EMG coherence of foot and ankle muscles increases with a postural challenge in men.

BACKGROUND: The functional role of intrinsic foot muscles in the control of standing balance is often overlooked in rehabilitation, partly because the interactions with ankle muscles are poorly understood. RESEARCH QUESTION: How does coactivation of Flexor Digitorum Brevis (FDB) and soleus (SOL) vary across standing tasks of increasing difficulty. METHODS: Postural sway (Centre of Pressure, CoP) and the electromyographic (EMG) activity of FDB, SOL, Medial Gastrocnemius (MG) and Tibialis Anterior (TA) were measured during bipedal standing, tandem stance, one-legged balance, and standing on toes. Coherence of the rectified EMG signals for SOL and FDB in two bandwidths (0-5 and 10-20 Hz) was calculated as a coactivation index. RESULTS AND SIGNIFICANCE: The CoP sway and the EMG activity of all muscles was greater (P<0.05) for the three difficult tasks. Significant coherence between the SOL and FDB EMG activity was found in both frequency regions: 0-5 and 10-20 Hz. The coherence integral increased with the difficulty of the postural task, especially in the 10-20 Hz band. The findings underscore the important role of FDB in the control of standing balance across tasks and its coactivation with SOL. Clinical recommendations to improve balance control need to consider the interaction between the plantar flexor and intrinsic-foot muscles.

Anatomical variation of quadratus plantae in relation with flexor digitorum brevis.

The muscles of the sole have been traditionally categorized into four layers, but it is more practical to divide them into peripheral and central groups. The peripheral groups include medial and lateral groups. The central plantar muscles are more numerous and divided into superficial and deep layers. During routine dissection in the Department of Anatomy, All India Institute of Medical Sciences Bibinagar, Hyderabad, variations are been observed in the plantar intrinsic muscle in the left foot & right foot of a 53-year-old male cadaver. This is the first cadaveric report of a combination of discrepancies especially the inter-tendinous connection between quadratus plantae and flexor digitorum brevis. Similar observations in the literature were not found by us. It is important to identify and study these dissimilarities of muscles of the sole for surgeons, anatomists, radiologists and orthopaedics as these muscles and tendons are used in foot reconstructive procedures, and for the treatment of some congenital anomalies.

Separated muscle belly of the flexor digitorum brevis for the fifth toe: a case report.

This case report describes a variation of the flexor digitorum brevis (FDB) with a separated muscle belly and tendon at the fifth toe. The narrow tendon and muscle belly for the fifth toe arose from the intermuscular septum between the FDB and abductor digiti minimi adjacent to the arising fibers of the FDB, separating from its other fibers. The tendon and muscle belly for the fifth toe became wider at the base of the metatarsal bones and narrower as it coursed toward the toes in a fusiform shape. The tendon and muscle belly for the fifth toe became thin at the midfoot and coursed just beneath the flexor digitorum longus tendon and entered the digital tendinous sheath. FDB variations including that described herein should be considered when performing various surgical procedures and evaluating the biomechanics of the foot.

Trans-scale thermal signaling in biological systems.

Biochemical reactions in cells serve as the endogenous source of heat, maintaining a constant body temperature. This process requires proper control; otherwise, serious consequences can arise due to the unwanted but unavoidable responses of biological systems to heat. This review aims to present a range of responses to heat in biological systems across various spatial scales. We begin by examining the impaired thermogenesis of malignant hyperthermia in model mice and skeletal muscle cells, demonstrating that the progression of this disease is caused by a positive feedback loop between thermally driven Ca2+ signaling and thermogenesis at the subcellular scale. After we explore thermally driven force generation in both muscle and non-muscle cells, we illustrate how in vitro assays using purified proteins can reveal the heat-responsive properties of proteins and protein assemblies. Building on these experimental findings, we propose the concept of 'trans-scale thermal signaling'.

Advanced Methodology for Rapid Isolation of Single Myofibers from Flexor Digitorum Brevis Muscle.

