Hamstring muscle activation and morphology are significantly altered 1–6 years after anterior cruciate ligament reconstruction with semitendinosus graft

Abstract

Purpose
Harvest of the semitendinosus (ST) tendon for anterior cruciate ligament reconstruction (ACLR) causes persistent hypotrophy of this muscle even after a return to sport, although it is unclear if hamstring activation patterns are altered during eccentric exercise. It was hypothesised that in comparison with contralateral control limbs, limbs with previous ACLR involving ST grafts would display (i) deficits in ST activation during maximal eccentric exercise; (ii) smaller ST muscle volumes and anatomical cross-sectional areas (ACSAs); and (iii) lower eccentric knee flexor strength.

Methods
Fourteen athletes who had successfully returned to sport after unilateral ACLR involving ST tendon graft were recruited. Median time since surgery was 49 months (range 12–78 months). Participants underwent functional magnetic resonance imaging (MRI) of their thighs before and after the Nordic hamstring exercise (NHE) and percentage change in transverse (T2) relaxation time was used as an index of hamstring activation. Muscle volumes and ACSAs were determined from MRI and distal ST tendons were evaluated via ultrasound. Eccentric knee flexor strength was determined during the NHE.

Results
Exercise-induced T2 change was lower for ST muscles in surgical than control limbs (95% CI − 3.8 to − 16.0%). Both ST muscle volume (95% CI − 57.1 to − 104.7 cm³) and ACSA (95% CI − 1.9 to − 5.0 cm²) were markedly lower in surgical limbs. Semimembranosus (95% CI 5.5–14.0 cm³) and biceps femoris short head (95% CI 0.6–11.0 cm³) volumes were slightly higher in surgical limbs. No between-limb difference in eccentric knee flexor strength was observed (95% CI 33 N to − 74 N).

Conclusion
ST activation is significantly lower in surgical than control limbs during eccentric knee flexor exercise 1–6 years after ACLR with ST graft. Lower levels of ST activation may partially explain this muscle’s persistent hypotrophy post ACLR and have implications for the design of more effective rehabilitation programs.

Level of evidence
IV.

Full text

1 23
Knee Surgery, Sports Traumatology,
Arthroscopy
ISSN 0942-2056
Knee Surg Sports Traumatol Arthrosc
DOI 10.1007/s00167-019-05374-w
Hamstring muscle activation and
morphology are significantly altered 1–
6years after anterior cruciate ligament
reconstruction with semitendinosus graft
Daniel J.Messer, Anthony J.Shield,
Morgan D.Williams, Ryan G.Timmins
& Matthew N.Bourne

1 23
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Knee Surgery, Sports Traumatology, Arthroscopy
https://doi.org/10.1007/s00167-019-05374-w
KNEE
Hamstring muscle activation andmorphology are significantly
altered 1–6years afteranterior cruciate ligament reconstruction
withsemitendinosus graft
DanielJ.Messer1,2· AnthonyJ.Shield1,2· MorganD.Williams3· RyanG.Timmins4· MatthewN.Bourne5
Received: 26 March 2018 / Accepted: 24 January 2019
© European Society of Sports Traumatology, Knee Surgery, Arthroscopy (ESSKA) 2019
Abstract
Purpose Harvest of the semitendinosus (ST) tendon for anterior cruciate ligament reconstruction (ACLR) causes persistent
hypotrophy of this muscle even after a return to sport, although it is unclear if hamstring activation patterns are altered dur-
ing eccentric exercise. It was hypothesised that in comparison with contralateral control limbs, limbs with previous ACLR
involving ST grafts would display (i) deficits in ST activation during maximal eccentric exercise; (ii) smaller ST muscle
volumes and anatomical cross-sectional areas (ACSAs); and (iii) lower eccentric knee flexor strength.
Methods Fourteen athletes who had successfully returned to sport after unilateral ACLR involving ST tendon graft were
recruited. Median time since surgery was 49months (range 12–78months). Participants underwent functional magnetic
resonance imaging (MRI) of their thighs before and after the Nordic hamstring exercise (NHE) and percentage change in
transverse (T2) relaxation time was used as an index of hamstring activation. Muscle volumes and ACSAs were determined
from MRI and distal ST tendons were evaluated via ultrasound. Eccentric knee flexor strength was determined during the
NHE.
