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Abstract
Introduction Objective outcome measures that can quantify the force generating capacity of the lower limb are required to allow clinicians to accurately measure functional status and treatment adaptations over time. The aim of this prospective observational cohort study is to: (1) evaluate the acceptability of the isometric mid-thigh pull (IMTP) test as a measure of functional strength with military personnel undergoing residential hip pain rehabilitation; (2) compare the peak force values recorded against the updated Army physical employment standards (PES) assessment criteria and (3) assess if the minimum PES required of military personnel has the potential to inform clinical decision making and return to duty criteria within UK Defence Rehabilitation.
Methods Acceptability was assessed against patient’s adherence to the testing procedures and test burden. Clinician acceptability was assessed against ease of administration and safety of test procedure. Hip pain was recorded before, immediately following and 1 hour after testing. Net peak force was recorded using portable force plates.
Results Full patient and clinician acceptability to IMTP testing procedures were demonstrated. Minimal changes in visual analogue scale (VAS) pain scores were demonstrated between baseline values at rest and follow-up. Despite being medically downgraded and functionally compromised due to chronic hip pain, 100% of patients met the PES expected on entry to the British Army and 79% met the PES expected at the end of basic training.
Conclusion The IMTP provides rehabilitation clinicians with an objective quantifiable measure of maximum muscle strength that can be used early in the rehabilitation care pathway. Based on our finding, it is unclear if the current British Army PES can be used as a criterion standard in Defence Rehabilitation. Therefore, further research focused on generating clinically relevant patient-specific IMTP score criteria, with a larger sample of diverse diagnostic sub-groups is required.
- rehabilitation medicine
- sports medicine
- hip
Data availability statement
Data are available on reasonable request. All data relevant to the study are included in the article or uploaded as supplementary information.
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Key messages
There is an absence of objective outcome measures that can be used to quantify lower-limb muscle strength across all tiers of UK Defence Rehabilitation.
The isometric mid-thigh pull (IMTP) is part of the Army role fitness test (RFT) and has the potential to inform clinical decisions during rehabilitation.
Full acceptability of the IMTP by patients and clinicians were demonstrated during this evaluation, with no increased symptomatic hip pain or any adverse events reported.
Peak forces generated by patients were significantly greater than the minimum IMTP values required during RFT, questioning the usefulness of current physical employment standards to inform rehabilitation.
The value of integrating the IMTP into other musculoskeletal injury care pathways across UK Defence Rehabilitation remains unclear.
Introduction
The overall aim of treatment at each level of the UK Defence Rehabilitation care pathway is to restore optimal function and regain full occupational employability to maintain force readiness.1 Muscle strength is widely acknowledged as a component of treatment closely associated with functional improvements in musculoskeletal rehabilitation.2 The availability of objective measures of functional strength are required to allow clinicians to accurately measure functional status and treatment adaptations over time. The traditional method of measuring maximal muscle strength in healthy adult populations involves the use of 1-repetition maximum (RM) testing.3 However, contraindications associated with dynamic and provocative maximal muscle testing procedures have led to their inconsistent use, modifications or complete withdrawal from UK Defence Rehabilitation practice.
A 2020 UK Defence Rehabilitation research priorities survey identified ‘rehabilitation outcomes’ and ‘hip pain’ among the highest rated research topics scored by 165 rehabilitation practitioners employed across all three tiers of Defence Primary Healthcare.4 Hip-related pain is known to be a major cause of disability negatively impacting quality of life, function, employment and physical capacity.5 The International Hip-related Pain Research Network consensus panel also recommend that further research should ‘investigate, report and improve the measurement properties of tests of muscle strength and functional performance’.6 Current outcome measures of strength changes include dynamometry assessed manual testing and patient self-report. However, these methods are limited as a means of monitoring the link between strength improvements and changes in functional capacity over time. Therefore, further research is required examining and reporting the use of objective methods of hip muscle strength and function in symptomatic military personnel.