Isolated individual myofibers are valuable experimental models that can be used in various conditions to understand skeletal muscle physiology and pathophysiology at the tissue and cellular level. This report details a time- and cost-effective method for isolation of single myofibers from the flexor digitorum brevis (FDB) muscle in both young and aged mice. The FDB muscle was chosen for its documented history in single myofiber experiments. By modifying published methods for FDB myofiber isolation, we have optimized the protocol by first separating FDB muscle into individual bundles before the digestion, followed by optimizing the subsequent digestion medium conditions to ensure reproducibility. Morphological and functional assessments demonstrate a high yield of isolated FDB myofibers with sarcolemma integrity achieved in a shorter time frame than previous published procedures. This method could be also adapted to other types of skeletal muscle. Additionally, this highly reproducible method can greatly reduce the number of animals needed to yield adequate numbers of myofibers for experiments. Thus, this advanced method for myofiber isolation has the potential to accelerate research in skeletal muscle physiology and screening potential therapeutics "ex vivo" for muscle diseases and regeneration.

Association of the intrinsic foot muscles and plantar fascia with repetitive rebound jumping and jump landing in adolescent athletes: An ultrasound-based study.

BACKGROUND: The characteristics of foot structure in adolescents and adults are different, affecting sports performance and leading to the progression of foot and lower extremity disorders. RESEARCH QUESTION: This study aimed to investigate the relationship between the intrinsic foot muscles (IFM) and plantar fascia morphology and the repetitive rebound jumping and jump landing ability in adolescent athletes. METHODS: A total of 60 adolescent athletes (35 boys and 25 girls) participated in this study. B-mode ultrasonography was used to obtain images of the IFM and plantar fascia morphology [thickness and cross-sectional area (CSA) of the abductor hallucis (AbH), flexor hallucis brevis (FHB), flexor digitorum brevis (FDB), and thickness of the plantar fascia]. The repetitive rebound jump performance was evaluated using the Optojump™ system. Participants were instructed to jump five times continuously with one leg, jumping as high as possible with minimal ground contact time. The jump landing was assessed by measuring the dynamic posture stability index (DPSI) using forward one-legged jump landings. RESULTS: The thickness and CSA of the AbH and FDB were positively correlated with the jump height and reactive jump index. The DPSI score was significantly correlated with the thickness of the AbH, but not with other IFMs or plantar fascia. In the multiple regression analysis, only the thickness of the FDB was associated with the jump height and reactive jump index, indicating that FDB thickness might facilitate adolescent athletes to jump higher with minimal contact time in repetitive rebounding movements. SIGNIFICANCE: The IFM (especially FDB) should be focused on when examining sports performance in adolescent athletes.

Posture dependent ankle and foot muscle responses evoked by Achilles' tendon vibration.

To investigate the link between the triceps surae and the intrinsic muscles of the foot, often underestimated in posture maintenance, we asked how Achilles' tendon vibration modulates the EMG activity of the soleus and flexor digitorum brevis (FDB) muscles during different postural tasks: sitting, standing and forward leaning. Young healthy participants (n = 19, age = 24 ± 7.4 years) stood for 60 s in three visually controlled postures, while vibration (1.5-1.8 mm, 80 Hz) was bilaterally applied over the Achilles' tendon during the middle 20 s. Center of Pressure (CoP) and EMG activity of the soleus and FDB muscle were summarized in 5 s epochs and compared across time (before, during and after vibration) and postural tasks. Achilles' tendon vibration shifted the CoP position forward in sitting and backward in standing and leaning and increased the root mean square of the CoP velocity to a greater extent in standing and leaning compared to sitting. Soleus and FDB EMG amplitude also increased in response to vibration. These responses were posture dependent, being greater in standing (soleus: 57 %, FDB: 67 % relative to pre-vibration) compared to sitting (soleus: 36 %, FDB: 27 % relative to pre-vibration) and leaning (soleus: 26 %, FDB: 8% relative to pre-vibration). After vibration offset, both soleus and FDB showed sustained activation across all three postures. Results highlight the presence of Ia afferent projections from the soleus to the α motor neurons of the FDB muscle triggered by Achilles' tendon vibration. This link is posture dependent serving a functional role in standing and forward leaning in the presence of externally applied perturbations.

Impact of intertendinous connections between the flexor digitorum brevis and longus on percutaneous tenotomy for the treatment of claw toes: an anatomic and ultrasound study.