Results Exercise-induced T2 change was lower for ST muscles in surgical than control limbs (95% CI − 3.8 to − 16.0%).
Both ST muscle volume (95% CI − 57.1 to − 104.7cm3) and ACSA (95% CI − 1.9 to − 5.0cm2) were markedly lower in
surgical limbs. Semimembranosus (95% CI 5.5–14.0cm3) and biceps femoris short head (95% CI 0.6–11.0cm3) volumes
were slightly higher in surgical limbs. No between-limb difference in eccentric knee flexor strength was observed (95% CI
33N to − 74N).
Conclusion ST activation is significantly lower in surgical than control limbs during eccentric knee flexor exercise 1–6years
after ACLR with ST graft. Lower levels of ST activation may partially explain this muscle’s persistent hypotrophy post ACLR
and have implications for the design of more effective rehabilitation programs.
Level of evidence IV.
Keywords Imaging· Magnetic resonance· Physical therapy· Rehabilitation· Injury prevention
Electronic supplementary material The online version of this
article (https ://doi.org/10.1007/s0016 7-019-05374 -w) contains
supplementary material, which is available to authorized users.
* Anthony J. Shield
aj.shield@qut.edu.au
1 School ofExercise andNutrition Sciences, Institute
ofHealth andBiomedical Innovation, Queensland University
ofTechnology, Victoria Park Road, Kelvin Grove, Brisbane,
QLD, Australia
2 Institute ofHealth andBiomedical Innovation, Queensland
University ofTechnology, Brisbane, Australia
3 School ofHealth, Sport andProfessional Practice, Faculty
ofLife Sciences andEducation, University ofSouth Wales,
Wales, UK
4 School ofExercise Sciences, Australian Catholic University,
Melbourne, Australia
5 School ofAllied Health Sciences, Menzies Health Institute
Queensland, Griffith University, GoldCoast, QLD, Australia
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Introduction
Anterior cruciate ligament (ACL) ruptures are debilitating
injuries that can lead to chronic deficits in medial hamstring
volumes [13, 16, 23], knee flexor [13, 16, 23] and internal
rotator strength [13], and knee stability [1], at least some of
which may contribute to altered gait [1, 25] and early onset
of knee osteoarthritis [4]. ACL reconstruction (ACLR) sur-
gery is thought necessary to restore knee stability for sports
participation [20] and it often involves autografts from the
semitendinosus (ST), with or without the gracilis. How-
ever, despite the fact that ST tendons have been reported to
eventually regenerate and make attachments to the tibia or
other knee flexor muscle sheaths in a majority of cases [13,
16, 24], surgery typically results in long lasting ST muscle
hypotrophy along with the aforementioned strength deficits.
Persistent deficits in hamstring muscle size and strength
following ACLR with ST graft may be at least partly
explained by chronic neuromuscular inhibition of the donor
muscle. For example, medial hamstring surface electromyo-
graphic (sEMG) activity is diminished in limbs with pre-
vious ACLR during eccentric knee flexor exercise [3] and
hopping [8]. However, limitations in spatial resolution of
sEMG makes it impossible to determine whether only the ST
muscle activity has changed. Functional magnetic resonance
imaging (fMRI) offers a high resolution means of assessing
spatial patterns of muscle use during exercise [10], which,
as far as the authors are aware, has only once been employed
to examine the hamstrings after ACLR involving ST grafts
[24]. Takeda etal. [24] assessed hamstring muscle use after
concentric exercise for the knee flexors 7–32months after
surgery and reported almost identical ST muscle activation
between surgical and control limbs. However, neuromuscu-
lar inhibition of the hamstrings may be larger in supramaxi-
mal eccentric than concentric contractions [18] and fMRI
has never been applied to eccentric exercise after ST grafts.
The primary purpose of this investigation was to explore
the extent and pattern of hamstring muscle activation dur-
ing intense eccentric exercise in individuals with a previous
unilateral ACLR involving ST autograft. Secondary goals
were to examine hamstring muscle volumes and anatomi-
cal cross-sectional areas (ACSAs), ST muscle length and
eccentric knee flexor strength. It was hypothesised that in
comparison with contralateral control limbs, limbs with a
previous ACLR would display (i) deficits in ST activation
(according to T2 changes assessed via fMRI) during the
eccentric Nordic hamstring exercise (NHE); (ii) smaller ST
muscle volumes, ACSAs and lengths; and (iii) lower eccen-
tric knee flexor strength.
