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Biomechanical and clinical outcomes in response to inpatient multidisciplinary hip and groin rehabilitation in UK military personnel
  1. Richard Allan1,
  2. R P Cassidy1,2,
  3. R J Coppack1,3,
  4. T Papadopoulou2,4 and
  5. A N Bennett1,5
  1. 1 Academic Department of Military Rehabilitation, Defence Medical Rehabilitation Centre Stanford Hall, Stanford-on-Soar, UK
  2. 2 Centre for Lower Limb Rehabilitation, Defence Medical Rehabilitation Centre Stanford Hall, Stanford-on-Soar, UK
  3. 3 Centre for Sport, Exercise and Osteoarthritis Research Versus Arthritis, University of Bath, Bath, UK
  4. 4 British Association of Sport and Exercise Medicine, Doncaster, UK
  5. 5 National Heart and Lung Institute, Faculty of Medicine, Imperial College London, London, UK
  1. Correspondence to Dr Richard Allan, Academic Department of Military Rehabilitation, Defence Medical Rehabilitation Centre Stanford Hall, Stanford-on-Soar LE12 5QW, UK; Richard.Allan104{at}


Introduction Hip and groin related pain is a common complaint among the military population across UK Defence Rehabilitation and addressing associated biomechanical dysfunction is a key treatment goal. Personnel are exposed to complex occupational loads, therefore assessing movement during demanding tasks may expose biomechanical deficits. Observing biomechanical and clinical outcomes in response to treatment is therefore an important consideration. The aims were to examine clinical and biomechanical outcomes prior to (T1) and 12 weeks post (T2) inpatient rehabilitation and explore the influence of pathological subgroup.

Methods Prospective cohort study as part of a clinical service evaluation of 25 patients undergoing treatment for hip and groin related pain. Three-dimensional motion capture (3DM) during a single-leg squat, hip strength and patient-reported outcome measures were collected at T1 and T2.

Results Increased abductor and external rotator strength with reductions in contralateral pelvic drop (1.9°; p=0.041) at T2 on the injured side. Pain reduced by 9.6 mm (p=0.017) but no differences were found for Non-Arthritic Hip Score (NAHS). No statistically significant differences were found between pathological subgroups. Kinematic profiles display variation between diagnostic subgroups and response to treatment.

Conclusion Strength and pain improved with treatment in this service evaluation although biomechanical adaptation and NAHS remain inconclusive. Small and uneven sample size prevents a firm conclusion regarding the effect of pathological subgroupings, however, the data can be considered hypothesis generating for future, larger studies to integrate 3DM for monitoring response to rehabilitation in pathological subgroups to support clinical decision making.

  • musculoskeletal disorders
  • rehabilitation medicine
  • hip
  • sports medicine

Data availability statement

Data are available upon reasonable request. Data are available to personnel, with the relevant security clearance, working within a UK government organisation. To request the data included in this study, please contact the corresponding author.

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Key messages

  • Residential multidisciplinary exercise-based rehabilitation for patients with non-arthritic hip and groin pain increases bilateral hip strength and reduces pain.

  • Despite these gains, biomechanical adaptation and the effect on patient-reported outcome measures remain inconclusive.

  • Variable response to treatment suggestive of influence of pathology with extra-articular patient’s displaying increased gains in strength and Non-Arthritic Hip Score but less biomechanical adaptation.

  • Further understanding of biomechanical dysfunction in relation to specific diagnostic subgroups and whether targeting rehabilitation based on diagnosis can improve outcomes is required.


Hip and groin related pain is a common, complex complaint among the military population, encompassing a plethora of extra-articular and intra-articular pathologies, which can lead to the necessity for operative management. Irrespective of diagnosis, they remain a limiting factor to the duties of active serving personnel, requiring extensive rehabilitation. A recent engagement exercise of clinical practitioners employed across all three tiers of Defence Primary Healthcare identified ‘hip pain’ as one of its highest rated research priorities.1 Residential multidisciplinary team (MDT) exercise-based rehabilitation for patients with non-arthritic hip and groin pain has a long tradition in the UK military. Core treatment involves optimising neuromuscular control, increasing joint range of motion (RoM) and improving the strength and stability of the hip and surrounding muscles to improve physical and functional performance and reduce pain.2 Recently, physiotherapy-led treatment has been stated as a key recommendation for individuals with hip-related pain.3 Despite the well-founded importance of the rehabilitation approach, there is dearth of knowledge regarding how treatment response translates to biomechanical changes within these individuals.

