a moderate muscle length. So eccentric-only loading and training at long muscle lengths are additive.
This dual mechanism for improving fascicle length probably explains why the (eccentric-only) Nordic curl, which produces a peak contraction at a moderate muscle length, produces similar changes in muscle fascicle length to the conventional (eccentric-concentric) 45-degree back extension, which produces a peak contraction at long muscle lengths.
However, things are perhaps not entirely this simple, as the 45-degree back extension is a hip extension exercise, while the Nordic curl is a knee flexion exercise.
So the 45-degree back extension probably also produces smaller mechanical loading on the hamstrings, because it shares some of the work of hip extension between the hamstrings and the other hip extensors, including the adductor magnus and the gluteus maximus.
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