Measurement of Murine Neuromuscular Function Using the In Situ Preparation.

The presence and progression of a neuromuscular pathology can impact on the contractile force production of a muscle. Hence, measurements of force production can be an important tool for the evaluation of disease progression. In this chapter, we describe how to perform in situ function testing on the tibialis anterior muscle using a murine model. Performing neuromuscular in situ function testing allows force measurements to be recorded in a physiologically relevant environment.

Morphological variability of the leg muscles: potential traps on ultrasound that await clinicians.

BACKGROUND: Although muscles and their tendons are not considered the most morphologically variable structures, they still manifest a substantial diversity of variants. The aim of this study is to increase awareness of some of the many possible variants found during ultrasound imaging of one lower limb compartment, the leg, that could potentially mislead clinicians and lead to misdiagnosis. MATERIALS AND METHODS: PubMed was used for a comprehensive literature search for morphological variations. Relevant papers were included, and citation tracking was used to identify further publications. RESULTS: Several morphological variants of muscles of the leg have been described over many years, but this study shows that the occurrence of further variations in ultrasound imaging requires further investigations. CONCLUSIONS: The incidence of additional structures including muscles and tendons during ultrasound examination can cause confusion and lead to misinterpretation of images, misdiagnosis, and the introduction of unnecessary and inappropriate treatments.

Effects of Visual-Motor Illusion via Image Videos Showing Increased Exercise Intensity on the Tibial Anterior during Sit-to-Stand Movement: A Study of Healthy Participants.

Visual-motor illusion (VMI) elicits kinesthetic sensation from visual stimulation. We have previously performed ankle motion VMI with resistance applied to the ankle joint on the paralyzed side (power-VMI (P-VMI)) and ankle motion VMI without resistance (standard-VMI (S-VMI)) to activate the tibialis anterior (TA) muscle in stroke-paralyzed patients and compared sit-to-stand (STS) durations, but these studies did not measure TA activity during the STS movement. The purpose of this study was to evaluate the effects of different intensities of visual stimuli presented during VMI on TA and STS movement. Healthy right-footed adults (n = 18) observed two different VMI videos of ankle dorsiflexion, including S-VMI and P-VMI, with an observation time of 2 min each. STS movement was evaluated before and after watching each video. Each participant performed both S-VMI and P-VMI interventions on the same day. Only P-VMI enhanced the integrated electromyogram of the TA, increased the angular velocities of the trunk forward inclination and the ankle dorsiflexion, and shortened the STS duration. Our results indicate that P-VMI facilitates the activation of TA during STS, and we believe that we have clarified the intervention mechanism of VMI.

Electrical stimulation to promote muscle and motor unit force and endurance after spinal cord injury.

Fatigue is a common feature of paralysed skeletal muscle, hindering performance when subjected to functional electrical stimulation (ES) for movement. We asked whether (1) 20 Hz ES for 5% of each day (2.5 s on and 2.5 s off for 3 h) increases tibialis anterior and medial gastrocnemius muscle and motor unit (MU) endurance after paralysis by hemisection and deafferentation (HSDA), and (2) muscle length or loading affects their isometric contractile properties. The daily 5% ES increased muscle endurance, largely independent of muscle length or loading, but to a lesser extent than the daily 50% ES (2.5 s on and 2.5 s off for 24 h). The former was effective in counteracting the decline and slowing of muscle force promoted by the 50% ES. The altered muscle properties were confirmed at the MU level in final experiments once the properties had plateaued. Fast-fatigable MUs were converted to fatigue-intermediate and -resistant MUs that finally comprised ∼80% as compared to ∼10% of the total MU number in the daily 5% ES and the control normal groups, respectively. We conclude that the daily 5% ES regimen counteracts the fatigue of paralysed muscle without compromising contractile force, and thereby, is effective in conditioning muscle for effective movement. KEY POINTS: We asked whether 20 Hz electrical stimulation (ES) for 5% of each day (2.5 s on and 2.5 s off for 3 h; 5% ES) preserves medial gastrocnemius and tibialis anterior muscle and MU isometric contractile forces and increases their endurance after paralysis. Daily 5% ES promoted increased muscle endurance irrespective of the muscle length or loading but to a lesser extent than daily 50% ES (20 Hz ES 2.5 s on and 2.5 s off for 24 h). 5% ES was effective in counteracting decline and slowing of muscle force that resulted from 50% ES. Motor units (MUs) were converted from fast fatigable to fatigue intermediate and resistant MUs, comprising ∼80% as compared to ∼10% in the control normal groups. We conclude that the 5% ES regimen counteracts the fatigue of paralysed muscle without compromising contractile force, and thereby is effective in conditioning the muscle for effective movement.