Materials andmethods
All participants provided written informed consent to par-
ticipate in this study, which was approved by the Queens-
land University of Technology Human Research Ethics
Committee (Approval Number: 1600000882). Fourteen
recreationally active participants (five men, mean age,
27.2 ± 4.0 years; mean height, 181.4 ± 3.2 cm; mean
body weight, 80.4 ± 6.1 kg; and nine women, mean age,
25.0 ± 5.3years; mean height, 168.9 ± 5.3cm; mean body
weight, 65.3 ± 12.5kg), with a history of unilateral ACLR
were recruited for this study. The median time since surgery
was 49months (range 12–78months) at the time of testing.
All had undergone rehabilitation under the supervision of a
qualified physiotherapist and had returned to their pre-injury
levels of training and competition. Inclusion criteria were:
(i) age between 18 and 35years, (ii) history of unilateral
ACLR autograft from the ipsilateral semitendinosus, and
(iii) ≥ 12months post-ACLR surgery. Exclusion criteria
were (i) any contraindications to MRI, (ii) complex knee
injuries with additional ligament surgery or meniscal injury,
and (iii) any history of a hamstring injury to the operated or
non-operated contralateral limb. Prior to testing, all partici-
pants completed a cardiovascular screening questionnaire to
ensure it was safe for them to exercise, and a standardised
MRI questionnaire to ensure it was safe for them to enter
the magnetic field.
Familiarization
Participants performed a familiarisation session of the NHE
at least 5days (range 5–12days) before experimental testing.
Upon arrival at the laboratory, participants were provided
with a demonstration of the NHE. Subsequently, participants
performed several practice repetitions (typically two sets
of five repetitions) whilst receiving verbal feedback from
investigators.
Experimental procedures
Upon arriving at the imaging facility, participants were
seated at rest for at least 15min before data collection.
Panoramic ultrasound images were then acquired for the
hamstrings on both limbs. Finally, participants underwent
an fMRI scan of their thighs before and immediately after
performing the NHE.
Exercise protocol andeccentric strength testing
Participants performed the NHE on a NordBord (Vald Per-
formance, Brisbane, Australia) as per previous studies [7,
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Knee Surgery, Sports Traumatology, Arthroscopy
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15]. Participants completed five sets of ten repetitions of
the NHE with 1-min rests between sets. During the rest
periods, participants lay prone to minimise activation of
the knee flexors. Investigators provided strong verbal sup-
port throughout the exercise session to encourage maximal
effort. All participants completed the 50 repetitions and
were returned to the scanner immediately following the
cessation of exercise (< 1min). Post-exercise scans began
within 189.7 ± 24s (mean ± SD). The NordBord measures
forces at the ankles via load cells (sampling at 50Hz) that
are attached to ankle hooks placed immediately superior to
the lateral malleoli. Eccentric strength was determined for
each limb from the peak force (N) produced during the first
set (ten repetitions) of the exercise session.
Ultrasound imaging
The distal ST tendons and adjacent muscle fascicles of both
limbs were imaged via grey-scale ultrasound (US) images
taken with an iU22 Philips scanner (Philips Healthcare,
Eindhoven, Netherlands) equipped with a high resolution
L12MHz linear transducer. All scanning was performed by
a single sonographer with > 20years of musculoskeletal
experience. The sonographer was not blinded to the ACLR
limb. Participants lay in the prone position to allow the pos-
terior thigh to be examined in the longitudinal and transverse
planes. A standardised, pre-programmed general musculo-
skeletal setting was selected for the grey-scale US scanning
protocol. Distal ST muscles and their tendons were com-
pared for the absence or presence of grey scale abnormality
(normal/abnormal). The sonographer made notes based on
the following criteria; (1) integrity of distal semitendinosus
tendon and appearance of adjacent muscle fascicles com-
pared to those from semimembranosus and biceps femo-
ris long head (normal, partial loss of fibrillary pattern or
echogenic complete loss of fibrillary pattern), (2) absence or
presence of the surgical tendon scar (absent, thinned, normal
reconstituted or hypertrophic), (3) observation of maturity
of tendon scar (echogenic, mixed, hypoechoic or fluid), (4)
colour Doppler imaging indicative of vascularity of the
post-surgical harvest site graded using the semi-quantitative
method (none 0%, scant 1–24%, mild 25–49%, moderate
50–74% or severe 75–100%). All images and worksheets
were recorded and stored with the picture archiving and
communication system (PACs).