Three-dimensional motion capture (3DM) is considered the gold standard for obtaining objective biomechanical data. Within hip and groin pathology, it has primarily been used under walking conditions to investigate femoroacetabular impingement (FAI)4–6 and non-arthritic pathologies such as gluteal tendinopathy and chronic hip joint pain.7 8 Variation in outcomes suggests that the demands of walking may be of limited value to identify functional and biomechanical deficits as it fails to push the joint and surrounding musculature to the boundaries of its capability.4

The prevalence of hip and groin related pathologies in young, athletic populations has highlighted a need to better understand the demands of more functional tasks such as running9 and lateral hopping.10 The single-leg squat (SLS) is another task routinely used to assess movement control as it reflects a commonly performed yet demanding movement during unilateral loading. It requires increased multiplanar RoM, motor control and muscular strength,11 12 and it has recently been identified as a valid and reliable test for evaluating physical function in patients with non-arthritic hip pain.13 Biomechanical deficits/compensatory strategies could therefore be better exposed during these movements. Furthermore, as the occupational demands of military personnel align closer to this population, the SLS offers a clinically and functionally relevant means for assessing biomechanics.

Outcomes regarding the biomechanical assessment of an SLS in hip and groin pathology have been inconsistent. FAI patients have demonstrated a 6° reduction in peak hip adduction compared with asymptomatic controls,14 whereas increased hip adduction has been reported in patients with chronic hip pain.15 This suggests that different movement strategies exist between pathologies and should be considered when assessing squat performance. Inconsistency in the instruction of how the task should be performed has also been recognised as a factor for the inability to identify biomechanical parameters, which distinguishes the presence of pathology from healthy individuals.16 Despite this, increased frontal plane motion throughout the kinematic chain including frontal plane projection angle, hip adduction, knee AB, ipsilateral trunk lean and contralateral pelvic drop has been recommended to offer the greatest differences.

Hip abductors, external rotators and extensors and core strength have all been previously correlated with abnormal frontal plane motion during the SLS,17 18 suggesting that weakening of these muscles play a role in suboptimal performance. This has led to hip-strengthening programmes designed to target movement dysfunction to be investigated.10 15 19 In relation to SLS performance, hip abductor and external rotator strength gains accompanied a reduction in hip adduction, internal rotation and contralateral pelvic drop in healthy individuals19 while improvements in pain, function, adduction motion and abductor strength have been reported in individuals with chronic hip joint pain.15

Considering all non-arthritic hip and groin patients attending the Defence Medical Rehabilitation Centre, Stanford Hall, are the recipients of a similar semistructured MDT rehabilitation, understanding response to treatment is an important crucial, potentially offering insight into the requirement for condition-specific treatments. The aim of this service evaluation was twofold; (1) examine the clinical and biomechanical outcomes prior to and 12 weeks post rehabilitation; (2) examine whether outcomes are influenced by subgroup.


Study design

This study used a within-subject, repeated-measures design in a laboratory setting as part of a clinical service evaluation. Patients presenting with non-arthritic hip and groin pathology were assessed and screened for eligibility in a specialist MDT clinic by a sport and exercise medicine consultant (TP), physiotherapist (RPC) and exercise rehabilitation instructor. No exclusion criteria based on injury diagnosis, severity or the amount of previous MDT rehabilitation was used, however, patients were excluded if they had a recent stress fracture; were less than 3 months postoperative; had psychosocial factors and/or experienced high pain/irritability. Patients were included if they were cleared by the MDT based on their ability to mobilise unaided, fully weight bear on single leg standing and had no orthopaedic restrictions/contraindications for testing. Patients were classified into three groups by the MDT based on initial clinical examination and history; patients with extra-articular pathology and patients with intra-articular pathology. A total of 25 patients who undertook the 7-day MDT residential rehabilitation programme between January 2019 and March 2020 were included in the analysis. These 25 patients formed a representative sample of the 81 patients who attended DMRC during this period. The 7 day MDT programme adopted an intensive abbreviated version of a 3-week rehabilitation programme previously published,2 which targets hip RoM, core and trunk muscle function, strength and neuromotor control of the deep hip stabiliser muscles, pain control and improving function in daily living. Patients completed a series of assessments; 0–100 mm Visual Analogue Scale for Pain (VAS-P), Non-Arthritic Hip Score (NAHS); hip strength (handheld dynamometer (HHD)) and 3DM on the first day of admission (T1) and 12-week post-treatment (T2). Only patients with complete T1 and T2 data were included for analysis. Appropriate approval was provided prior to commencing this service evaluation, and all participants gave informed consent to data capture.

