Modifying the Mobility Scale for Acute Stroke (MSAS) for All Stroke Phases (MSAllS): Measurement Properties and Clinical Application.

OBJECTIVE: To develop and examine the measurement properties and interpretability of the Mobility Scale for "All" Stroke Phases (MSAllS) as a potential single outcome measure to capture improvements in physical function throughout the stroke continuum. DESIGN: Retrospective cross-sectional study. SETTING: Inpatient rehabilitation unit. PARTICIPANTS: People after stroke at discharge from rehabilitation (N=309). INTERVENTION: Not applicable. MAIN OUTCOME MEASURE(S): We developed MSAllS by extending the highest MSAS level (walk 10 m independently) with 4 gait speed levels. To establish a clinical anchor, we extracted a 4-level discharge outcome. To assess the distributional properties and internal consistency of MSAllS, we evaluated its ceiling effects and calculated the Cronbach alpha, respectively. To assess structural validity, we performed a confirmatory factor analysis. To assess (i) its convergent validity with the FIM and (ii) its predictive validity with the clinical anchor, we used Spearman's rank correlations. To evaluate the clinical interpretability of MSAllS, we used an item-response theory-based method to estimate MSAllS thresholds associated with the clinical anchor. RESULTS: The MSAllS had lower ceiling effects compared with MSAS (0% vs 25%). Internal consistency of MSAllS was excellent (α=0.94). Structural validity of MSAllS demonstrated a good fit (Comparative Fit Index=0.95; Tucker-Lewis Index=0.92; Root Means Square Error of Approximation=0.17). MSAllS demonstrated a moderate correlation (rho=0.66) with FIM score and with the clinical anchor (rho=0.75). MSAllS thresholds for increasing levels of the clinical anchor were 22 (20.8 to 23.6) - at least moderate assistance with walking/transfers, 28 (27.5 to 29.4) - at most supervision with walking, and 33 (32.5 to 33.4) - able to walk unassisted. CONCLUSION: The MSAllS showed adequate measurement properties and clinical interpretability. MSAllS has the potential to be a single universal measure to evaluate physical function after stroke but further evaluation of clinical interpretability is required.

Brief report: Passive mechanical properties of gastrocnemius in multiple sclerosis and ankle contracture.

BACKGROUND: Ankle contracture is common in people with multiple sclerosis (MS) but the mechanisms of contracture are not clear. This study aimed to identify the mechanisms of contracture in MS by comparing passive muscle length and stiffness at known tension, separated into contributions by muscle fascicles and tendons, between people with MS who had contracture and healthy people. METHODS: Passive length-tension curves of the gastrocnemius muscle-tendon unit were derived from passive ankle torque and angle using a published biomechanical method. Ultrasound images of medial gastrocnemius muscle fascicles were used to partition length-tension curves into fascicle and tendon components. Lengths and stiffness of the muscle-tendon unit, muscle fascicles and tendons were compared between groups with linear regression. FINDINGS: Data were obtained from 15 participants with MS who had contracture [age 53 (12) years, mean (SD)] and 25 healthy participants [48 (20) years]. Participants with MS had clinically significant ankle contracture, and had shorter fascicles at slack length (between-groups mean difference -0.8 cm, 95% CI -1.2 to -0.4 cm, p < 0.001) and at 100 N (-0.7 cm, 95% CI -1.3 to -0.1 cm, p = 0.02) compared to healthy participants. There were no differences between groups in all other outcomes. INTERPRETATION: Tension-referenced comparisons of passive muscle length and stiffness show that people with MS who had contracture had shorter fascicles at low and high tension compared to healthy people, but there were no changes to the muscle-tendon unit or tendon. Further studies are needed to identify the causes and mechanisms of contracture in neurological conditions.

Rigid dressings versus soft dressings for transtibial amputations.