MRI
All MRI scans were performed using a 3-T imaging sys-
tem (Phillips Ingenia, © Koninklijke Phillips N.V). Par-
ticipants were positioned supine in the magnet bore with
their knees fully extended, hips in neutral and straps secured
around both limbs to prevent undesired movement. Scans
of both lower limbs began at the level of the femoral head
and finished immediately distal to the tibial plateau. Par-
ticipants were positioned in the centre of the magnet bore
with a 32-channel spinal coil placed over the anterior thighs.
Prior to exercise, participants underwent two MRI scanning
sequences of both upper limbs simultaneously to generate
T2-weighted and mDixon axial images. T2-weighted imag-
ing was repeated immediately after exercise. T2-weighted
images were acquired using a Carr–Purcell–Meiboom–Gill
spin echo pulse sequence (Table1) as per previous work [7,
15]. To ensure the signal intensity profile of T2-weighted
images was not disturbed by abnormal fluid shifts, partici-
pants were instructed to avoid strength training of the lower
limbs for 72h prior to data acquisition and were seated for
15min [6, 17] before pre-exercise imaging. Axial mDixon
images were taken using a T1-weighted 3-dimensional (3D)
fast field echo (FFE) sequence (Table1). The images were
acquired in 4 stations (water only, fat only, in-phase and
out-of-phase) with 180 slices per station. The FFE sequence
provided smooth 3-D images allowing for improved visibil-
ity of the muscles’ outer margins for manual segmentation.
Muscle activation
To determine the extent of hamstring muscle activation
during the NHE, T2 relaxation times were measured in
consecutive multi-echo T2-weighted images acquired
before and after exercise (see Fig.1). The percentage
changein T2 relaxation timewasemployed as an index of
Table 1 T2-weighted and
mDixon slice positioning and
image acquisition parameters
Scan position and acquisition parameters T2-weighted mDixon
Slice thickness (mm) 10 3.6
Interslice gap (mm) 10 0
Field of view (mm) 220 × 360 350 × 450
Relaxation time (ms) 2500 3.2
Echo time (ms) 8, 16, 24, 32, 40, 48 1.1, 2.1
Number of echoes 6 2
Voxel size (mm) 0.9 × 0.9 × 10 1.8 × 1.8 × 3.6
Total acquisition time for each scan (s) 348 28
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muscle activation.All images were transferred to a Win-
dows computer in the digital imaging and communica-
tions in medicine (DICOM) file format. For all hamstring
muscles, the T2 relaxation time was measured in five axial
slices that corresponded to 30, 40, 50, 60 and 70% of thigh
length [defined as the distance between the inferior margin
of the ischial tuberosity (0%) and the superior border of
the tibial plateau (100%)] [6]. In the pre- and post- exer-
cise scans, the signal intensity of each hamstring muscle
in both limbs was measured using image analysis software
(Sante Dicom Viewer and Editor, Cornell University).
The signal intensity was measured in each slice using a
0.5–10cm2 circular region of interest (ROI) [14], which
was placed in a homogenous area of contractile tissue in
the centre of each muscle belly (avoiding aponeurosis,
fat, tendon, bone and blood vessels). The size of each
ROI varied due to the cross-sectional area and amount of
homogeneous muscle tissue identifiable in each slice of
interest. The signal intensity represented the mean value
of all pixels within the ROI and was measured across six
echo times (8, 16, 24, 32, 40 and 48ms). The T2 relaxa-
tion times where determined as per previous work [7, 15].
Fig. 1 a Tracings of hamstring
muscles in a mDixon image and
a T2-weighted image b before
and; c immediately after 50 rep-
etitions of the Nordic Hamstring
Exercise. BFLH biceps femoris
long head, BFSH biceps femoris
short head, ST semitendinosus,
SM semimembranosus. For
all images, the right side of
the image corresponds to the
participant’s left side as per
radiology convention
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Muscle volume, anatomical cross‑sectional area
andmuscle length
Muscle volume, anatomical cross-sectional area (ACSA) and
muscle length for each of the hamstrings [biceps femoris
long head (BFLH), biceps femoris short head (BFSH), ST and
semimembranosus (SM)] were determined for both limbs
from mDixon images using manual segmentation. Muscle
boundaries were identified and traced on each image where
the desired structure was present using image analysis soft-
ware (Sante DICOM Viewer and Editor, Cornell Univer-
sity) (see Fig.1). Volumes were determined for each muscle
by multiplying the summed cross-sectional areas (CSAs)
(from all slices containing the muscle of interest) by the slice
thickness [21]. Maximum ACSA was determined by finding
the 3.6mm slice with the greatest CSA and averaging this
along with the two slices immediately cranial and caudal
(five slices). To determine muscle length, the total number
of slices containing muscle tissue for each muscle of interest
were summed and then multiplied by the slice thickness to
represent the total length of each respective muscle belly. All
traces were performed by the same investigator (DM) who
was blinded to participant identity and limb history (surgical
vs control)throughout all analyses.