Hip muscle strength

Hip strength was measured on both sides for extension (EXT), abduction (AB) and external rotation (ER) using a microFET3 (Hoggan Health Industries, West Jordan, Utah, USA) HHD.2 Designed to invoke less stress on the musculoskeletal system compared with other forms of testing, an isometric ‘make-test’ was adopted throughout.20 Patients were tested on a clinical examination couch in a seated, supine or prone position depending on the movement being measured. Once positioned, patients were asked to perform four consecutive 5 s maximum voluntary isometric contraction against the fixed resistance of the HHD held by the examiner. A 30 s rest period was provided between each trial. Measures were reported in Newtons (N) with the highest value used in the analysis.

Biomechanical assessment

Forty-eight retroreflective markers (14 mm diameter) were placed on the skin over anatomical landmarks establishing an eight-segment model including the feet, shank, thigh, pelvis, trunk.21 A 20-camera 3DM system (Vicon MX system, Oxford Metrics, Oxford, England) captured data at 100 Hz. Identical protocols were employed at T1 and T2, and all testing was completed by the same operator (RA).

Following static and RoM calibrations, patients performed an SLS to 60° knee flexion and returned to an upright position over a 4 s metronome paced cycle.21 Patients were instructed to hold their non-stance limb at approximately 90° knee flexion, maintain an upright trunk and pelvis and have their hands by their side. Patients were given two to three practice squats, with a goniometer set to 60° aligned with the knee joint centre in the sagittal plane for reference. This standardisation is designed to reduce variability in individual strategy, increasing validity that observed movement patterns are the result of injury and response to treatment.16 After a 2 min rest, five repetitions were captured on each leg. For unilateral patients, the asymptomatic limb was tested prior to the symptomatic limb while bilateral patients were given individual preference. When required, selection of test leg was made based on symptom presentation by the lead physiotherapist (RPC). Test order was mirrored at T2.

Data processing

Data was labelled in Vicon Nexus (V.2.7) and processed in Visual 3D (C-Motion V.6.0, Rochelle, USA). Marker trajectories were filtered using a fourth order Butterworth low-pass filter with a cut-off frequency of 6 Hz22 and gaps were interpolated using a third order least squares fit.23 Three-trial averages of the second, third and fourth repetitions of the kinematic variables at the time of peak knee flexion were analysed, including hip adduction, knee flexion, pelvic tilt, pelvic lateral tilt (obliquity), anterior trunk lean and lateral trunk lean. Kinematic data from the trunk, pelvis, hip and knee were time-normalised to represent a percentage of the squat cycle, defined as the time between two consecutive points of minimum knee flexion angle.21

Statistical analysis

This was a clinical service evaluation; therefore, statistical analysis was exploratory in nature with no formal sample size calculation performed. All data were checked for normality via Shapiro-Wilk tests, prior to analysis. A mixed-model between-within subject’s analysis of variance was conducted to examine the main effect of treatment and the influence of diagnosis. Where significant interactions were present, post hoc pair-wise comparisons were made using the Bonferroni adjustment. Statistical analysis was performed using SPSS software (SPSS). Significance was set at p<0.05.


Table 1 and Figure 1 present the demographic and clinical data of the population. No significant differences were found between the subgroups for demographics, patient-reported outcome measures (PROMs) or strength at T1.