Pharmacological responsiveness of periodic limb movements in patients with restless legs syndrome: a systematic review and meta-analysis.

STUDY OBJECTIVES: Periodic limb movements during sleep (PLMS) are a frequent finding in restless legs syndrome, but their impact on sleep is still debated, as well the indication for treatment. We systematically reviewed the available literature to describe which drug categories are effective in suppressing PLMS, assessing their efficacy through a meta-analysis, when this was possible. METHODS: The review protocol was preregistered on PROSPERO (CRD42021175848), and the systematic search was conducted on and EMBASE (last searched on March 2020). We included original human studies, which assessed PLMS modification on drug treatment with a full-night polysomnography, through surface electrodes on each tibialis anterior muscle. When at least 4 studies were available on the same drug or drug category, we performed a random-effect model meta-analysis. RESULTS: Dopamine agonists like pramipexole and ropinirole resulted the most effective, followed by l-dopa and other dopamine agonists. Alpha2delta ligands are moderately effective as well opioids, despite available data on these drugs are much more limited than those on dopaminergic agents. Valproate and carbamazepine did not show a significant effect on PLMS. Clonazepam showed contradictory results. Perampanel and dypiridamole showed promising but still insufficient data. The same applies to iron supplementation. CONCLUSIONS: Dopaminergic agents are the most powerful suppressors of PLMS. However, most therapeutic trials in restless legs syndrome do not report objective polysomnographic findings, there is a lack of uniformity in presenting results on PLMS. Longitudinal polysomnographic interventional studies, using well-defined and unanimous scoring criteria and endpoints on PLMS are needed. CITATION: Riccardi S, Ferri R, Garbazza C, Miano S, Manconi M. Pharmacological responsiveness of periodic limb movements in patients with restless legs syndrome: a systematic review and meta-analysis. J Clin Sleep Med. 2023;19(4):811-822.

Muscle herniae: Conservative and surgical management. Systematic review.

BACKGROUND: Muscle herniae are often unrecognized. The primary objective of this systematic review is to evaluate the outcomes of conservative and surgical management for muscle herniae. The secondary objective is to define the most appropriate management for muscle herniae depending on aetiology and size of the fascial defect. METHODS: The PRISMA guidelines were used to organize this systematic review to assess the different management modalities and identify possible criteria useful to guide the management of muscle herniae. An electronic search of PubMed and Scopus databases was performed. RESULTS: A total of 132 patients were identified. Conservative management was carried out in 22 (16.7%) patients, and 110 (83.3%) patients underwent surgical procedures. Pain was reported in 3/22 (13.6%) patients managed conservatively. Post-surgical pain was reported in 0/5 (0%) patients treated with autologous graft repair, 1/15 (6.7%) patient with mesh repair, 2/13 (15.4%) patients with direct repair and 11/77 (14.3%) patients with fasciotomy. Return to normal activity was possible in 16/22 (72.7%) patients treated conservatively, 5/5 (100%) patients undergoing autologous graft repair, 13/15 (86.7%) with mesh repair, 62/77 (80.52%) with fasciotomy and 4/12 (33.3%) with direct repair. CONCLUSION: In congenital muscle herniae, fasciotomy should be considered the surgical choice to prevent complications. In post-traumatic muscle hernia, a small fascial defect can be treated with the direct suture repair, while mesh repair and autologous graft repair should be considered the most appropriate procedures to avoid severe complications such as compartment syndrome.