Statistical analysis
Data were analysed using JMP Version 10.02 (SAS Institute,
2012). Hamstring muscle volume, ACSA, lengthand pre-
and post-exercise T2 values were reported as means ± SDs.
Clinical interpretation of ultrasound images was reported
descriptively. A repeated measures linear mixed model fitted
with the restricted maximum likelihood (REML) method
was used to compare transient exercise-induced percentage
changes in T2 relaxation times and resting values of muscle
volume, ACSA and muscle length for each hamstring mus-
cle. For this analysis, muscle (BFLH, BFSH, ST, SM), limb
(surgical/control) and muscle by limb interaction were the
fixed factors with participant identity (ID), participant ID
by muscle and participant ID by limb as the random fac-
tors. When a significant main effect was detected post hoc
Student’s t tests with Bonferroni corrections were used to
determine which comparisons differed. Student’s t tests
were used for between-limb comparisons of muscle volumes
and ACSAs for the total lateral (BFLH + BFSH) and medial
(ST + SM) hamstrings, the whole hamstrings and eccentric
knee flexor strength. Comparisons were reported as mean
differences with 95% CIs and α was set at p < 0.05. For all
analyses, Cohen’s d was reported as a measure for the effect
size, with the levels of effect being deemed small (d = 0.20),
medium (d = 0.50) or large (d = 0.80).
As this is the first study to explore hamstring mus-
cle activation during eccentric exercise in individuals
following ACLR, it was not possible to base sample size
estimates on previously reported effect sizes. However,
previous studies exploring differences in strength and ST
muscle volume have reported effect sizes of 1.0–1.97 when
comparing surgical to non-surgical limbs [13]. There-
fore, conservative sample size estimates were based on
anticipated effect sizes of 0.7 and a sample size of 14 was
deemed sufficient to provide a statistical power of ≥ 0.8
when p < 0.05.
Results
Between limb comparisons
T2 relaxation time changes followingeccentric exercise
A muscle by limb interaction was found (p < 0.001) for the
percentage change in T2 relaxation time following the NHE.
The average exercise-induced T2 change in surgical limb ST
muscles was a third less (− 9.9%; 95% CI − 3.8 to − 16.0%;
p = 0.004; d = 0.93) than controls. No significant differ-
ences in T2 changes were observed between the surgical and
control limbs for SM (− 2.2%; 95% CI − 10.0 to 6.1%; n.s;
d = 0.33), BFLH (− 0.9%; 95% CI − 5.8 to 4.1%; n.s; d = 0.24)
or BFSH (0.6%; 95% CI − 2.4 to 3.5%; n.s; d = 0.10) (Fig.2).
Fig. 2 Percentage change in fMRI T2 relaxation times of each ham-
string muscle following the Nordic hamstring exercise. Values are
displayed as the mean percentage change compared to values at rest.
*Indicates significant difference between limbs (p = 0.004). Data are
presented as mean values (± SD). BFLH biceps femoris long head,
BFSH biceps femoris short head, ST semitendinosus, SM semimem-
branosus
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Hamstring muscle volumes
A muscle by limb interaction was detected for muscle vol-
ume (p < 0.001). The surgical limb ST volume was 45%
lower (80.9cm3; 95% CI − 57.1 to − 104.7cm3; p < 0.001;
d = 1.52) than control limbs. Surgical SM volume was
greater (9.7 cm3; 95% CI 5.5–14.0 cm3; p < 0.001;
d = 0.20) than control limbs. Between limb differences
for both BFSH (5.9cm3; 95% CI 0.6–11.0cm3; p = 0.032;
d = 0.25) and BFLH (7.5cm3; 95% CI − 1.4–16.0 cm3;
n.s; d = 0.17) volumes were small and trivial (Fig.3a).