Figure 1

Change in strength and patient-reported outcome measures between prerehabilitation assessment (T1) and 12-week review (T2). Improved outcome for strength and Non-Arthritic Hip Score (NAHS) are denoted by a positive change while an improved outcome for VAS-P is denoted by a negative change. AB, abduction; ER, external rotation; EXT, extension; VAS-P, Visual Analogue Scale for Pain.

Table 1

Demographic, PROMs and function scores of hip and groin patients pre versus post inpatient MDT rehabilitation


T1 and T2 mean±SD participant VAS-P were 37.8±20.7 mm and 28.2±23.4 mm, respectively (Table 1). Main effect of treatment was significant (F1, 23 = 4.7, p=0.04, Embedded Image = 0.017) corresponding to a change of −9.6 mm (95% CI: –18.5 to –0.5) at T2. Interaction effect of diagnostic subgroup was not significant (F1, 23 = 0.4, p=0.85, Embedded Image = 0.002).

Non-Arthritic Hip Score

T1 and T2 mean±SD scores were 67.2±16.7 and 70.7±18.1, respectively (Table 1). No significant main effect of treatment (F1, 23 = 1.7, p=0.21, Embedded Image = 0.07) or interaction effect of diagnostic subgroup (F1, 23 = 0.4, p=0.52, Embedded Image = 0.002) was found for NAHS.

Hip strength

Main effect of treatment was significant for AB (F1, 23 = 5.0, p=0.035, Embedded Image = 0.18) and ER (F1, 23 = 5.9, p=0.023, Embedded Image = 0.21) strength between T1 and T2. This corresponded to a strength increase of 12.5N (95% CI: 1 to 24.0) and 22.8N (95% CI: 3.4 to 42.2) for AB and ER, respectively, in the symptomatic side at T2 (Table 1). No significant interaction effect of diagnostic subgroup was found for AB (F1, 23 = 0.3, p=0.873, Embedded Image = 0.001) or ER (F1, 23 = 2.7, p=0.11, Embedded Image = 0.12) although patients with extra-articular pathologies demonstrated greater strength increases (Figure 1). A significant 17.9N increase (95% CI: 1.9 to 33.9) in AB strength (F1, 23 = 5.3, p=0.03, Embedded Image = 0.19) and 18.2N (95% CI: 1.7 to 34.7) ER (F1, 23 = 5.2, p=0.032, Embedded Image = 0.19) was also found on the contralateral side. No interaction effects were found. No significant main or interaction effects were found for EXT strength.

Main effect comparing affected/contralateral strength at T1 was significant for EXT (F1, 23 = 6.33, p=0.019, Embedded Image = 0.22) corresponding to an increase of 22.5N (95% CI: 4.0 to 41.0) in the contralateral hip at T1 compared with the symptomatic hip. There was no significant interaction effect of diagnostic subgroup for EXT (F2, 26 = 0.07, p=0.8, Embedded Image = 0.03). No other main or interaction effects were found.

At T2, the main effect comparing affected/contralateral strength was again significant for EXT (F1, 23 = 5.3, p=0.031, Embedded Image = 0.19) corresponding to an increase of 17.5N (95% CI: 1.8 to 33.1) in the contralateral hip at T2 compared with the symptomatic hip. No other main or interaction effects were significant for any other strength outcome at T2.


On the symptomatic side, patients improved squat depth closer to the required 60° by 3.3° (95% CI: 0.7 to 5.4) at T2 compared with T1 (F1, 23 = 7.3, p=0.013, Embedded Image = 0.24) (Table 2). No interaction effect of diagnostic subgroup was found (F1, 23 = 0.72, p=0.4, Embedded Image = 0.03).

Table 2

Kinematic variables at peak knee flexion during the SLS in subgroups of the Hip & Groin cohort (T1 and T2)*

Despite a 2.4° reduction (95% CI: 0.3 to 4.6) in hip adduction in the symptomatic side at T2 compared with T1, this was not statistically significant (F1, 23 = 4.1, p=0.054, Embedded Image = 0.15). No significant interaction effect of diagnostic subgroup was found (F1, 23 = 0.008, p=0.984, Embedded Image = 0.000). Despite the insignificant change, the hip adduction profile shows a general improvement at T2 with variation between subgroups in response to treatment (Figure 2).