BACKGROUND: Dressings are part of the routine postoperative management of people after transtibial amputation. Two types of dressings are commonly used; soft dressings (e.g. elastic bandages, crepe bandages) and rigid dressings (e.g. non-removable rigid dressings, removable rigid dressings, immediate postoperative protheses). Soft dressings are the conventional dressing choice as they are cheap and easy to apply, while rigid dressings are costly, more time consuming to apply and require skilled personnel to apply the dressings. However, rigid dressings have been suggested to result in faster wound healing due to the hard exterior providing a greater degree of compression to the stump. OBJECTIVES: To assess the benefits and harms of rigid dressings versus soft dressings for treating transtibial amputations. SEARCH METHODS: In December 2018 we searched the Cochrane Wounds Specialised Register, the Cochrane Central Register of Controlled Trials (CENTRAL), Ovid MEDLINE, Ovid Embase, EBSCO CINAHL Plus, Ovid AMED and PEDro to identify relevant trials. To identify further published, unpublished and ongoing studies, we also searched clinical trial registries, the grey literature, the reference lists of relevant studies and reviews identified in prior searches. We used the Cited Reference Search facility on ThomsonReuters Web of Science and contacted relevant individuals and organisations. There were no restrictions with respect to language, date of publication or study setting. SELECTION CRITERIA: We included randomised controlled trials (RCTs) and quasi-RCTs that enrolled people with transtibial amputations. There were no restrictions on the age of participants and reasons for amputation. Trials that compared the effectiveness of rigid dressings with soft dressings were the main focus of this review. DATA COLLECTION AND ANALYSIS: Two review authors independently screened titles, abstracts and full-text publications for eligible studies. Two review authors also independently extracted data on study characteristics and outcomes, and performed risk of bias and GRADE assessments. MAIN RESULTS: We included nine RCTs and quasi-RCTs involving 436 participants (441 limbs). All studies recruited participants from acute and/or rehabilitation hospitals from seven different countries (the USA, Australia, Indonesia, Thailand, Canada, France and the UK). In all but one study, it was clearly stated that amputations were secondary to vascular conditions.Primary outcomes Wound healing We are uncertain whether rigid dressings decrease the time to wound healing compared with soft dressings (MD -25.60 days; 95% CI -49.08 to -2.12; one study, 56 participants); very low-certainty evidence, downgraded twice for very high risk of bias and once for serious imprecision. It is not clear whether rigid dressings increase the proportion of wounds healed compared with soft dressings (RR 1.14; 95% CI 0.74 to 1.76; one study, 51 participants); very low-certainty evidence, downgraded twice for very high risk of bias and twice for very serious imprecision.Adverse events It is not clear whether rigid dressings increase the proportion of skin-related adverse events compared with soft dressings (RR 0.65; 95% CI 0.32 to 1.32; I2 = 0%; six studies, 336 participants (340 limbs)); very low-certainty evidence, downgraded twice for very high risk of bias and once for serious imprecision.It is not clear whether rigid dressings increase the proportion of non skin-related adverse events compared with soft dressings (RR 1.09; 95% CI 0.60 to 1.99; I2 = 0%; six studies, 342 participants (346 limbs)); very low-certainty evidence, downgraded twice for very high risk of bias and once for serious imprecision. In addition, we are uncertain whether rigid dressings decrease the time to no pain compared with soft dressings (MD -0.35 weeks; 95% CI -2.11 to 1.41; one study of 23 participants); very low-certainty evidence, downgraded twice for very high risk of bias and twice for very serious imprecision.Secondary outcomesWe are uncertain whether rigid dressings decrease the time to walking compared with soft dressings (MD -3 days; 95% CI -9.96 to 3.96; one study, 56 participants); very low-certainty evidence, downgraded twice for very high risk of bias and twice for very serious imprecision. We are also uncertain whether rigid dressings decrease the length of hospital stay compared with soft dressings (MD -30.10 days; 95% CI -49.82 to -10.38; one study, 56 participants); very low-certainty evidence, downgraded twice for very high risk of bias and once for serious imprecision. It is also not clear whether rigid dressings decrease the time to readiness for prosthetic prescription and swelling compared with soft dressings, as results are based on very low-certainty evidence, downgraded twice for very high risk of bias and once/twice for serious/very serious imprecision. None of the studies reported outcomes on patient comfort, quality of life and cost. AUTHORS' CONCLUSIONS: We are uncertain of the benefits and harms of rigid dressings compared with soft dressings for people undergoing transtibial amputation due to limited and very low-certainty evidence. It is not clear if rigid dressings are superior to soft dressings for improving outcomes related to wound healing, adverse events, prosthetic prescription, walking function, length of hospital stay and swelling. Clinicians should exercise clinical judgement as to which type of dressing they use, and consider the pros and cons of each for patients (e.g. patients with high risk of falling may benefit from the protection offered by a rigid dressing, and patients with poor skin integrity may have less risk of skin breakdown from a soft dressing).

Reliability and Validity of Measurement Tools for Residual Limb Volume in People With Limb Amputations: A Systematic Review.