Medial hamstring muscle volume of the surgical limbs
was 18% lower (− 71.3cm3; 95% CI − 48.9 to − 93.6cm3;
p < 0.001; d = 0.78) than controls (Fig.3a). Lateral ham-
string volume did not differ significantly (13.4cm3; 95%
CI − 8.9 to 35.7cm3; n.s; d = 0.21) between surgical and
control limbs. Total hamstring muscle volume was 9%
lower (− 57.9cm3; 95% CI − 38.0 to − 77.6cm3; p < 0.001;
d = 0.39) in surgical than control limbs.
Hamstring muscle ACSA
A main effect was observed for muscle ACSA between
limbs (p < 0.001). ACSA of the ST was 28% lower
(− 3.5cm2; 95% CI − 1.9 to − 5.0cm2; p < 0.001; d = 0.89)
in surgical than control limbs, but ACSA of BFSH was 9%
larger (0.7cm2; 95% CI 0.2–1.2cm2; p = 0.008; d = 0.28)
in the surgical than control limbs (Fig.3b). No between-
limb differences were observed for SM (0.4cm2; 95% CI
− 8.2 to 9.1cm2; n.s; d = 0.15) or BFLH ACSA (0.3cm2;
95% CI − 46.8 to 47.3 cm2; n.s; d = 0.07). The com-
bined ACSA for the surgical medial hamstrings was 11%
lower (− 3.1cm2; 95% CI − 1.2 to − 4.9cm2; p = 0.001;
d = 0.49) than the control limbs (Fig.3b). For the lateral
hamstrings, the combined ACSA was 5% greater in surgi-
cal than control limbs, although this difference was not
statistically significant (1.0cm2; 95% CI − 0.8 to 2.8cm2;
n.s; d = 0.17). The combined total of all hamstring mus-
cle ACSAs was not different in surgical and control limbs
(− 2.1cm2; 95% CI − 5.4 to 1.2cm2; n.s; d = 0.17).
Hamstring muscle length
A main effect was observed for muscle length between
limbs (p < 0.001). ST muscles of the surgical limb
were 23% shorter (− 7.2cm; 95% CI − 4.8 to − 9.5cm;
p < 0.001; d = 1.99) than control limbs (Fig. 3c). No
between-limb length differences were observed for the
remaining homonymous hamstring muscle pairs (all p
values n.s; all d values < 0.10).
Fig. 3 a Mean volumes, b anatomical cross-sectional areas (ACSAs)
and c lengths of hamstring muscles in surgical and control limbs. Val-
ues were measured at rest. Data are presented as mean values (± SD).
For between limb muscle comparisons, *p < 0.001, **p = 0.001 and
*#p < 0.05. BFLH biceps femoris long head, BFSH biceps femoris short
head, ST semitendinosus, SM semimembranosus, Hams hamstrings,
Medial Hams medial hamstrings, Lateral Hams lateral hamstrings
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Comparison oftendon andmuscle morphology
ofsemitendinosus betweenlimbs
Of the 14 surgical ST tendons, seven showed partial and
four showed a complete loss of fibrillary pattern while three
appeared normal under ultrasound. All ST tendons from
control limbs appeared normal (Supplementary file 1a). Dis-
tal ST muscle fascicles were abnormal only in the surgical
limbs. Ultrasound of the tendon harvest site showed variable
degrees of scarring, (Supplementary file 1b) while ten surgi-
cal tendons exhibited no vascularity in the region of the scar,
three displayed ‘scant’ and one displayed ‘mild’ vascularity.
Eccentric knee flexor strength
Differences ineccentric knee flexor strength, as determined
from the highest forces generated in the first set of the NHE,
were small and trivial (− 21 N; 95% CI 33 N to − 74 N; n.s;
d = 0.26) between surgical (289 ± 87 N) and control limbs
(310 ± 71 N) (Fig.4). Three participant’s strength tests were
not recorded due to equipment failure during testing.