Figure 2

Hip adduction profile during a single-leg squat task for patients with intra-articular pathology (orange) and extra-articular pathology (blue). Solid line indicate T1 and dashed line indicate T2. Values represent the time-normalised group mean and SD.

Main effect of treatment was not significant for lateral trunk lean (F1, 23 = 0.3, p=0.588, Embedded Image = 0.01) corresponding to a 0.3° (95% CI: −0.9 to 1.5) reduction in lateral trunk flexion at T2 compared with T1 on their symptomatic side (Figure 3). No significant interaction effect of diagnostic subgroup between sessions was found (F1, 23 = 0.28, p=0.869, Embedded Image = 0.001).

Figure 3

Lateral trunk lean profile during a single-leg squat task for patients with intra-articular pathology (orange) and extra-articular pathology (blue)). Solid line indicate T1 and dashed line indicate T2. Values represent the time-normalised group mean and SD.

On the symptomatic side, a significant 1.9° (95% CI: −0.08 to 2.1) reduction in contralateral pelvic drop was found at T2 (F1, 23 = 4.7, p=0.041, Embedded Image = 0.17). No significant interaction effect of diagnostic subgroup was found (F1, 23 = 0.27, p=0.872, Embedded Image = 0.001). No other significant differences in kinematics between sessions were found. Mean kinematic pelvic obliquity profile indicates variances between subgroups with extra-articular pathologies demonstrating more prolonged contralateral pelvic drop during squat ascent (Figure 4).

Figure 4

Pelvic obliquity profile during a single-leg squat task for patients with intra-articular pathology (orange) and extra-articular pathology (blue). Solid line indicate T1 and dashed line indicate T2. Values represent the time-normalised group mean and SD. Positive direction indicates increased contralateral pelvic drop, negative direction indicates contralateral pelvic rise.

On the contralateral side, a 2.9° increase (95% CI: 0.5 to 5.3) in knee adduction angle at T2 was found (F1, 23 = 4.1, p=0.022, Embedded Image = 0.21). No other main effect of treatment or interaction effects of subgroup were found.


Improving strength and stability of the hip and lumbopelvic musculature is one of the core treatment goals of inpatient MDT for hip and groin related problems across UK Defence Rehabilitation. Hip abductor and external rotator strength gains over time indicate that treatment had a beneficial effect on these measures across the population. Although no significant effect of diagnosis was found, the data indicates variation between diagnostic subgroups, with patients with extra-articular pathology demonstrating larger gains in hip strength across all three measures both in the symptomatic and in contralateral side at T2 compared with patients with intra-articular pathology.

At the group level, inpatient treatment showed a significant beneficial effect on pain. Despite a reduction of 9.6 mm, this change was below the minimally clinically important difference of 14.8 mm previously reported in a similar population although in which pain was assessed 2 years postoperative treatment.24 Considering the shorter term post-treatment timeframe in our data, the reduction in VAS gives a useful indicator of treatment effect although longer term adherence to rehabilitation programmes may further improve these pain levels. For NAHS, the minimal and non-significant reductions were also lower than the clinically relevant detected improvement of 15.9 stated by Bennett et al.25 These findings may be attributable to the combination of small sample size and the heterogeneous nature of the cohort. PROMs in this service evaluation are however supported by Kemp et al 26 who concluded that although physiotherapy-led interventions led to improved function and strength there was less of an impact on pain and quality of life. In this service, evaluation PROMs showed a highly variable response with the intra-articular subgroup demonstrating greater improvement in VAS-P while extra-articular pathologies had a greater increase in NAHS. The contrasting responses in core treatment outcomes indicate that a greater understanding of their relationship with biomechanical changes is required.

Adaptations in frontal plane kinematics in the form of reduced contralateral pelvic drop were found at T2 alongside non-significant reductions in hip adduction, suggesting that proximal frontal plane control is improved by inpatient treatment irrespective of pathology. Despite the lack of statistically significant interaction effects in the discrete variables, the data offers insight into differing biomechanical presentation and response to treatment between diagnostic subgroups. Patients with extra-articular pathology appear to prolong both hip adduction and contralateral pelvic drop during the ascent phase of the SLS when compared with those with intra-articular pathology, indicating that altered strategies are adopted to perform the task. Previously reported research has shown a 6° reduction in peak hip adduction in patients with FAI than people without14 attributing this as an attempt to reduce painful impingement and joint load by restricting the medial collapse of the thigh. The combined presence of faster hip AB and contralateral pelvic rise during SLS ascent in those with intra-articular pathology supports this movement strategy aimed towards minimising an impinged hip position. The intra-articular pathology did however display a more abducted knee during the SLS, which may reflect a more distal compensation strategy to perform the task.