In a recent clinical commentary investigating the integration of strength training into UK Defence Rehabilitation practice,1 it was recommended that rehabilitation practitioners adopt elements of the British Army soldier conditioning review and updated (Army) physical employment standards (PES). The whole career role fitness test continuum (during entry selection, basic training and in-service) could be used to inform exercise prescription/assessment during rehabilitation care. When measured using portable force plates, the isometric mid-thigh pull (IMTP, Figure 1) is a test that can assess multiple derivatives of maximal lower-limb muscle strength including, peak force, rate of force development and limb asymmetry. However, peak force (kg) is the only output derived from the IMTP within the Army PES, due to limitations with testing equipment. The IMTP test has demonstrated good to excellent reliability in measuring maximum strength.7 Crucially, the IMTP is simple to explain and considered safe to administer.8 Where pain is the primary limiting factor to progress, the potential benefit of isometric performance testing is that by eliminating painful joint movement under loading it may yield a more consistent standardised and reliable method of functional strength testing. There is also some evidence of a pain modulation response induced during isometric muscle contractions, although this may be subject to a dose-response effect.9 Therefore, the IMTP has the potential to be employed as an alternative to dynamic maximal lower-limb muscle strength assessment during all stages of the rehabilitation care pathway.
Image of the isometric mid-thigh pull (IMTP) test.
In September 2020, the IMTP was integrated into the existing biomechanics performance assessment of the 7-day residential hip and groin rehabilitation course (Table 1) at the Defence Medical Rehabilitation Centre (DMRC), Stanford Hall.10 Despite its relevance and appeal to UK Defence Rehabilitation, no data are available documenting the acceptability of the IMTP as a clinical outcome measure in musculoskeletal injury (MSKI) rehabilitation. Therefore, the aim of this evaluation is to: (1) document the acceptability of introducing the IMTP as a measure of functional strength with military personnel undergoing residential hip pain rehabilitation; (2) compare the peak forces generated by this patient group against the updated Army PES assessment criteria; (3) assess if the minimum PES required of military personnel has the potential to inform clinical decision making and return to duty criteria within UK Defence Rehabilitation. An overview of the IMPT within the updated Army PES is included as an online supplemental annex to this article.
Supplemental material
Components of treatment within the 7-day residential hip and groin course at Defence Medical Rehabilitation Centre
Methods
In accordance with current policy surrounding the use of clinical data for education, evaluation and audit purposes, permission was granted for publication by the DMRC Caldecott guardian. All anonymised data were treated confidentially in accordance with Caldecott principles. The patient information and clinical data routinely collected in support of clinical practice at DMRC was used in this evaluation. During their biomechanics performance assessment, all patients signed an informed consent form providing permission for their anonymised data to be shared. The entire biomechanics performance assessment includes IMTP testing; hand-held dynamometry assessed isometric hip strength in all planes of motion and a qualitative assessment of single leg knee bend using a movement screening tool. This article conforms to the STrengthening the Reporting of OBservational Studies in Epidemiology statement checklist for the reporting of observational studies.11
Study design
This was a prospective observational clinical cohort study of an existing biomechanical performance assessment.
Setting
All data were collected at the UK DMRC Stanford Hall between October 2020 and March 2021.
Participants
Patients included in this cohort study were screened to undergo IMTP testing as part of their routine care by the consultant-led multidisciplinary team (MDT) during the course admission clinic on day 1 of the 7-day residential course (Table 1). The eligibility criteria employed by the MDT ensure maximal isometric testing will not affect the patient’s full participation in the rehabilitation care programme. This eligibility criteria includes: (1) ability to fully weight bear; (2) no surgical interventions within the past 3 months; (3) no unresolved stress fracture of the hip/pelvis; (4) no uncontrolled hip pain during planned functional tasks (eg, walking, straight line running, stair climbing, etc); (5) walk without the use of aids and (6) pain is stable or regressing at rest.