Discussion
The most important finding of this study was that 1–6years
after surgical intervention, the graft donor ST is activated
significantly less than the homonymous muscle in the con-
trol limb during the NHE, an exercise known to place high
demands on this muscle [7]. Deficits in ST muscle size and
length and ultrasound evidence consistent with chronic ST
tendon unloading were also apparent in surgical limbs. BFSH
volume and ACSA and SM volume were slightly higher in
surgical than control limbs and there were only minor defi-
cits in total hamstrings volume (9%) while the total ham-
strings ACSA was not significantly different. These modest
differences in total muscle size may explain the statistically
insignificant between-limb differencein eccentric knee
flexor strength, despite large deficits in ST ACSA (~ 28%).
To the authors’ knowledge, this is the first fMRI study to
explore hamstring muscle activation during eccentric exer-
cise in recipients of ACLR involving ST grafts.
One previous study used fMRI to evaluate hamstring
activation after ACLR [24] and it showed no difference in
exercise-induced T2 changes in ST muscles of surgical and
contralateral limbs after concentric isokinetic knee flexion
exercise. It is possible that the greater demands imposed by
the supramaximal eccentric exercise in this study revealed
muscle activation deficits while submaximal concentric
exercise as employed by Takeda and colleagues [24] could
not.
Deficits in ST volume and ACSA after ACLR involv-
ing ST grafts have previously been reported [13, 16, 23]. In
contrast to this study, Konrath etal. [13] reported that BFLH
muscles were and BFSH muscles were not larger in surgical
than control limbs. BFSH muscles in the surgical limbs in
the current study were larger than those in control limbs,
while there was no significant between-limb difference in
BFLH size. It is possible that the larger BF muscles in surgi-
cal limbs have experienced compensatory hypertrophy after
ST tendon grafts, although this is obviously impossible to
prove in retrospective studies like these. Differences in rela-
tive hamstring muscle volumes between studies [13, 16, 23]
may reflect variable rehabilitation strategies or subsequent
training of the participants in each study. Alternatively, the
diversity of relative hamstring volumes may reflect differ-
ences that pre-dated surgery. Like Konrath etal. [13], this
study showed that SM volume but not ACSA was larger in
surgical than control limbs and that the summed volumes
and ACSAs of the medial hamstrings were in deficit in sur-
gical limbs. These observations have implications for inter-
nal knee rotation strength, which has been reported to be in
deficit long after ACLR with ST grafts [13].
The persistent deficit in medial hamstring muscle mass
after ACLR with ST graft is a concern given the role of
these muscles in countering external tibial rotation tor-
ques and knee valgus moments [9], both of which may be
risk factors for ACL injury [2, 20]. Given the devastat-
ing effects of ST grafts, it may be beneficial to develop
rehabilitation strategies that target the SM, the only other
internal rotator of the knee that also acts as a hip extensor.
Bourne etal. [5] reported that 10weeks of hip exten-
sion strength training resulted in significant SM hyper-
trophy while training with the NHE (in which overload
is largely limited to the knee) did not. So hip-extension
Fig. 4 Peak eccentric knee flexor force measured at the ankles dur-
ing the Nordic hamstring exercise. Bars depict the average peak knee
flexor forces, while the dots represent each participant’s responses.
Strength is reported in absolute terms (N)
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Knee Surgery, Sports Traumatology, Arthroscopy
1 3
exercises may be effective in compensating for the medial
hamstrings size deficits that this study and others have
reported [13]. In uninjured athletes, the ST hypertrophies
significantly in response to both hip extensor and knee
flexor strength training, with a trend towards greater
responses after the knee-oriented exercise [5]. However,
it is doubtful that similar benefits occur after ST grafts,
because the persistent deficits in ST muscle size shown
here and by others [13, 16] are evident 1–6years after
surgery despite the completion of standard rehabilitation
programs and successful return to sport. The present find-
ings of relatively low levels of post-surgical ST activa-
tion in the demanding NHE also suggest that this muscle
receives limited stimulus for adaptation, even during a
supramaximal exercise known to preferentially target
this muscle [5, 7]. It should also be considered that ST
tendon regeneration after ACLR may take approximately
18months [19] and may not occur at all in 10–50% of
patients [13, 16, 23]. Rehabilitation during this time and
for individuals with no tendon regeneration would pre-
sumably not load the ST significantly. Future studies may
examine the effectiveness of hip-extension exercises in
promoting SM hypertrophy, improving knee internal rota-
tion strength and altering dynamic lower limb function
during running gait after ACLR with ST grafts.