Minimally invasive fasciotomy for symptomatic tibialis anterior muscle hernia.

BACKGROUND: Symptomatic muscle herniae are an uncommon cause of chronic exercise induced leg pain. The most common site for muscle hernia is the tibialis anterior muscle. This study evaluates the outcome of a minimal incision fasciotomy in patients with a symptomatic muscle hernia of the tibialis anterior muscle, and their return to normal daily activities including sport. METHODS: The study reports mid-term results in a series of 22 consecutive patients (17 males and 5 females, median age: 22 years) with a unilateral tibialis anterior MH who had undergone minimally invasive fasciotomy between 2008 and 2019. Clinical outcomes were assessed with SF-36 and European Quality of Life-5 Dimensions scale (EQ-5D). The ability to participate in sport before and after surgery, and the time to return to training (RTT) and to sport (RTS) were recorded. RESULTS: At a median follow up after surgery of 23 months, both questionnaires showed a statistically significant improvement (P < 0.005). At the latest follow up, 16 of patients (73%) had returned to pre-injury or higher levels of sport/activity. The median time to return to training and to return to sport was 7 and 11 weeks respectively. No severe complications and no recurrence of symptoms were recorded. CONCLUSION: Minimally invasive fasciotomy is effective and safe for patients suffering from muscle hernia of the tibialis anterior muscle with good results in the mid-term. LEVEL OF EVIDENCE: IV.

Shear Wave Elastography for Detecting Calf Muscle Stiffness: An Effective Tool for Assessing Sarcopenia.

OBJECTIVES: To explore the feasibility of shear wave elastography for evaluating sarcopenia. METHODS: The shear wave velocities (SWV) of the tibialis anterior, medial gastrocnemius, and soleus were measured in 130 subjects in the Second Affiliated Hospital of Fujian Medical University from January 2021 to June 2022. Consistency was evaluated in 20 cases using the intraclass correlation coefficient. According to the 2019 Asian Working Group for Sarcopenia（AWGS） diagnostic criteria, the patients were divided into a healthy and a sarcopenia group. The differences in SWV between the two groups were compared, and their correlation between calf muscles and muscle mass, grip strength, and pace were analyzed. The diagnostic cutoff value of calf muscle SWV for sarcopenia was obtained using receiver operating characteristic (ROC) curves, and the diagnostic efficacy of different ROC curves was compared. RESULTS: The SWV inter-group and intra-group correlation coefficients of the three lower limbs muscles were all greater than 0.85. Moreover, the corresponding SWV in the sarcopenia group were significantly smaller than those in the healthy control group (P < .05). Further, SWV were positively correlated with the appendicular skeletal muscle mass index (ASMI), grip strength, and gait speed. Finally, the SWV of the anterior tibialis and medial gastrocnemius muscles were 3.02 and 2.26 m/s, respectively, and their diagnostic efficacy for sarcopenia did not differ significantly (Z = 0.190, P = .8497). CONCLUSION: SWE can be used to detect the hardness of the anterior tibialis and medial gastrocnemius, calculate their muscle mass as an effective tool to evaluate sarcopenia.

The Muscle Thickness Assessment Using Ultrasonography is a Useful Alternative to Skeletal Muscle Mass by Bioelectrical Impedance Analysis.