Testing procedure
Prior to commencing the IMTP test, patient information and injury characteristics were collected (Table 2). Following a standardised progressive 5 min warm up on a stationary bike (Watt Bike, Nottingham, UK), the recommended IMTP testing procedures were adopted.12 Patients were positioned in an upright standing posture on a pair of portable force plates (ForceDecks, Vald Performance, Queensland, Australia) located on the base plate of the mid-thigh pull rig (Absolute Performance, Cardiff, UK). The bar height is then adjusted to allow the patient to obtain an optimal knee angle (125° to 145°) and hip (140° to 150°) angle (Figure 1). Goniometers were used to verify joint angles. Each patient was asked to perform a specific IMTP warm up to become familiarised with pulling on the mid-thigh pull bar (3 s efforts at 50%, 75% and 90% of maximal effort, separated by 1 min recovery). Patients were afforded 2 min rest before completing three maximal attempts each lasting 5 s with 2 min passive recovery between attempts. Standardised instructions were provided before each maximal effort. Patients were encouraged to drive their feet into the ground as hard and as fast as possible. To achieve a stable force baseline patients were directed to ‘get ready’, by providing a small amount of pretension to the bar for 1 s prior to the maximum effort.13 Participants were then given the countdown ‘3, 2, 1, Drive!’ Verbal encouragement was provided during each trial. If any discomfort was felt in the hip during the assessment, individuals were instructed to stop immediately and the test was terminated. Lifting straps were used to make sure grip strength was not a limiting factor during each pull. The integrated analysing software V.2.1 (Forcedecks, Vald Performance) used a sampling frequency of 1000 Hz. Each maximum effort was visually inspected to ensure the force time traces were acceptable (ie, no countermovement or unstable baseline trace appeared prior to the effort and verified that peak force did not occur at the end of effort).12
Patient demographics and injury characteristics
Outcome measures
Patient demographic/characteristics and injury-specific information were recorded by members of the MDT on admission.
Isometric mid-thigh pull test acceptability
A series of questions created for use in the biomechanical assessment process of hip and groin patients at DMRC were used to determine the acceptability of the IMTP test. Questions explored the patient’s willingness to exert a maximum effort, adherence to the test procedure, acceptable/minimal patient burden, ease of integration with other biomechanical tests. Clinician acceptability was determined against the eligibility criteria and ease of administration. Questions used have not yet been formally examined for test–retest reliability, construct validity and internal consistency. However, they do provide some face validity for use within UK Defence Rehabilitation.
Clinician endorsement of the IMTP test was assessed against the usefulness of the final patient report to inform current and future clinical decision making. The patient report reviewed by the MDT included values that reflect peak force, rate of force development and limb asymmetry.
Pain
Self-reported symptomatic hip pain were determined using a 100 mm visual analogue scale (VAS), where 0 represents ‘no pain’ and 100 represents ‘worst possible pain’. VAS scores were collected immediately before (during rest), immediately after the IMTP test and at 1 hour following the entire biomechanical performance assessment clinic.
Peak force
Peak force is the highest value of force generated during the test and is used to determine maximal isometric strength. Comparisons in peak force values will be made against the minimum PES required in the IMTP during the role fitness test at entry (RFT-E, 50 kg) and role fitness test at the end of basic training (RFT-BT, 95 kg).
Data analysis
Due to the small numbers included in this preliminary evaluation, we did not subject the data to formal statistical testing. Raw descriptive scores to gain a preliminary understanding of study outcomes (patient acceptability, pain (VAS), net peak force) were used for simple comparison with the Army PES testing criteria. Individual force plate raw data files were exported from the integrated software and used for analysis purposes. Net peak force (N) is converted to kg by dividing the sum of net peak force by 9.81.
Results
Patient demographic and injury characteristic
From a total of 36 patients screened, 19 patients met the eligibility criteria and were referred for routine biomechanics performance assessment and included in this study. Patient demographic and injury characteristics are reported in Table 2. The mean age was 34±8 years, mean body mass 75±11 kg, 63% were male, 27% were female, 58% served in the British Army, 37% in the RAF and 5% in the Navy. Mean duration of service was 12±7 years prior to admission. The mean duration of symptoms prior to admission to DMRC was 5±4 years. According to the Joint Medical Employability Standards, 47% of patients had diagnosed hip pain that resulted in a medically limited deployability status and 53% were deemed medically non-deployable.
IMPT acceptability
Full patient acceptability and adherence to IMTP testing procedures were demonstrated. Results from patient feedback revealed instructions and administration of the test were simple to understand and follow. Patients expressed a willingness to exert a maximum isometric contraction in the upright test position. Patients described the inclusion of the IMTP to be no additional burden when used alongside other existing biomechanical tests. Clinicians also reported the IMTP easy to administer (requiring 15 min in total), and simple to explain. Importantly, clinicians report that force plate-derived peak force values were clinically valuable.