PURPOSE: Selective percutaneous tenotomy of the flexor digitorum longus (FDL) is a treatment for claw toes that gives astonishingly good functional results despite tendon sacrifice. However, the involution of the FDL tendon stump after tenotomy is unknown. The aim of our study was to assess the involution of the tendon stump after selective percutaneous tenotomy of the FDL. METHODS: The study included two parts. In the clinical part, an ultrasound analysis of 15 FDL tenotomies in 7 patients was carried out 3 months post-surgery. In the anatomic part, the feet of 10 bodies donated to science were dissected and examined anatomically. RESULTS: The proximal stump of the FDL was located near the base of the proximal phalanx and moved synchronously with the flexor digitorum brevis (FDB).Separating the FDB and FDL revealed a large tissue connection between the plantar surface of the tendinous chiasm of the FDB and the dorsal part of the FDL. These connections had significant resistance ranging from 2 to 9 Newtons depending on the toe. Tenotomy of the FDL followed by proximal traction of it led to retraction of the stump up to the base of the proximal phalanx and transfer of its action to the FDB by tensioning the intertendinous structure. Histologically, these structures were mostly comprised of tendon connective tissue. Their vascular component was small. CONCLUSION: The presence of this intertendinous connection leads, in the case of isolated tenotomy of the FDL, to equivalent transfer of the latter to the FDB.

Morphological variability is greater at developing than mature mouse neuromuscular junctions.

The neuromuscular junction (NMJ) is the highly specialised peripheral synapse formed between lower motor neuron terminals and muscle fibres. Post-synaptic acetylcholine receptors (AChRs), which are found in high density in the muscle membrane, bind to acetylcholine released into the synaptic cleft of the NMJ, thereby enabling the conversion of motor action potentials to muscle contractions. NMJs have been studied for many years as a general model for synapse formation, development and function, and are known to be early sites of pathological changes in many neuromuscular diseases. However, information is limited on the diversity of NMJs in different muscles, how synaptic morphology changes during development, and the relevance of these parameters to neuropathology. Here, this crucial gap was addressed using a robust and standardised semi-automated workflow called NMJ-morph to quantify features of pre- and post-synaptic NMJ architecture in an unbiased manner. Five wholemount muscles from wild-type mice were dissected and compared at immature (post-natal day, P7) and early adult (P31-32) timepoints. The inter-muscular variability was greater in mature post-synaptic AChR morphology than that of the pre-synaptic motor neuron terminal. Moreover, the developing NMJ showed greater differences across muscles than the mature synapse, perhaps due to the observed distinctions in synaptic growth between muscles. Nevertheless, the amount of nerve to muscle contact was consistent, suggesting that pathological denervation can be reliably compared across different muscles in mouse models of neurodegeneration. Additionally, mature post-synaptic endplate diameters correlated with fibre type, independently of muscle fibre diameter. Altogether, this work provides detailed information on healthy pre- and post-synaptic NMJ morphology from five anatomically and functionally distinct mouse muscles, delivering useful reference data for future comparison with neuromuscular disease models.

The contractile properties of motor units in the rat flexor digitorum brevis muscle have continuous distribution.

The majority of motor unit studies were performed predominantly on calf muscles, where three types of units: S, FR and FF were found. These muscles are involved in postural activity, walking, running and jumping. The properties of foot muscles that perform other functions, e.g. scratching (in animals), and are purely co-active with calf muscles, are poorly known. The aim of the present study was to investigate the contractile properties of motor units in the flexor digitorum brevis. Fifty-six motor units were studied in male Wistar rats. Several methods of fast/slow motor unit categorization, presence of sag, contraction time values, and 20 Hz index, did not allow the separation of the studied motor units into discrete clusters. Therefore, motor units were divided into two groups: fatigable and resistant to fatigue, based on the fatigue index with the border value of 0.5 (although the distribution of the index was not bimodal). The fatigable motor units were stronger and faster compared to the resistant ones. In conclusion, the distribution of motor unit contractile properties in the studied foot muscle was continuous and indicated a lack of three separate physiological types of motor units that usually occurs for the majority of hindlimb muscles. This discrepancy appears to be associated with differences in the typical forms of motor unit activity in distinct muscles.

The glucocorticoid-induced leucine zipper mediates statin-induced muscle damage.