Purpose: Muscle mass, a key index for the diagnosis of sarcopenia, is currently assessed using the appendicular skeletal muscle mass index (ASMI) by bioelectrical impedance analysis (BIA). Muscle thickness (MT) assessed by ultrasonography (US) may be a better determinant and/or predictor of muscle condition than ASMI. Thus, we compared it to the ASMI determined by the BIA. Patients and Methods: Our study included 165 ambulatory older adults (84 males, 81 females, mean age: 76.82 years). The ASMI by the BIA method, MT by US, and the distribution of body mass index (BMI) and body fat percentage (BFP) were examined using defined values for men and women. These were used as the basis for examining the association of MT and ASMI with handgrip strength (HGS), leg muscle strength (LMS), gait speed (GS), and echo intensity (EI). We compared HGS, LMS, GS, and EI for high and low ASMI among lower BMI or BFP. The same was also done for MT assessed by US. Results: MT, as well as ASMI, was strongly associated with HGS and LMS. There was a correlation between MT and GS and EI but not between ASMI and GS and EI. There were significant differences in the prevalence between high ASMI and high MT or low ASMI and low MT in those with lower BMI or BFP. In non-overweight participants, HGS, LMS, GS, and EI were significantly higher in those with high MT than in those with low MT; however, there were no significant differences in them between those with high and low ASMI. Conclusion: In the non-overweight group, the MT assessment by US showed a stronger relationship to muscle strength and muscle quality than the ASMI assessment by BIA. The MT assessment using US is a useful alternative to BIA-assessed ASMI, especially in non-overweight participants.

Running on the Edge: Rupture of the Tibialis Anterior Muscle During an Ultramarathon.

A 47-year-old male ultramarathon runner presented with persistent discomfort in the anterior region of the left leg. The patient reported a snapping sensation in his left leg four weeks earlier while running an ultramarathon, followed by ecchymosis and functional impairment the next day. Physical examination revealed a palpable area of fibrosis in his anterior left leg. Ultrasound of the left leg identified a partially organized rupture of the distal third of the tibialis anterior muscle. The rupture had an extension of 36 x 10 x 27mm with associated muscle edema. The patient was treated non-surgically with a customized rehabilitation program and later returned to ultramarathon running. This case illustrates the importance of proper differential diagnosis and individualized rehabilitation programs to achieve optimum clinical and functional results.

Reliability of TMS measurements using conventional hand-hold method with different numbers of stimuli for tibialis anterior muscle in healthy adults.

Objective: The objective of this study was to determine the reliability of corticomotor excitability measurements using the conventional hand-hold transcranial magnetic stimulation (TMS) method for the tibialis anterior (TA) muscle in healthy adults and the number of stimuli required for reliable assessment. Methods: Forty healthy adults participated in three repeated sessions of corticomotor excitability assessment in terms of resting motor threshold (rMT), slope of recruitment curve (RC), peak motor evoked potential amplitude (pMEP), and MEP latency using conventional TMS method. The first two sessions were conducted with a rest interval of 1 h, and the last session was conducted 7-10 days afterward. With the exception of rMT, the other three outcomes measure elicited with the block of first 3-10 stimuli were analyzed respectively. The within-day (session 1 vs. 2) and between-day (session 1 vs. 3) reliability for all four outcome measures were assessed using intraclass correlation coefficient (ICC), standard error of measurement, and minimum detectable difference at 95% confidence interval. Results: Good to excellent within-day and between-day reliability was found for TMS-induced outcome measures examined using 10 stimuli (ICC ≥ 0.823), except in pMEP, which showed between-day reliability at moderate level (ICC = 0.730). The number of three stimuli was adequate to achieve minimum acceptable within-day reliability for all TMS-induced parameters and between-day reliability for MEP latency. With regard to between-day reliability of RC slope and pMEP, at least seven and nine stimuli were recommended respectively. Conclusion: Our findings indicated the high reliability of corticomotor excitability measurement by TMS with adequate number of stimuli for the TA muscle in healthy adults. This result should be interpreted with caveats for the specific methodological choices, equipment setting, and the characteristics of the sample in the current study. Clinical Trial Registration: http://www.chictr.org.cn, identifier ChiCTR2100045141.

