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Editorial
Hamstrings are most susceptible
to injury during the early stance
phase of sprinting
John W Orchard
The fi rst publications which considered
the time of onset in the gait cycle for
hamstring strains concluded that early
stance was the highest risk period.1,2 The
rationale proposed was that external joint
moments were much higher in stance
phases than swing (table 1) because of
the presence of high hip and knee joint
reaction forces secondary to the ground
reaction force (GRF). 3–6 Ralph Mann’s
original argument has been rejected
by many subsequent authors, perhaps
because of the dogma that muscles probably strain during eccentric contractions
(table 1). This is a widely held belief
despite experimental muscle strains
being able to be produced during concentric (shortening) contractions.7
Table 1 shows that the high knee flexion moment (sum of angular forces) in
late swing occurs because the hamstrings
are highly active and angular forces in
the opposite direction (ie, due to quadriceps activity and external forces) are
minimal. The hamstring does work hard
in late swing to reverse the inertia of the
shank angular movement in the opposite
direction, but inertia (momentum) itself
is not a force. However, in early stance
the hamstring is also working hard
but against potentially large opposing
forces.1–4,6
A ‘smoking gun’ video of a case of
calf strain has shown the exact time of
disruption of gastrocnemius fibres during late stance/push-off, 8 which is conveniently a stance phase but also when
the calf is contracting eccentrically.
No such direct evidence is available for
hamstring strains. Cases which claim to
show a hamstring strain occurring in late
swing 9,10 do so using indirect evidence,
looking at change in body mechanics (athlete reaction) to a strain and speculating
when the injury occurred prior to the fi rst
available indication. For example, in the
Heiderscheit et al case, the fi rst indication
of change of mechanics is actually seen
Correspondence to John W Orchard, School of
Public Health, University of Sydney, Sydney 2006,
NSW, Australia; johnorchard@msn.com.au
88
in the mid-stance phase.9 The authors
conclude, based on their assumptions
of neuromuscular delay that the injury
‘probably’ occurred during late swing,
although the range of possible times for
actual injury they give includes early
stance. This uncertainty over length of
neuromuscular delay means that we are
back to speculation and personal (expert)
opinion about when the muscle strain
occurred in this case.
Just as there is no conclusive video
showing the time that actual fibres tear
in a hamstring strain, there have been
no studies measuring the in vivo forces
of hamstring muscles or tendons in
human sprinting. However, there have
been studies directly measuring in vivo
forces in Achilles tendon in sprinting11
and patellar tendon in jumping12 and
hopping.13 In these studies, the actual in
vivo forces in Achilles and patella tendon are far higher in the stance phases
(of sprinting, jumping and hopping) than
in the swing phases.11,12 In addition, the
in vivo force in the patella tendon during
hopping increases slightly in the early
concentric phase compared with the late
eccentric phase.13 In modeling for running injuries, it is considered that the
stance phase is when the vast majority
of forces must be absorbed by tissues,14
which is not surprising when one considers that a human leg is about 10% of body
weight whereas GRFs during sprinting
are more than 300% of body weight.
One could concede that the hamstring
muscles’ job starts in late swing and its
failure to do its job properly in this phase
might result in a strain. However, it is
erroneous to assume that the job failure and the injury are the same thing. A
faulty parachute fails to activate in midair, but the subject dies upon hitting the
ground. A gunshot kills when the bullet
hits the target, not when the trigger is
pulled. Eccentric strengthening may help
prevent hamstring strains but it does not
necessarily mean the force(s) causing
injury are acting in this phase.
That the GRF is the likely culprit for
hamstring failure in sprinting (as it is
for other tissue failure in other running
injuries)14 is further illustrated by the
low risk of muscle strains during openchain (only) activities. Upper limb athletes performing open-chain activities
(and hence with far less external force/
GRF involvement) do not tend to strain
muscles despite high angular velocities
in the upper limb joints. Ten-metre platform divers also do not tend to strain
hamstring muscles in the pike position
despite stretch and high angular velocity.
Neither do trapeze artists or aerial skiers mid-jump. Water skiers do—but only
when they are holding the rope when the
boat is pulling in the other direction and
providing the opposing force. It is likely
that it is opposing forces (which are greatest secondary to external moments) that
lead to muscle strains. These are potentially much greater in early stance than
late swing (table 1, items 3–6).
Although now in the minority, there
are still researchers who support the
original thesis of Ralph Mann1–6: it is
a strong force (moment, torque) in the
opposite direction(s) (causing hip flexion
and knee extension) that strains a hamstring, which is most likely to be due to
the GRF. If this is happening in vivo in
human sprinting when the hamstring
is contracting concentrically, then so be
it. Although this would surprise some
authors, it probably should not as experimental strains in the laboratory can be
created during concentric contractions7
and in vivo musculotendinous forces
may be equal or even higher in the early
concentric phase compared with the late
eccentric phase of contractions.13
Competing interests None.
Provenance and peer review Not commissioned;
externally peer reviewed.
Accepted 14 August 2011
Published Online First 18 September 2011
Br J Sports Med 2012;46:88–89.
doi:10.1136/bjsports-2011-090127
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Br J Sports Med February 2012 Vol 46 No 2
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Editorial
Table 1 Comparison of hamstring and opposing forces in late swing and early stance
Opposing moments
Phase
Hamstring action and moments
Internal (quadriceps and hip flexors)
External
Late swing
Strong eccentric contraction (whilst at submaximal length) to decelerate the shank,
producing in particular a high knee flexion
moment.
Vastus muscles begin to activate at the end of
swing producing a weak (early) contraction.15
Hip flexors are almost inactive.15
Minimal opposing forces (the light force of air
resistance would actually assist hamstring in
decelerating the shank).
Early stance
Strong concentric contraction to resist
opposing forces and help produce a hip extensor
moment.1–4,6
Vastus muscles increase their activation in early
stance, producing a stronger contraction, whilst
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the knee and hip joints, due to forefoot-strike).1–4,6
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89
Downloaded from bjsm.bmj.com on September 5, 2012 - Published by group.bmj.com
Hamstrings are most susceptible to injury
during the early stance phase of sprinting
John W Orchard
Br J Sports Med 2012 46: 88-89 originally published online September
18, 2011
doi: 10.1136/bjsports-2011-090127
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