Statins, the most prescribed class of drugs for the treatment of hypercholesterolemia, can cause muscle-related adverse effects. It has been shown that the glucocorticoid-induced leucine zipper (GILZ) plays a key role in the anti-myogenic action of dexamethasone. In the present study, we aimed to evaluate the role of GILZ in statin-induced myopathy. Statins induced GILZ expression in C2C12 cells, primary murine myoblasts/myotubes, primary human myoblasts, and in vivo in zebrafish embryos and human quadriceps femoris muscle. Gilz induction was mediated by FOXO3 activation and binding to the Gilz promoter, and could be reversed by the addition of geranylgeranyl, but not farnesyl, pyrophosphate. Atorvastatin decreased Akt phosphorylation and increased cleaved caspase-3 levels in myoblasts. This effect was reversed in myoblasts from GILZ knockout mice. Similarly, myofibers isolated from knockout animals were more resistant toward statin-induced cell death than their wild-type counterparts. Statins also impaired myoblast differentiation, and this effect was accompanied by GILZ induction. The in vivo relevance of our findings was supported by the observation that gilz overexpression in zebrafish embryos led to impaired embryonic muscle development. Taken together, our data point toward GILZ as an essential mediator of the molecular mechanisms leading to statin-induced muscle damage.

A Mechanism for Statin-Induced Susceptibility to Myopathy.

This study aimed to identify a mechanism for statin-induced myopathy that explains its prevalence and selectivity for skeletal muscle, and to understand its interaction with moderate exercise. Statin-associated adverse muscle symptoms reduce adherence to statin therapy; this limits the effectiveness of statins in reducing cardiovascular risk. The issue is further compounded by perceived interactions between statin treatment and exercise. This study examined muscles from individuals taking statins and rats treated with statins for 4 weeks. In skeletal muscle, statin treatment caused dissociation of the stabilizing protein FK506 binding protein (FKBP12) from the sarcoplasmic reticulum (SR) calcium (Ca2+) release channel, the ryanodine receptor 1, which was associated with pro-apoptotic signaling and reactive nitrogen species/reactive oxygen species (RNS/ROS)-dependent spontaneous SR Ca2+ release events (Ca2+ sparks). Statin treatment had no effect on Ca2+ spark frequency in cardiac myocytes. Despite potentially deleterious effects of statins on skeletal muscle, there was no impact on force production or SR Ca2+ release in electrically stimulated muscle fibers. Statin-treated rats with access to a running wheel ran further than control rats; this exercise normalized FKBP12 binding to ryanodine receptor 1, preventing the increase in Ca2+ sparks and pro-apoptotic signaling. Statin-mediated RNS/ROS-dependent destabilization of SR Ca2+ handling has the potential to initiate skeletal (but not cardiac) myopathy in susceptible individuals. Importantly, although exercise increases RNS/ROS, it did not trigger deleterious statin effects on skeletal muscle. Indeed, our results indicate that moderate exercise might benefit individuals who take statins.

Pedicled and Free Tissue Transfers.

Tissue defects that result from diabetic foot infections are often complex and necessitate reconstructive soft-tissue surgery to achieve closure. Intrinsic muscle flaps of the foot require attention to major vascular pedicles and are useful for closing smaller ulcerations. Microvascular free flaps are beneficial for large defects and provide long-term survivability. Perioperative planning is an important aspect of caring for diabetic patients requiring reconstructive surgery. These techniques are valuable tools for use in efforts to preserve a functional limb in this patient population.

Superficial course of the medial plantar nerve: case report.

The medial and lateral plantar nerves are branched from the tibial nerve and move to the tip of the toes. A variation of medial plantar nerve was found on the left side of a 78-year-old Korean male cadaver. The tibial nerve was divided into the lateral and medial plantar nerves beneath the plantar flexor. The medial plantar nerve passed deep to plantar aponeurosis and superficial to the flexor digitorum brevis. It gave off a common plantar digital nerve and then divided into three proper plantar digital nerves near the metatarsal bases. In this article, we report a superficial course of the medial plantar nerve and describe its unique morphology and discuss the clinical significance of this variation.

Intrinsic foot muscles contribute to elastic energy storage and return in the human foot.