Concentric needle jitter: Reference values in stimulated Tibialis Anterior muscle.

Calculating the reference values for jitter parameters utilizing a disposable concentric needle have been already done for the most often tested muscles. Jitter, expressed as the mean consecutive difference (MCD), was measured in the Tibialis Anterior (TA), not routinely tested muscle. Jitter measurement was taken using the intramuscular microaxonal stimulation technique in 32 healthy subjects. The mean MCD and the mean MCD of the 27th value from the 32 subjects had a normal distribution and were 19.79 ± 2.72 µs and 26.88 ± 3.56 µs, respectively. The suggested limit for the mean MCD is ≥ 26 µs and for the individual values is > 34 µs.

Relationship between attachment site of tibialis anterior muscle and shape of tibia: anatomical study of cadavers.

BACKGROUND: Tibialis anterior (TA) muscle is the largest dorsiflexor of the ankle joint and plays an important role during gait movement. However, descriptions of the TA attachment site are inconsistent even among major anatomy textbooks, and its origin, especially the attachment site for the tibia, has not been reported in detail. This study is the first experimental attempt to investigate the origin of the TA in detail, paying particular attention to the relationship with the shape of the tibia, including sex differences. METHODS: Forty legs (20 males, 20 females) from twenty Japanese cadavers were examined. Gross anatomical examination of the TA's attachment site to the tibia and the tibia's shape was performed. RESULTS: The location of the distal end of the TA's attachment on tibia was significantly more distal in males than in females (p < 0.01). The anterior border of the tibia had a gentle S-like curve, with a medially convex curve in the proximal region and a laterally convex curve in the distal region in frontal plane. The most protruding point of the distal curve of the anterior border located significantly more proximal in females than in males (p = 0.02). CONCLUSIONS: There were sex differences in the distal end of the attachment site on tibia of the TA and the shape of the tibia. Consequently, the variations in the attachment site of TA were considered to provide for differences in function of TA. In males, the TA may enable advantageous power exertion, whereas in females it may work efficiently for dorsiflexion of ankle, respectively. Sex differences in TA's attachment site and the shape of the tibia may be involved in gait movement as well as frequency of lower leg disorders such as chronic exertional compartment syndrome.

Tibialis Anterior Muscle: Pedicled Bone Graft for Defect of the Tibia.

Purpose: To report the result of the bone graft retrograde pedicled with the tibialis anterior muscle belly for the treatment of bone defect of tibia. Methods: Between 2017 and 2020, the bone graft pedicled with the tibialis anterior muscle was applied for the treatment of 11 patients with a cortical defect and nonunion of the tibia due to trauma and one patient with segmental bone defect caused by tramatic osteomyelitis. The tibialis anterior has the largest muscle belly, but has not been commonly used due to functional disability following use as transposition muscle flap. New anatomic and clinical data confirm that the distal half of this group of muscles can be transposed with the tendon left intact with no noticeable loss of muscle function. Based on the vascular nutrition of this muscle, we performed a retrograde pedicled bone graft using the muscle belly, thus preserving the function of this muscle. Result: The operation was relatively simple and early bone unions have been achieved without any complications. Conclusion: The bone graft pedicled with the tibialis anterior muscle is useful for the treatment of bone defect of the tibia.

Alexa Fluor 488-conjugated cholera toxin subunit B optimally labels neurons 3-7 days after injection into the rat gastrocnemius muscle.