Pain
Compared with mean resting VAS pain scores (13±12 mm), symptomatic hip pain increased by 7 mm (on a 0–100 mm scale) immediately following the IMTP test (20±20 mm) and increased by 9 mm from baseline to 1 hour following the biomechanical performance assessment clinic (22±20 mm).
Strength derivatives from IMTP
Mean recorded net peak force was 1281±440 N (range, 502–2059 N) (Table 3). A comparison of these peak force values against the PES are provided in Table 3 and Figure 2. Figure 2 demonstrates each patient’s maximal force generating ability during the IMTP test compared with the minimum PES expected of British Army personnel at entry (50 kg) and the end of basic training (95 kg) for personnel applying for ground close combat roles (Infantry and Royal Armoured Corps). The patients’ mean net peak forces were 131±45 kg. Despite being functionally compromised and medically downgraded due to chronic hip pain, 100% of patients met the PES expected on entry to the British Army (mean 2.6 times greater than this minimum PES) and 79% met the PES expected at the end of basic training (mean 1.4 times greater than this minimum PES).
Peak force values from the isometric mid-thigh pull test and comparisons to the minimum standards required during role fitness test whole career continuum
Peak force values of each patient plotted against the minimum isometric mid-thigh pull standards required during the role fitness test on entry to the British Army (dashed line) and after basic training (solid line).
Discussion
Following a MSKI, rehabilitation practitioners must individually tailor their exercise programmes to meet the functional needs of the individual and the occupational standards expected by the British Armed Forces. Clear clinical reasoning relating to effective progression of therapeutic strength training is essential to achieve a positive clinical outcome.14 A significant challenge lies in designing optimal rehabilitation programmes that facilitate both neurological and muscular adaptations while concurrently accommodating biological healing, recovery and patient safety.15 Figure 3 presents a model that describes the challenges of applying ‘mainstream’ strength and conditioning principles into a complex multimodal rehabilitation programme. There is currently an urgent need within UK Defence Rehabilitation and the wider rehabilitation/sport and exercise medicine community for an available strength-based rehabilitation outcome measure at the lower end of the functional continuum (green triangle, Figure 3).1
Theoretical model by Coppack and Ladlow of the challenges integrating ‘mainstream’ versus ‘therapeutic’ strength training principles across the rehabilitation functional continuum. Horizontal axis=functional rehabilitation continuum. Blue triangle=‘mainstream’ S&C principles and knowledge; green triangle=‘therapeutic’ S&C principles and knowledge; shaded rectangle=overlap during functional recovery. Vertical arrows indicate proportion of knowledge and evidence applied at different stages of recovery. Taken with permission from Ladlow et al 1.
We have communicated a theoretical rationale for the use of the IMTP as a rehabilitation outcome measure for individuals following lower-limb MSKI. This service evaluation is the first to demonstrate the acceptability, to both patients and clinicians, of integrating the IMTP into a residential rehabilitation care pathway. Patients reported confidence in their willingness to apply a maximum effort (within the limitations of their injury). Mean VAS scores taken before and after the IMTP test were all <34 mm which reflects ‘mild MSK pain interference’.16 Increases in mean VAS pain scores of 7 mm and 9 mm are below the 13 mm criteria for an acute minimal clinical important difference in pain.17 These findings are suggestive of minimal changes in symptomatic pain following the IMTP test. After excluding dynamic maximal muscle testing from the 7-day hip pain course, the IMTP only requires 15 min of the overall assessment time. However, the inclusion of the IMTP does provide clinicians with valuable quantifiable information on the force generating capacity of the injured lower limb.
The IMTP provides rehabilitation clinicians with an objective quantifiable measure of maximum muscle strength that can be used early in the rehabilitation care pathway when maximal dynamic contractions are contraindicated. Intuitively, having the ability to quantify progress in derivatives of maximal muscle strength earlier in the rehabilitation process could inform and support decisions on exercise progression, thereby optimising early adaptations to therapeutic strength training.