BACKGROUND: Measurements of residual limb volume often guide decisions on the type and timing of prosthetic prescription. To help inform these decisions, it is important that clinicians use measurement tools that are reliable and valid. PURPOSE: The aim of this systematic review was to investigate the reliability and validity of measurement tools for residual limb volume in people with limb amputations. DATA SOURCES: A comprehensive search on MEDLINE, EMBASE, CINAHL, Scopus, and Web of Science was performed on July 11, 2016. STUDY SELECTION: Studies were included if they examined the reliability or validity of measurement tools for residual limb volume, were conducted on humans, and were published in English. DATA EXTRACTION: Data were extracted from 11 reliability and 4 validity studies and included study characteristics, volumetric estimates, and reliability and validity estimates. The quality of the studies was also rated. DATA SYNTHESIS: Data from 2 studies (38 participants) indicated good to excellent intrarater (intraclass correlation coefficient [ICC] ≥0.88) and interrater (ICC ≥0.88) reliability and high between-session reliability (coefficient of variation [CV] = 10%) for water displacement volumetry. One study (28 participants) reported excellent intrarater and interrater reliability (ICC ≥0.93) for the circumferential method, and data from 2 studies (19 participants) indicated high between-session reliability for the optical surface scanner (CV ≤9.8%). Three studies (26 participants) indicated good to excellent between-session reliability results for computed tomography (CV = 9.2%-10.9%). One study (7 participants) showed moderate within-session reliability (CV = 50%). Using water displacement volumetry as the gold standard, 2 studies (79 participants) indicated excellent validity for the circumferential method ( r ≥0.92; ICC ≥0.92). All studies reporting measures of reliability or validity were performed with people who had transtibial amputations. LIMITATIONS: Only studies published in English and in which water displacement volumetry was used as the gold standard were included in this review. The reliability and validity of the quality rating scale used in this review have not been tested. CONCLUSIONS: On the basis of a limited number of moderate- to high-quality studies with small sample sizes, circumferential and water displacement methods were found to be reliable, and the circumferential method was found to be valid in people with transtibial amputations. There are inadequate data for drawing conclusions about volume measurement methods in people with other types of limb amputations.

Quality of Clinical Practice Guidelines for Management of Limb Amputations: A Systematic Review.

BACKGROUND: The quality of clinical practice guidelines (CPGs) is important to ensure guideline adoption by clinicians. PURPOSE: The aim of this review was to identify CPGs for the management of limb amputations, appraise the quality of CPGs, and synthesize recommendations from comprehensive CPGs of high quality. DATA SOURCES: MEDLINE, EMBASE, CINAHL, PEDro, guideline-specific websites, websites for associations or networks for people with amputations, and Google Scholar were searched from April 2007 to April 2017. STUDY SELECTION: Publications were included if they were CPGs or consensus statements/standards endorsed by a certified organization, covered the management of limb amputations, were freely accessible, and were written in English. DATA EXTRACTION: Two reviewers independently screened titles and abstracts for eligible CPGs and rated the quality of CPGs using the Appraisal of Guidelines Research and Evaluation (AGREE-II) instrument. DATA SYNTHESIS: Of the 15 included CPGs, 11 were of low to moderate quality and 4 were of high quality. Mean (or median) domain scores on AGREE-II were as follows: 83% for domain 1 (scope and purpose), 61% for domain 2 (stakeholder involvement), 7% for domain 3 (rigor of development), 65% for domain 4 (clarity and presentation), 24% for domain 5 (applicability), and 21% for domain 6 (editorial independence). Strong recommendations from comprehensive and high-quality CPGs were few and focused on the development of individualized treatment plans, exercises for improving physical function and the ability to perform activities of daily living, and the assessment of physical function and prognostic factors. LIMITATIONS: CPGs that were not written in English were excluded. Final recommendations from CPGs might differ if different criteria were used. Low domain scores on the AGREE-II might be due to poor reporting rather than poor methodology in the CPG development process. CONCLUSIONS: Few CPGs for the management of limb amputations were of high quality, and few recommendations were strong. To improve the quality of future CPGs, guideline developers should report funding and competing interests of members, provide information to aid in the practical application of CPGs, and use a systematic approach to search for evidence and derive strength of recommendations.

Reliability and validity of the iSense optical scanner for measuring volume of transtibial residual limb models.