Neural tract tracing is used to study neural pathways and evaluate neuronal regeneration following nerve injuries. However, it is not always clear which tracer should be used to yield optimal results. In this study, we examined the use of Alexa Fluor 488-conjugated cholera toxin subunit B (AF488-CTB). This was injected into the gastrocnemius muscle of rats, and it was found that motor, sensory, and sympathetic neurons were labeled in the spinal ventral horn, dorsal root ganglia, and sympathetic chain, respectively. Similar results were obtained when we injected AF594-CTB into the tibialis anterior muscle. The morphology and number of neurons were evaluated at different time points following the AF488-CTB injection. It was found that labeled motor and sensory neurons could be observed 12 hours post-injection. The intensity was found to increase over time, and the morphology appeared clear and complete 3-7 days post-injection, with clearly distinguishable motor neuron axons and dendrites. However, 14 days after the injection, the quality of the images decreased and the neurons appeared blurred and incomplete. Nissl and immunohistochemical staining showed that the AF488-CTB-labeled neurons retained normal neurochemical and morphological features, and the surrounding microglia were also found to be unaltered. Overall, these results imply that the cholera toxin subunit B, whether unconjugated or conjugated with Alexa Fluor, is effective for retrograde tracing in muscular tissues and that it would also be suitable for evaluating the regeneration or degeneration of injured nerves.

Predictors of functional recovery following surgery for foot drop due to degenerative lumbar disease.

OBJECTIVE: Surgery for foot drop secondary to lumbar degenerative disease is not always associated with postoperative functional improvement. It is still unclear whether early decompression results in better functional recovery and how soon surgery should be performed. This study aimed to evaluate predicting factors that affect short- and long-term recovery outcomes and to explore the relationship between timing of lumbar decompression and recovery from foot drop in an attempt to identify a cutoff time from symptom onset until decompression for optimal functional improvement. METHODS: The authors collected demographic, clinical, and radiographic data on patients who underwent surgery for foot drop due to lumbar degenerative disease. Clinical data included tibialis anterior muscle (TAM) strength before and after surgery, duration of preoperative motor weakness, and duration of radicular pain until surgery. TAM strength was recorded at the immediate postoperative period and 1 month after surgery while long-term follow-up on functional outcomes were obtained at ≥ 2 years postsurgery by telephone interview. Data including degree and duration of preoperative motor weakness as well as the occurrence of pain and its duration were collected to analyze their impact on short- and long-term outcomes. RESULTS: The majority of patients (70%) showed functional improvement within 1 month postsurgery and 40% recovered to normal or near-normal strength. Univariate analysis revealed a trend toward lower improvement rates in patients with preoperative weakness of more than 3 weeks (33%) compared with patients who were operated on earlier (76.5%, p = 0.034). In a multivariate analysis, the only significant predictor for maximal strength recovery was TAM strength before surgery (OR 6.80, 95% CI 1.38-33.42, p = 0.018). Maximal recovery by 1 month after surgery was significantly associated with sustained long-term functional improvement (p = 0.006). CONCLUSIONS: Early surgery may improve the recovery rate in patients with foot drop caused by lumbar degenerative disease, yet the strongest predictor for the extent of recovery is the severity of preoperative TAM weakness. Maximal recovery in the short-term postoperative period is associated with sustained long-term functional improvement and independence.

Intramuscular Coherence of the Lower Flexor Muscles during Robotic Ankle-Assisted Gait.

A close-fitting assisted walking device (RE-Gait) designed to assist ankle movements might be a novel approach for acquiring the forefoot rocker function in the gait cycle. The purpose of the present study was to investigate the effects of using RE-Gait by evaluating the intramuscular coherence (IMC) of the two parts of the tibialis anterior muscles (TA), which could indicate whether a common synaptic drive is present. Seventeen healthy volunteers walked on a treadmill at a comfortable speed before, during, and immediately after 15-minute RE-Gait intervention. After RE-Gait intervention, IMC of the two parts of the TA muscles in the beta frequency band in the initial swing phase was significantly enhanced during RE-Gait intervention. In addition, IMCs in the beta and low-gamma frequency bands were significantly correlated with the enhancement ratio of the step length. These results suggest that robotic ankle plantar flexion and dorsiflexion assistance in the initial swing phase may be effective for improving gait function with enhancement of the functioning of the sensorimotor loop.

Alterations of tibialis anterior muscle activation pattern in subjects with type 2 diabetes and diabetic peripheral neuropathy.