The human foot is uniquely stiff to enable forward propulsion, yet also possesses sufficient elasticity to act as an energy store, recycling mechanical energy during locomotion. Historically, this dichotomous function has been attributed to the passive contribution of the plantar aponeurosis. However, recent evidence highlights the potential for muscles to modulate the energetic function of the foot actively. Here, we test the hypothesis that the central nervous system can actively control the foot's energetic function, via activation of the muscles within the foot's longitudinal arch. We used a custom-built loading apparatus to deliver cyclical loads to human feet in vivo, to deform the arch in a manner similar to that observed in locomotion. We recorded foot motion and forces, alongside muscle activation and ultrasound images from flexor digitorum brevis (FDB), an intrinsic foot muscle that spans the arch. When active, the FDB muscle fascicles contracted in an isometric manner, facilitating elastic energy storage in the tendon, in addition to the energy stored within the plantar aponeurosis. We propose that the human foot is akin to an active suspension system for the human body, with mechanical and energetic properties that can be actively controlled by the central nervous system. NEW & NOTEWORTHY The human foot is renowned for its ability to recycle mechanical energy during locomotion, contributing up to 17% of the energy required to power a stride. This mechanism has long been considered passive in nature, facilitated by the elastic ligaments within the arch of the foot. In this paper, we present the first direct evidence that the intrinsic foot muscles also contribute to elastic energy storage and return within the human foot. Isometric contraction of the flexor digitorum brevis muscle tissue facilitates tendon stretch and recoil during controlled loading of the foot. The significance of these muscles has been greatly debated by evolutionary biologists seeking to understand the origins of upright posture and gait, as well as applied and clinical scientists. The data we present here show a potential function for these muscles in contributing to the energetic function of the human foot.

Superficial course of the medial plantar nerve: case report / 대한해부학회지

The medial and lateral plantar nerves are branched from the tibial nerve and move to the tip of the toes. A variation of medial plantar nerve was found on the left side of a 78-year-old Korean male cadaver. The tibial nerve was divided into the lateral and medial plantar nerves beneath the plantar flexor. The medial plantar nerve passed deep to plantar aponeurosis and superficial to the flexor digitorum brevis. It gave off a common plantar digital nerve and then divided into three proper plantar digital nerves near the metatarsal bases. In this article, we report a superficial course of the medial plantar nerve and describe its unique morphology and discuss the clinical significance of this variation.

Isolation, Culture, and Immunostaining of Skeletal Muscle Myofibers from Wildtype and Nestin-GFP Mice as a Means to Analyze Satellite Cell.

Multinucleated myofibers, the functional contractile units of adult skeletal muscle, harbor mononuclear Pax7+ myogenic progenitors on their surface between the myofiber basal lamina and plasmalemma. These progenitors, known as satellite cells, are the primary myogenic stem cells in adult muscle. This chapter describes our laboratory protocols for isolating, culturing, and immunostaining intact myofibers from mouse skeletal muscle as a means for studying satellite cell dynamics. The first protocol discusses myofiber isolation from the flexor digitorum brevis (FDB) muscle. These short myofibers are plated in dishes coated with PureCol collagen (formerly known as Vitrogen) and maintained in a mitogen-poor medium (± supplemental growth factors). Employing such conditions, satellite cells remain at the surface of the parent myofiber while synchronously undergoing a limited number of proliferative cycles and rapidly differentiate. The second protocol discusses the isolation of longer myofibers from the extensor digitorum longus (EDL) muscle. These EDL myofibers are routinely plated individually as adherent myofibers in wells coated with Matrigel and maintained in a mitogen-rich medium, conditions in which satellite cells migrate away from the parent myofiber, proliferate extensively, and generate numerous differentiating progeny. Alternatively, these EDL myofibers can be plated as non-adherent myofibers in uncoated wells and maintained in a mitogen-poor medium (± supplemental growth factors), conditions that retain satellite cell progeny at the myofiber niche similar to the FDB myofiber cultures. However, the adherent myofiber format is our preferred choice for monitoring satellite cells in freshly isolated (Time 0) myofibers. We conclude this chapter by promoting the Nestin-GFP transgenic mouse as an efficient tool for direct analysis of satellite cells in isolated myofibers. While satellite cells have been often detected by their expression of the Pax7 protein or the Myf5nLacZ knockin reporter (approaches that are also detailed herein), the Nestin-GFP reporter distinctively permits quantification of satellite cells in live myofibers, which enables linking initial Time 0 numbers and subsequent performance upon culturing. We additionally point out to the implementation of the Nestin-GFP transgene for monitoring other selective cell lineages as illustrated by GFP expression in capillaries, endothelial tubes and neuronal cells. Myofibers from other types of muscles, such as diaphragm, masseter, and extraocular, can also be isolated and analyzed using protocols described herein. Collectively, this chapter provides essential tools for studying satellite cells in their native position and their interplay with the parent myofiber.