At this time, the only IMTP-related standards available against which to compare derivative of maximal lower-limb muscle strength is from the Army RFT-E (50 kg) and RFT-BT (95 kg). Our findings demonstrate that the majority of patients with hip pain, who are medically downgraded and considered functionally compromised, already meet these IMTP-specific PES. It is therefore our opinion that the existing IMTP-related PES are too low for use as a criterion measure to form the basis of facilitating clinical decision making during the rehabilitation care pathway of individuals with hip pain. The usefulness of current Army PES as a criterion measure with other lower-limb MSK is unclear and requires further consideration. It may be useful to consider adding the IMTP to the battery of tests performed later in the RFT whole career continuum. This would provide clinically meaningful baseline data on all new recruits. Should any of these recruits subsequently experience a lower-limb MSKI, this baseline value could prove useful to rehabilitation practitioners responsible for making informed decision on the appropriate timing of progressing functional activity while minimising re-injury risk. Data describing IMTP normative values of UK Armed Forces personnel in productive service could be used as a reference to compare functional status and is recommended as a focus for future research.
Limitations
This service evaluation was delivered using chronic patients with hip pain. Our findings that the IMTP is an acceptable rehabilitation outcomes measure during MSK rehabilitation cannot be immediately transferred to MSK conditions affecting other joints (eg, knee, ankle and spine). However, as the hip joint contributes significantly to the forces being applied during this assessment, and this maximal isometric contraction did not result in any symptomatic impairment, it is reasonable to suggest that the IMTP could prove to be acceptable outcome measure during the rehabilitation of other MSKI.
This cohort study employed a small number (n=19) of military personnel with chronic hip pain from all service branches (Army, RAF and Navy). We only compared the PES of a single service (Army). However, in the absence of available IMTP-related PES from the RAF and Navy, using the Army PES allows a useful military-specific comparison to be made until more single service data are available.
Conclusion
Contraindications and challenges with provocative maximal muscle testing procedures have led to inconsistencies in the assessment of strength training adaptations across UK Defence rehabilitation. Objective and reliable measures of functional strength are required to allow clinicians to accurately monitor the patient’s functional status over the entire course of the rehabilitation care pathway. This study is the first to present preliminary data reporting the use of an IMTP test with objective strength derivatives in UK Military personnel undergoing MSKI rehabilitation. Full patient acceptability, adherence to IMTP test procedures and minimal patient burden were demonstrated. Functionally compromised patients with hip pain in this study produced higher mean net peak force values (131±45 kg) than the criterion values used in the Army PES without a significant change in pain scores. Therefore, while we have demonstrated the potential utility of using the IMTP test across all stages of functional recovery, the usefulness of employing the current Army PES criterion values to inform clinical decision making remains unclear. Further research focused on generating patient-specific IMTP score criteria, with a larger sample of diverse diagnostic subgroups is required.
Data availability statement
Data are available on reasonable request. All data relevant to the study are included in the article or uploaded as supplementary information.
Ethics statements
Patient consent for publication
Acknowledgments
The authors would like to acknowledge Dr Richard Allen for his role with data collection during the wider biomechanics performance assessment clinic at DMRC Stanford Hall and Mrs Fiona Miller for her administrative support. We would also like to acknowledge other clinical team members, including Laura Learmonth and Matt Birkett.
References
Supplementary materials
Supplementary Data
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Footnotes
Twitter @JBilzon
Contributors VW, RJC and PL conceived the study idea. VW, RJC, RC, TP and PL conceptualised study design. TP had oversight of patient’s rehabilitation care pathway. VW managed data collection and performed the data analysis. VW, RJC and PL wrote the first draft manuscript. All authors contributed to the interpretation of the findings, read, critically reviewed and approved the final version.
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.
Supplemental material This content has been supplied by the author(s). It has not been vetted by BMJ Publishing Group Limited (BMJ) and may not have been peer-reviewed. Any opinions or recommendations discussed are solely those of the author(s) and are not endorsed by BMJ. BMJ disclaims all liability and responsibility arising from any reliance placed on the content. Where the content includes any translated material, BMJ does not warrant the accuracy and reliability of the translations (including but not limited to local regulations, clinical guidelines, terminology, drug names and drug dosages), and is not responsible for any error and/or omissions arising from translation and adaptation or otherwise.