Contrary to this study’s hypothesis, there were no
significant differences in eccentric knee flexor strength
between surgical and control limbs, although there was
considerable between-subject variability. The literature
regarding knee flexor strength after ACLR is mixed, with
most studies reporting persistent deficits [16, 26] and oth-
ers showing none [22]. The study by Timmins etal. [26]
is the most similar to the current study because it also
assessed eccentric forces during the NHE. By contrast,
they observed a ~ 14% strength deficit in surgical limbs,
with an effect size approximately twice as big as the one
reported here (d = 0.51 vs 0.26). Future work should
investigate the impact of different ACLR graft techniques
(hamstring vs bone–patellar-tendon–bone grafts) on knee
flexor muscle use after rehabilitation and successful
return to sport [11, 12].
The limitations of this study include its lack of internal
knee rotation strength measurements and the large range
in times since surgery; the latter of which could conceiv-
ably influence compensatory muscle hypertrophy in the
postoperative limb. Variability in participant rehabilita-
tion and sports participation before and after the injury
and surgery is also likely to have impacted these find-
ings. Finally, while there was no control group (without
a history of ACLR) in this study, the activation patterns
of the control limbs are very similar to those previously
observed in uninjured limbs [6, 7, 15].
Conclusion
In conclusion, this is the first fMRI study to show ST acti-
vation is significantly reduced during eccentric exercise
1–6years after ACLR with ST graft. Diminished ST activa-
tion may partially explain this muscle’s persistent hypotro-
phy and have implications for the design of more effective
rehabilitation programs.
Acknowledgements The authors acknowledge the facilities, and the
scientific and technical assistance of the staff at the Imaging at Olympic
Park Centre, Melbourne.
Author contributions DM was the principal investigator and was
involved with study design, recruitment, analysis and manuscript write
up. AS, MW, RT and MB were involved with the study design, analysis
and manuscript preparation. All authors had full access to all of the
data (including statistical reports and tables) in the study and can take
responsibility for the integrity of the data and the accuracy of the data
analysis.
Funding This study was funded by a Grant from the Institute of Health
and Biomedical Innovation at the Queensland University of Technology
(Approval number: Human Research Ethics Committee 1600000882).
Compliance with ethical standards
Conflict of interest AS is listed as a co-inventor on a patent filed
for the knee-flexor testing device employed in this study (PCT/
AU2012/001041.2012) as well as being a minority shareholder in Vald
Performance Pty Ltd, the company responsible for comercialisng the
device. MW reports receiving fees from Vald Performance, for work
on that company’s research committee but not related to the current
study.MB has previously been employed by Vald Performance and
has previouslyreceived funding from them for research unrelated to
the current study.All authors have completed the Unified Competing
Interest form at http://www.icmje .org/coi_discl osure .pdf (available on
request from the corresponding author) and declare that (1) the Insti-
tute of Health and Biomedical Innovation, Queensland University of
Technology funded this study; (2) DM andRT have no relationships
with companies that might have an interest in the submitted work in
the previous 3years; (3) their spouses, partners, or children have no
financial relationships that may be relevant to the submitted work; and
(4) DM, MW, RT and MB have no non-financial interests that may be
relevant to the submitted work.
Transparency declaration The lead author* (DM) affirms that this
manuscript is an honest, accurate, and transparent account of the study
being reported; that no important aspects of the study have been omit-
ted; and that any discrepancies from the study as planned (and, if rel-
evant, registered) have been explained. * = The manuscript’s guarantor.
Copyright declaration The Corresponding Author has the right to
grant on behalf of all authors and does grant on behalf of all authors,
a worldwide licence to the Publishers and its licensees in perpetuity,
in all forms, formats and media (whether known now or created in
the future), to (i) publish, reproduce, distribute, display and store the
Contribution, (ii) translate the Contribution into other languages, create
adaptations, reprints, include within collections and create summaries,
extracts and/or, abstracts of the Contribution, (iii) create any other
derivative work(s) based on the Contribution, (iv) to exploit all subsidi-
ary rights in the Contribution, (v) the inclusion of electronic links from
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Knee Surgery, Sports Traumatology, Arthroscopy
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the Contribution to third party material where-ever it may be located;
and, (vi) licence any third party to do any or all of the above.
Data sharing Consent was not obtained for data sharing but the pre-
sented data are anonymised and risk of identification is low.
Ethical approval All participants provided written, informed consent
for this study, which was approved by the Queensland University of
Technology Human Research Ethics Committee.
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