Length-dependent Ca2+ activation in skeletal muscle fibers from mammalians.

We tested the hypotheses that 1) a decrease in activation of skeletal muscles at short sarcomere lengths (SLs) is caused by an inhibition of Ca(2+) release from the sarcoplasmic reticulum (SR), and 2) the decrease in Ca(2+) would be caused by an inhibition of action potential conduction from the periphery to the core of the fibers. Intact, single fibers dissected from the flexor digitorum brevis from mice were activated at different SLs, and intracellular Ca(2+) was imaged with confocal microscopy. Force decreased at SLs shorter than 2.1 µm, while Ca(2+) concentration decreased at SLs below 1.9 µm. The concentration of Ca(2+) at short SL was lower at the core than at the peripheries of the fiber. When the external concentration of Na(+) was decreased in the experimental media, impairing action potential conduction, Ca(2+) gradients were observed in all SLs. When caffeine was used in the experimental media, the gradients of Ca(2+) were abolished. We concluded that there is an inhibition of Ca(2+) release from the sarcoplasmic reticulum (SR) at short SLs, which results from a decreased conduction of action potential from the periphery to the core of the fibers.

Comparative Label-Free Mass Spectrometric Analysis of Mildly versus Severely Affected mdx Mouse Skeletal Muscles Identifies Annexin, Lamin, and Vimentin as Universal Dystrophic Markers.

The primary deficiency in the membrane cytoskeletal protein dystrophin results in complex changes in dystrophic muscles. In order to compare the degree of secondary alterations in differently affected subtypes of skeletal muscles, we have conducted a global analysis of proteome-wide changes in various dystrophin-deficient muscles. In contrast to the highly degenerative mdx diaphragm muscle, which showed considerable alterations in 35 distinct proteins, the spectrum of mildly to moderately dystrophic skeletal muscles, including interosseus, flexor digitorum brevis, soleus, and extensor digitorum longus muscle, exhibited a smaller number of changed proteins. Compensatory mechanisms and/or cellular variances may be responsible for differing secondary changes in individual mdx muscles. Label-free mass spectrometry established altered expression levels for diaphragm proteins associated with contraction, energy metabolism, the cytoskeleton, the extracellular matrix and the cellular stress response. Comparative immunoblotting verified the differences in the degree of secondary changes in dystrophin-deficient muscles and showed that the up-regulation of molecular chaperones, the compensatory increase in proteins of the intermediate filaments, the fibrosis-related increase in collagen levels and the pathophysiological decrease in calcium binding proteins is more pronounced in mdx diaphragm as compared to the less severely affected mdx leg muscles. Annexin, lamin, and vimentin were identified as universal dystrophic markers.

Flexor digitorum brevis tendon transfer to the flexor digitorum longus tendon according to Valtin in posttraumatic flexible claw toe deformity due to extrinsic toe flexor shortening.

Claw toe deformity after posterior leg compartment syndrome is rare but incapacitating. When the mechanism is flexor digitorum longus (FDL) shortening due to ischemic contracture of the muscle after posterior leg syndrome, a good treatment option is the Valtin procedure in which the flexor digitorum brevis (FDB) is transferred to the FDL after FDL tenotomy. The Valtin procedure reduces the deformity by lengthening and reactivating the FDL. Here, we report the outcomes of FDB to FDL transfer according to Valtin in 10 patients with posttraumatic claw toe deformity treated a mean of 34 months after the injury. Toe flexion was restored in all 10 patients, with no claw toe deformity even during dorsiflexion of the ankle.