A biomechanical analysis of the single-leg Roman chair hold and the Nordic hamstring curl

Exercise has been proposed as an effective primary and secondary prevention
strategy for many diseases.1 A recent meta-analysis of different sports found
the strongest evidence for the health benefits of football and running.2
However, hamstring injuries are one of the most common injuries in sports that
involve high-speed running such as football and they have a high risk of
recurrence.3-5 Specifically, the incidence of hamstring injuries has been
reported to range from 20.3-36.9 per 1000h of match play3 6 and the re-injury
rate has been reported to range from 12-33%.7 Reducing the number of hamstring
injuries can result in greater health benefits from exercise2 8-10 and improve
sports performance.11-13
Traditionally, it has been assumed that there is an eccentric hamstring muscle
fiber action (i.e., the fibers lengthen while being activated) during the swing
phase of high-speed running.14 Since improvements in force production following
training are largely specific to the contraction type15, exercises thought to
produce an eccentric muscle fiber action have been used to specifically
condition the hamstrings for high-speed running and hereby prevent hamstring
injuries.16 Although several studies have shown eccentric training to be
effective in reducing the number of hamstring injuries17, the overall
hamstrings injury rate has not decreased in the last 30 years18, but even
increased by an average of 2.3% per year from 2001-2014 in professional
football.19
It has recently been argued that there is no eccentric, but an isometric
hamstring muscle fiber action during the swing phase of high-speed running.14
Additionally, the authors suggested that an inability of the muscle fibers to
remain isometric due to a lack of isometric strength could lead to hamstring
injuries. Isometric hamstring exercises were therefore hypothesized to be more
specific and hence more effective than eccentric exercises in strengthening the
hamstrings for high-speed running and preventing injuries.17 However, no study
has investigated whether isometric hamstring exercises are indeed more
effective at improving running performance and in preventing hamstring injuries
than eccentric exercises.
Exercises that require a high muscle activity and force production are likely
the most effective at conditioning the hamstrings and preventing hamstring
injuries. For example, loading intensity has been found to be the primarily
stimulus for molecular responses in both muscle and tendinous tissue20 21 and
the findings of a meta-analysis suggest that loading intensity is the primarily
stimulus for improving neuromuscular function in Achilles and patella
tendinopathy.22 Therefore, by investigating the muscle activity and muscle
forces elicited by an exercise, it can be determined which exercise has the
highest potential to elicit training adaptations.23 Since non-invasive direct
measurements of muscle forces during movement is not possible in humans, muscle
forces have to be estimated using other methods such as computational
modelling24-26 or supersonic shear imaging.27 Furthermore, since the hamstring
muscle fibers likely function isometric in their optimum length during the
swing phase of high-speed running14, an isometric exercise would likely have
the greatest transfer to performance and injury prevention.17 However, the
behavior of the muscle fascicles during most hamstring exercises is unknown.
For example, although the Nordic hamstring curl (NHC) is arguably the most
widely used *eccentric* exercise, it is actually unknown whether this exercise
really requires an eccentric muscle fiber action and whether the force measured
during this exercise does therefore indeed represent eccentric force.
Similarly, although the *isometric* singe-leg Roman chair hold (SLRCH) has
recently shown promising results in reducing a risk factor for hamstring
injuries28, it is unknown whether this exercise is indeed isometric.
Therefore, the aim of this study is to investigate which exercise is
theoretically most effective in improving high-speed running performance and
preventing hamstring injuries by comparing the NHC to the SLRCH on their
required hamstring muscle activity, force and fascicle length changes.
Male athletes that participate in running-based sports will be used as
participants since these individuals are most likely to experience a hamstring
injury and it is therefore important to know which exercise is most beneficial
to prevent hamstring injuries in these individuals. Furthermore, B-mode
ultrasound images of the hamstrings are more clear when these muscles are
better conditioned. Since the hamstrings are highly involved in running29, it
can be expected that individuals that participate in running-based sports have
better conditioned hamstrings compared to individuals not participating in
running-based sports. Finally, participants who have already some experience in
sports are likely faster familiarized with the exercise than individuals who
have no or very little experience in sports.

Primary Objective: Investigate wether fascicle length changes of the biceps
femoris long head are larger than the typical error during the NHC and the
SLRCH.

Secondary Objective(s): compare the NHC and the SLRCH on their required
hamstring muscle activity and muscle force.

This study will be a cross-sectional study. All tests will be performed within
approximately 3 months in the Gait Lab of the department of Human Movement
Sciences of Maastricht University.

Two experimental sessions of approximately 1 hour for the first session and 3
hours for the second session, with at least 3 and maximum 7 days between the
sessions. Additionally, the procedures are non-invasive and the exercises are
unlikely to lead to injuries. The only discomfort that some participants will
possibly experience is delayed onset of muscle soreness 24-48 hours after the
exercises. However, the number of repetitions will be small so the total amount
of muscle soreness will also be limited. The direct benefit for the
participants is that they will receive information on the strength of their
hamstrings. Furthermore, the research will benefit the population by providing
new information on hamstring injury prevention and performance improvement.
Traveling costs will be reimbursed for all participants (¤ 0.19 per km).