BACKGROUND:: Residual limb volume is often measured as part of routine care for people with amputations. These measurements assist in the timing of prosthetic fitting or replacement. In order to make well informed decisions, clinicians need access to measurement tools that are valid and reliable. OBJECTIVES:: To assess the reliability and criterion validity of the iSense optical scanner in measuring volume of transtibial residual limb models. STUDY DESIGN:: Three assessors performed two measurements each on 13 residual limb models with an iSense optical scanner (3D systems, USA). Intra-rater and inter-rater reliability were calculated using intraclass correlation coefficients. Bland Altman plots were inspected for agreement. Criterion validity was assessed using a steel rod of known dimensions. Ten repeated measurements were performed by one assessor. A t-test was used to determine differences between measured and true rod volume. RESULTS:: Intra-rater reliability was excellent (range of intraclass correlation coefficients: 0.991-0.997, all with narrow 95% confidence intervals). While the intraclass correlation coefficients suggest excellent inter-rater reliability between all three assessors (range of intraclass correlation coefficients: 0.952-0.986), the 95% confidence intervals were wide between assessor 3 and the other two assessors. Poor agreement with assessor 3 was also seen in the Bland-Altman plots. Criterion validity was very poor with a significant difference between the mean iSense measurement and the true rod volume (difference: 221.18 mL; p < 0.001). CONCLUSIONS:: Although intra-rater reliability was excellent for the iSense scanner, we did not find similar results for inter-rater reliability and validity. These results suggest that further testing of the iSense scanner is required prior to use in clinical practice. CLINICAL RELEVANCE: The iSense offers a low cost scanning option for residual limb volume measurement. Intra-rater reliability was excellent, but inter-rater reliability and validity were such that clinical adoption is not indicated at present.

Prediction of Walking and Arm Recovery after Stroke: A Critical Review.

Clinicians often base their predictions of walking and arm recovery on multiple predictors. Multivariate prediction models may assist clinicians to make accurate predictions. Several reviews have been published on the prediction of motor recovery after stroke, but none have critically appraised development and validation studies of models for predicting walking and arm recovery. In this review, we highlight some common methodological limitations of models that have been developed and validated. Notable models include the proportional recovery model and the PREP algorithm. We also identify five other models based on clinical predictors that might be ready for further validation. It has been suggested that neurophysiological and neuroimaging data may be used to predict arm recovery. Current evidence suggests, but does not show conclusively, that the addition of neurophysiological and neuroimaging data to models containing clinical predictors yields clinically important increases in predictive accuracy.

Models containing age and NIHSS predict recovery of ambulation and upper limb function six months after stroke: an observational study.

QUESTIONS: What is the incidence of recovery of ambulation and upper limb function six months after stroke? Can measures such as age and the National Institutes of Health Stroke Scale (NIHSS) be used to develop models to predict the recovery of ambulation and upper limb function? DESIGN: Prospective cohort study. PARTICIPANTS: Consecutive sample of 200 people with stroke admitted to a Sydney Hospital. OUTCOME MEASURES: Ambulation was measured with item 5 of the Motor Assessment Scale (MAS); patients scoring ≥3 could ambulate independently. Upper limb function was measured with items 7 and 8 of the MAS; patients scoring ≥5 could move a cup across the table and feed themselves with a spoonful of liquid with the hemiplegic arm. RESULTS: Of the 114 stroke survivors who were unable to ambulate initially, 80 (70%) achieved independent ambulation at six months. Of the 51 stroke survivors who could not move a cup across the table initially, 21 (41%) achieved the upper limb task at six months. Of the 56 stroke survivors who were unable to feed themselves initially, 25 (45%) could feed themselves at six months. Models containing age and severity of stroke (measured with NIHSS) predicted recovery of ambulation and ability to move a cup across the table, whilst a model containing severity of stroke predicted ability to feed oneself. All prediction models showed good discrimination (AUC 0.73 to 0.84). CONCLUSION: More than two-thirds of people after stroke recovered independent ambulation and less than half recovered upper limb function at six months. Models using age and NIHSS can predict independent ambulation and upper limb function but these prediction models now require external validation before use in clinical practice.

Reliability and validity of ultrasound measurements of muscle fascicle length and pennation in humans: a systematic review.

Ultrasound imaging is widely used to measure architectural features of human skeletal muscles in vivo. We systematically reviewed studies of the reliability and validity of two-dimensional ultrasound measurement of muscle fascicle lengths or pennation angles in human skeletal muscles. A comprehensive search was conducted in June 2011. Thirty-six reliability studies and six validity studies met the inclusion criteria. Data from these studies indicate that ultrasound measurements of muscle fascicle lengths are reliable across a broad range of experimental conditions [intraclass correlation coefficient (ICC) and r values were always > 0.6, and coefficient of variation values were always < 10%]. The reliability of measurements of pennation angles is broadly similar (ICC and r values were always > 0.5 and coefficient of variation values were always < 14%). Data on validity are less extensive and probably less robust, but suggest that measurement of fascicle lengths and pennation angles are accurate (ICC > 0.7) under certain conditions, such as when large limb muscles are imaged in a relaxed state and the limb or joint remains stationary. Future studies on validity should consider ways to test for the validity of two-dimensional ultrasound imaging in contracted or moving muscles and the best method of probe alignment.

