The effectiveness of Nordic Hamstring Exercise for preventing hamstring re-injuries

Hamstring muscles are the most common anatomical site affected by injury in
sports, especially in high intensity and explosive sports i.e., athletics,
American football, rugby and football. Hamstring muscles have a high incidence
of re-injury Most re-injury occurred soon after Return to Play (RTP). A study
from Orchard et al (2020) showed that hamstring had the highest risk of
re-injury in the first match after RTP.22 The hamstring re-injury risk in the
first week was 9 % and the injury remained at elevated risk for 15 weeks. A
study from Brooks et al (2006) also showed that more than halve (59%) of
hamstring re-injury occurred within 1 month after RTP. Hamstring re-injury are
generally more severe compare to the initial injury (mean days lost : 25 vs 14
days).
Nordic hamstring exercise (NHE) program is an eccentric strengthening program
designed to prevent primary hamstring injury. A recent meta-analysis showed
that injury prevention programs that included the NHE could decrease the risk
of primary hamstring injuries by up to 51 % in football players. Not only in
football, but the evidence also suggests that NHE in isolation or as part of
the injury prevention program is effective for preventing hamstring injury
across multiple sports.
Although NHE is effective as primary hamstring injury prevention program, there
is still no high-quality study on hamstring re-injury prevention. Existing data
on re-injury risk reduction is limited to the study by Peterson. They showed
that a high dose NHE exercise program in pre-season followed by a once-weekly
low dose maintenance program during in-season effectively reduces the re-injury
rate by approximately 85% in football players that had sustained a previous
hamstring injury in the 12 months preceding the study. Although this indicates
that the NHE intervention is effective in reducing re-injury risk in the
subsequent season, the results cannot be generalized to a NHE intervention
following RTP after hamstring injury. Performing NHE directly following RTP
after a hamstring injury, the most vulnerable period for re-injuries is
different from the primarily pre-season program studied by Peterson et al.
Moreover, a NHE intervention after RTP is generally performed during in-season
period. Therefore, it remains unknown whether the NHE intervention directly
following RTP after hamstring injury can protect athletes for re-injuries.
Besides injury prevention aspect, evaluating NHE's effect on performance is
very important in this study. We need to know whether NHE may improve or reduce
performance. Enhancement of performance may help the adherence of the program,
but if the study shows the opposite result (reduce performance), the coach and
athletes can outweigh prevention vs. performance in their decision to use it or
not.
In summary, NHE exercises have proven to be effective as primary hamstring
injury prevention program. The evaluation of starting NHE directly after RTP
with the aim to prevent re- injuries (secondary prevention) has never been
done. In this study, we will primarily investigate the effectiveness of NHE as
a re-injury prevention program following RTP after hamstring injury. In a
sub-study we will evaluate the effect of NHE on physical performance
enhancement following RTP.

This study aims to investigate (1) the effectiveness of Nordic Hamstring
Exercise for preventing hamstring re-injuries (2) the effectiveness of Nordic
Hamstring Exercise for enhancing performance following Return to Play after
hamstring injury.

Monocenter, Randomized Controlled Trial (RCT) study in athletes at RTP
following hamstring injury. Randomization will be performed to determine
subjects who selected as the experiment (Nordics) and control (Control) group.

In this study, the subjects will be randomized to the Nordics group and Control
group. Participants in Nordics group perform the Nordic Hamstring Exercise
Program in addition to the normal routine training. The participants in Control
group will NOT carry out the program, but only their routine training.

The Nordics group perform 27 sessions of the hamstring exercise in a 10-week.
After 10 weeks, the exercise will conduct once a week until the end of the 12
months (52 weeks) follow-up. A total of intervention is 69 session.

The Nordic Hamstring Exercise:
Alternative 1 (with partner) : The subjects start in a kneeling position, with
his/her torso from the knees upward held rigid and straight. A partner applies
pressure to the athlete*s heels/lower legs to ensure that the feet stay in
contact with the ground throughout the movement. The subjects then attempt to
resist a forward-falling motion using his hamstring muscles to maximize loading
in the eccentric phase. The subjects are asked to break the forward fall for as
long as possible using the hamstrings. The subjects are asked to use their arms
and hands to buffer the fall, let the chest touch the surface, and immediately
get back to the starting position by pushing with their hands to minimize
loading in the concentric phase.

Alternative 2 (without partner):
The subjects are asked to hook the feet underneath the bottom of fixed
equipment (cable pull-down machine, a nautilus machine, or anything else that
has an opening a few inches off the ground). Remember to put something soft
under their knee. The subjects then attempt to resist a forward-falling motion
using his hamstring muscles to maximize loading in the eccentric phase. Break
the forward fall for as long as possible using the hamstrings. The subjects are
asked to use their arms and hands to buffer the fall, let the chest touch the
surface, and immediately get back to the starting position by pushing with
their hands to minimize loading in the concentric phase

Based on the current literature, there is no increased injury risk associated
with the study's prescribed exercises. In the initial stage, mild muscle
soreness may occur due to the high eccentric training load.
This study's benefit is that the participants can be involved in the hamstring
muscles' re-injury prevention program. The previous study showed that the
intervention (Nordic Hamstring Exercise) proved as primary injury prevention to
prevent acute hamstring injury.