The extent to which the hip adducts is primarily influenced by the strength of the hip abductors during weight bearing tasks, therefore it would be anticipated that strength gains in these muscles would lead to improved hip mechanics. Increases in hip abductor and ER strength alongside reductions in contralateral pelvic drop and hip adduction demonstrate this relationship and provide further support to the effects of strength-based training on the underlying biomechanics of functional movements.19

Despite frontal plane changes, proximal adaptation in the form of lateral trunk lean was only minimally reduced at T2 on the symptomatic side. Ipsilateral (lateral) trunk lean has been identified as a biomechanical adaptation to accommodate the contralateral pelvic drop introduced by hip abductor weakness.27 To reduce the mechanical demand on the hip abductors, the trunk shifts laterally over the affected side, positioning the centre of mass closer to the hip joint centre. Considering the reduction in contralateral pelvic drop observed at T2, a larger medialisation of the trunk would be expected to reflect the trunk-pelvis complex functioning in a more upright position.


Considerations are required for the interpretation of the current work. Study design and unequal subgroup sizes limit the conclusions regarding subgroup differences and response to treatment. This limitation was heavily influenced by the pragmatic approach taken as part of the service evaluation with the sample forming a good representation of the hip and groin population undergoing inpatient rehabilitation at DMRC. Despite this, these findings offer preliminary data to suggest that differences in presentation and response to treatment may be present between hip and groin related pathologies. In line with recommendations,3 future research would address these limitations by implementing a more robust study design providing greater control of subgroup classification and standardisation of subgroup sample size. Considering the nature of the military job role, future work should attempt to gain a better understanding to the barriers patients face when detached from the inpatient MDT environment, which will influence their adherence to rehabilitation programmes.


To our knowledge, this is the first study to describe clinical and biomechanical outcomes to inpatient hip and groin rehabilitation. In general, treatment resulted in significant improvements in pain and strength in the hip abductors and external rotators of the symptomatic and contralateral side with a concomitant adaptation in frontal plane kinematics. Despite strength and biomechanical changes, the effect on PROMs remains inconclusive. Understanding how these changes translate to function and quality of life requires further research. Although stratification based on pathology showed little statistical effect, this service evaluation suggests altered biomechanical presentation and response to treatment with patients with intra-articular diagnoses exhibiting larger reductions in frontal plane deficits. Irrespective of the findings, having a biomechanical performance assessment as part of the rehabilitation pathway is a novel capability, which provides an objective means of identifying biomechanical deficits, which can help focus clinical delivery and facilitate clinical decision making. These findings have important implications for UK Defence Rehabilitation as the heterogeneous response could suggest that a stratified approach to treatment based on diagnostic phenotype and biomechanical presentation may be required to improve outcomes and accelerate the return to service of military patients undergoing hip and groin rehabilitation.

Data availability statement

Data are available upon reasonable request. Data are available to personnel, with the relevant security clearance, working within a UK government organisation. To request the data included in this study, please contact the corresponding author.

Ethics statements

Patient consent for publication

Ethics approval

CC1 approval was granted for the publication of the findings of this SE (CC1-20200154).


We thank the staff and patients within the lower limb department at the Defence Medical Rehabilitation Centre (DMRC) whose hard work and effort made the outcomes presented within this manuscript possible. An additional thank you to Dr Vanessa Walters who provided assistance in the data interpretation.



  • Contributors RA, RJC, RPC, TP and ANB were involved in the conception of the work. RA conducted the data collection, analysis and interpretation. RA drafted the article. RPC and TP undertook all clinical work related to the article. RA, RJC, RPC, TP and ANB were involved in the critical revisions of the article. ANB granted final approval of the article.

  • Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

  • Competing interests None declared.

  • Provenance and peer review Not commissioned; externally peer reviewed.