Gastrocnemius muscle contracture after spinal cord injury: a longitudinal study.

OBJECTIVE: The aim of this study was to examine changes in passive length and stiffness of the gastrocnemius muscle-tendon unit in people after spinal cord injury. DESIGN: In a prospective longitudinal study, eight wheelchair-dependent participants with severe paralysis were assessed 3 and 12 mos after spinal cord injury. Passive torque-angle data were obtained as the ankle was slowly rotated through range at six knee angles. Differences in passive ankle torque-angle data recorded at different knee angles were used to derive passive length-tension curves of the gastrocnemius muscle-tendon unit. Ultrasound imaging was used to determine fascicle and tendon contributions to the muscle-tendon unit length-tension curves. RESULTS: The participants had ankle contractures (mean [SD] maximum passive ankle dorsiflexion angle, 88 [9] degrees) 3 mos after spinal cord injury. Ankle range did not worsen significantly during the subsequent 9 mos (mean change, -5 degrees; 95% confidence interval, -16 to 6 degrees). There were no changes in the mean slack length or the stiffness of the gastrocnemius muscle-tendon unit or in the slack lengths of the fascicles or the tendon between 3 and 12 mos after spinal cord injury. There were no consistent patterns of the change in slack length or stiffness with the changes in ankle range in the data from the individual participants. CONCLUSIONS: This study, the first longitudinal study of muscle length and stiffness after spinal cord injury, showed that the length and the stiffness of the gastrocnemius did not change substantially between 3 and 12 mos after injury.

Mechanisms of increased passive compliance of hamstring muscle-tendon units after spinal cord injury.

BACKGROUND: People with spinal cord injury sometimes develop abnormally compliant hamstring muscle-tendon units. This study investigated whether the increased muscle-tendon compliance is due to a change in the passive properties of the muscle fascicles or tendons, or to muscle tears. METHODS: Semimembranosus muscle fascicle lengths were measured from ultrasound images obtained from 15 spinal cord injured subjects and 20 control subjects while the hip was passively flexed with the knee extended. Semimembranosus muscles of spinal cord injured subjects were inspected for tears using ultrasound imaging. FINDINGS: The mean (SD) hip angle at 30 Nm was 97 (SD 24) degrees in spinal cord injured subjects and 70 (SD 11) degrees in control subjects, indicating that spinal cord injured subjects had very compliant hamstring muscle-tendon units. The ratio of change in fascicle length to change in muscle-tendon length was not statistically different between spinal cord injured subjects and control subjects: muscle fascicles lengthened by 0.30 (SD 0.24) mm/mm in spinal cord injured subjects and 0.42 (SD 0.29) mm/mm in control subjects. These data were used to show that there was evidence of increased tendon compliance of spinal cord injured subjects compared to control subjects, but no evidence of increased muscle fascicle compliance. No tears were observed in semimembranosus muscles of spinal cord injured subjects. INTERPRETATION: The increased hamstring muscle-tendon compliance apparent in some spinal cord injured subjects is due, at least in part, to increased tendon compliance. There was no evidence that the increased muscle-tendon compliance was due to muscle tears.

Passive mechanical properties of the gastrocnemius after spinal cord injury.

INTRODUCTION: In this study we compared passive mechanical properties of gastrocnemius muscle-tendon units, muscle fascicles, and tendons in control subjects and people with ankle contractures after spinal cord injury. METHODS: Passive gastrocnemius length-tension curves were derived from passive ankle torque-angle data obtained from 20 spinal cord injured subjects with ankle contractures and 30 control subjects. Ultrasound images of muscle fascicles were used to partition length-tension curves into fascicular and tendinous components. RESULTS: Spinal cord injured subjects had stiffer gastrocnemius muscle-tendon units (stiffness index: 74.8 ± 27.0 m(-1) ) than control subjects (54.4 ± 17.7 m(-1) ) (P = 0.004). Muscle-tendon slack lengths, as well as slack lengths and changes in length of fascicles and tendons, were similar in the two groups. CONCLUSIONS: People with ankle contractures after spinal cord injury have stiff gastrocnemius muscle-tendon units. It is not clear whether this reflects changes in properties of muscle fascicles or tendons.