Diabetic peripheral neuropathy (DPN) is associated with loss of motor units (MUs), which can cause changes in the activation pattern of muscle fibres. This study investigated the pattern of muscle activation using high-density surface electromyography (HD-sEMG) signals from subjects with type 2 diabetes mellitus (T2DM) and DPN. Thirty-five adults participated in the study: 12 healthy subjects (HV), 12 patients with T2DM without DPN (No-DPN) and 11 patients with T2DM with DPN (DPN). HD-sEMG signals were recorded in the tibialis anterior muscle during an isometric contraction of ankle dorsiflexion at 50% of the maximum voluntary isometric contraction (MVIC) during 30-s. The calculated HD-sEMG signals parameters were the normalised root mean square (RMS), normalised median frequency (MDF), coefficient of variation (CoV) and modified entropy (ME). The RMS increased significantly (p = 0.001) with time only for the DPN group, while the MDF decreased significantly (p < 0.01) with time for the three groups. Moreover, the ME was significantly lower (p = 0.005), and CoV was significantly higher (p = 0.003) for the DPN group than the HV group. Using HD-sEMG, we have demonstrated a reduction in the number of MU recruited by individuals with DPN. This study provides proof of concept for the clinical utility of this technique for identifying neuromuscular impairment caused by DPN.

Anatomical Variations of the Tibialis Anterior Tendon Insertion: An Updated and Comprehensive Review.

The tibialis anterior muscle originates on the lateral condyle of the tibia, on the upper two-thirds of the lateral surface of this bone, on the anterior surface of the interosseous membrane and on the deep surface of the fascia cruris. The distal attachment is typically at the medial cuneiform and first metatarsal. However, the tibialis anterior tendon can vary morphologically in both adults and fetuses. Different authors have created new classification systems for it. The main aim of this review is to present condensed information about the tibialis anterior tendon based on the available literature. Another aim is to compare classification systems and the results of previous studies.

Atrophy Resistant vs. Atrophy Susceptible Skeletal Muscles: "aRaS" as a Novel Experimental Paradigm to Study the Mechanisms of Human Disuse Atrophy.

OBJECTIVE: Disuse atrophy (DA) describes inactivity-induced skeletal muscle loss, through incompletely defined mechanisms. An intriguing observation is that individual muscles exhibit differing degrees of atrophy, despite exhibiting similar anatomical function/locations. We aimed to develop an innovative experimental paradigm to investigate Atrophy Resistant tibialis anterior (TA) and Atrophy Susceptible medial gastrocnemius (MG) muscles (aRaS) with a future view of uncovering central mechanisms. METHOD: Seven healthy young men (22 ± 1 year) underwent 15 days unilateral leg immobilisation (ULI). Participants had a single leg immobilised using a knee brace and air-boot to fix the leg (75° knee flexion) and ankle in place. Dual-energy X-ray absorptiometry (DXA), MRI and ultrasound scans of the lower leg were taken before and after the immobilisation period to determine changes in muscle mass. Techniques were developed for conchotome and microneedle TA/MG muscle biopsies following immobilisation (both limbs), and preliminary fibre typing analyses was conducted. RESULTS: TA/MG muscles displayed comparable fibre type distribution of predominantly type I fibres (TA 67 ± 7%, MG 63 ± 5%). Following 15 days immobilisation, MG muscle volume (-2.8 ± 1.4%, p < 0.05) and muscle thickness decreased (-12.9 ± 1.6%, p < 0.01), with a positive correlation between changes in muscle volume and thickness (R2 = 0.31, p = 0.038). Importantly, both TA muscle volume and thickness remained unchanged. CONCLUSION: The use of this unique "aRaS" paradigm provides an effective and convenient means by which to study the mechanistic basis of divergent DA susceptibility in humans, which may facilitate new mechanistic insights, and by extension, mitigation of skeletal muscle atrophy during human DA.