Passive mechanical properties of gastrocnemius muscles of people with ankle contracture after stroke.

OBJECTIVE: To investigate the mechanisms of contracture after stroke by comparing passive mechanical properties of gastrocnemius muscle-tendon units, muscle fascicles, and tendons in people with ankle contracture after stroke with control participants. DESIGN: Cross-sectional study. SETTING: Laboratory in a research institution. PARTICIPANTS: A convenience sample of people with ankle contracture after stroke (n=20) and able-bodied control subjects (n=30). INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Stiffness and lengths of gastrocnemius muscle-tendon units, lengths of muscle fascicles, and tendons at specific tensions. RESULTS: At a tension of 100N, the gastrocnemius muscle-tendon unit was significantly shorter in participants with stroke (mean, 436mm) than in able-bodied control participants (mean, 444mm; difference, 8mm; 95% confidence interval [CI], 0.2-15mm; P=.04). Muscle fascicles were also shorter in the stroke group (mean, 44mm) than in the control group (mean, 50mm; difference, 6mm; 95% CI, 1-12mm; P=.03). There were no significant differences between groups in the mean stiffness or length of the muscle-tendon units and fascicles at low tension, or in the mean length of the tendons at any tension. CONCLUSIONS: People with ankle contracture after stroke have shorter gastrocnemius muscle-tendon units and muscle fascicles than control participants at high tension. This difference is not apparent at low tension.

Half of the adults who present to hospital with stroke develop at least one contracture within six months: an observational study.

QUESTIONS: What is the incidence of contractures six months after stroke? Can factors measured within four weeks of stroke predict the development of elbow, wrist, and ankle contractures six months later? DESIGN: Prospective cohort study. PARTICIPANTS: Consecutive sample of 200 adults with stroke admitted to a Sydney hospital. OUTCOME MEASURES: Loss of range of motion in major joints of the body was measured using a 4-point ordinal contracture scale. In addition, elbow extension, wrist extension, and ankle dorsiflexion range of motion were measured using torque-controlled procedures. Potential predictors of contracture were age, pre-morbid function, severity of stroke, muscle strength, spasticity, motor function, and pain. Measurements were obtained within four weeks of stroke and at six months after stroke. RESULTS: 52% of participants developed at least one contracture. Incidence of contracture varied across joints from 12% to 28%; shoulders and hips were most commonly affected. Muscle strength was a significant predictor of elbow, wrist, and ankle joint range. Prediction models explained only 6% to 20% of variance in elbow, wrist, and ankle joint range. CONCLUSION: About half of all patients with stroke develop at least one contracture within six months of stroke. Incidence of contractures across all joints ranged from 12% to 28%. Muscle strength is a significant predictor of elbow, wrist, and ankle contractures but cannot be used to accurately predict contractures in these joints.

In vivo passive mechanical behaviour of muscle fascicles and tendons in human gastrocnemius muscle-tendon units.

Ultrasound imaging was used to measure the length of muscle fascicles in human gastrocnemius muscles while the muscle was passively lengthened and shortened by moving the ankle. In some subjects the muscle belly 'buckled' at short lengths. When the gastrocnemius muscle-tendon unit was passively lengthened from its shortest in vivo length by dorsiflexing the ankle, increases in muscle-tendon length were not initially accompanied by increases in muscle fascicle lengths (fascicle length remained constant), indicating muscle fascicles were slack at short muscle-tendon lengths. The muscle-tendon length at which slack is taken up differs among fascicles: some fascicles begin to lengthen at very short muscle-tendon lengths whereas other fascicles remain slack over a large range of muscle-tendon lengths. This suggests muscle fascicles are progressively 'recruited' and contribute sequentially to muscle-tendon stiffness during passive lengthening of the muscle-tendon unit. Even above their slack lengths muscle fascicles contribute only a small part (<~30%) of the total change in muscle-tendon length. The contribution of muscle fascicles to muscle-tendon length increases with muscle length. The novelty of this work is that it reveals a previously unrecognised phenomenon (buckling at short lengths), posits a new mechanism of passive mechanical properties of muscle (recruitment of muscle fascicles), and confirms with high-resolution measurements that the passive compliance of human gastrocnemius muscle-tendon units is due largely to the tendon. It would be interesting to investigate if adaptations of passive properties of muscles are associated with changes in the distribution of muscle lengths at which fascicles fall slack.