Involuntary sustained firing of plantar flexor motor neurones: effect of electrical stimulation parameters during tendon vibration.

PURPOSE: Simultaneous application of tendon vibration and neuromuscular electrical stimulation (NMES) induces an involuntary sustained torque. We examined the effect of different NMES parameters (intensity, pattern of stimulation and pulse width) on the magnitude of the evoked involuntary torque. METHODS: Plantar flexor torque was recorded during 33-s Achilles tendon vibration with simultaneous 20-Hz NMES bouts on triceps surae (n = 20; 13 women). Intensity was set to elicit 10, 20 or 30% of maximal voluntary contraction torque (MVC), pulse width was narrow (0.2 ms) or wide (1 ms), and the stimulus pattern varied (5 × 2-s or 10 × 1-s). Up to 12 different trials were performed in a randomized order, and then repeated in those who produced a sustained involuntary torque after the cessation of vibration. RESULTS: Six of 7 men and 5 of 13 women produced a post-vibration sustained torque. Eight of 20 participants did not complete the 30% trials, as they were perceived as painful. Torque during vibration at the end of NMES and the increase in torque throughout the trial were significantly higher in 20 than 10% trials (n = 11; 9.7 ± 9.0 vs 7.1 ± 6.1% MVC and 4.3 ± 4.5 vs 3.6 ± 3.5% MVC, respectively). Post-vibration sustained torque was higher in wide pulse-width trials (5.4 ± 5.9 vs 4.1 ± 4.3% MVC). Measures of involuntary torque were not different between 20 and 30% trials (n = 8). CONCLUSION: Bouts of 5 × 2-s NMES with wide pulse width eliciting 20% MVC provides the most robust responses and could be used to maximise the production of involuntary torque in triceps surae.

Quantitative assessment of neural elements in a rat model using nerve growth factor after remnant-preserving anterior cruciate ligament reconstruction: a histological and immunofluorescence pilot study.

BACKGROUND: Immunofluorescence analyses of anterior cruciate ligament (ACL) allografts following remnant-preserving ACL reconstruction using Achilles tendon allografts have provided evidence for the presence of neural elements. In this study, we aimed to examine the expression of neural elements and quantify the presence of neural cells in ACL remnants and Achilles allografts using nerve growth factor (NGF) therapy after remnant-preserving ACL reconstruction. METHODS: Experiments were conducted on 5 pairs of rats (approximately 8 weeks old and weighing 320 g at the time of surgery). Longitudinally, split Achilles tendons from the paired rats were freshly frozen and later defrosted with warm saline and allografted onto the right ACL of the other rat that was partially detached at the femoral attachment site. A sham operation was conducted on the left knee to be used as a control. NGF was injected into both knee joints every week for 6 weeks after surgery. The presence of neural cells in the ACL of the sham-operated knee, allografted Achilles tendon, and ACL remnant was examined 6 weeks post-surgery using H and E and immunofluorescent staining. RESULTS: H and E staining did not reveal neural cells in any of the three groups. However, immunofluorescence analysis showed the presence of nestin-positive neural elements in the normal ACL tissues as well as ACL remnants. Additionally, neural elements were examined in 7 of the 8 (87.5%) allograft tissues. Quantitative analysis showed no difference in the number or area of nuclei among the three groups. However, the number and area of neural cells in the Achilles allografts were significantly lower than those in the other two groups (p = 0.000 and p = 0.001, respectively). CONCLUSION: Our observations indicate that ACL remnants promote the new ingrowth and persistence of neural cells. We suggest that the ingrowth of neural elements can support the persistence and new ingrowth of mechanoreceptors, thereby enhancing the functional stability of knee joints. Moreover, the expression of neural cells in the Achilles allografts was lower than that in normal ACL tissues and ACL remnants in the quantitative evaluation, thereby confirming the essential role of ACL remnants in knee joint functionalization.

Exaggerated cardiovascular responses to muscle contraction and tendon stretch in UCD type-2 diabetes mellitus rats.

Patients with type-2 diabetes mellitus (T2DM) have exaggerated sympathetic activity and blood pressure responses to exercise. However, the underlying mechanisms for these responses, as well as how these responses change throughout disease progression, are not completely understood. For this study, we examined the effect of the progression of T2DM on the exercise pressor reflex, a critical neurocardiovascular mechanism that functions to increase sympathetic activity and blood pressure during exercise. We also aimed to examine the effect of T2DM on reflexive cardiovascular responses to static contraction, as well as those responses to tendon stretch when an exaggerated exercise pressor reflex was present. We evoked the exercise pressor reflex and mechanoreflex by statically contracting the hindlimb muscles and stretching the Achilles tendon, respectively, for 30 s. We then compared pressor and cardioaccelerator responses in unanesthetized, decerebrated University of California Davis (UCD)-T2DM rats at 21 and 31 wk following the onset of T2DM to responses in healthy nondiabetic rats. We found that the pressor response to static contraction was greater in the 31-wk T2DM [change in mean arterial pressure (∆MAP) = 39 ± 5 mmHg] but not in the 21-wk T2DM (∆MAP = 24 ± 5 mmHg) rats compared with nondiabetic rats (∆MAP = 18 ± 2 mmHg; P < 0.05). Similarly, the pressor and the cardioaccelerator responses to tendon stretch were significantly greater in the 31-wk T2DM rats [∆MAP = 69 ± 6 mmHg; change in heart rate (∆HR) = 28 ± 4 beats/min] compared with nondiabetic rats (∆MAP = 14 ± 2 mmHg; ∆HR = 5 ± 3 beats/min; P < 0.05). These findings suggest that the exercise pressor reflex changes as T2DM progresses and that a sensitized mechanoreflex may play a role in exaggerating these cardiovascular responses.NEW & NOTEWORTHY This is the first study to provide evidence that as type-2 diabetes mellitus (T2DM) progresses, the exercise pressor reflex becomes exaggerated, an effect that may be due to a sensitized mechanoreflex. Moreover, these findings provide compelling evidence suggesting that impairments in the reflexive control of circulation contribute to exaggerated blood pressure responses to exercise in T2DM.

Tendinosis-like changes in denervated rat Achilles tendon.

BACKGROUND: Tendon disorders are common and lead to significant disability and pain. Our knowledge of the 'tennis elbow', the 'jumpers knee', and Achilles tendinosis has increased over the years, but changes in denervated tendons is yet to be described in detail. The aim of the present study was to investigate the morphological and biochemical changes in tendon tissue following two weeks of denervation using a unilateral sciatic nerve transection model in rat Achilles tendons. METHODS: Tendons were compared with respect to cell number, nuclear roundness, and fiber structure. The non-denervated contralateral tendon served as a control. Also, the expression of neuromodulators such as substance P and its preferred receptor neurokinin-1 receptor, NK-1R, was evaluated using real-time qRT-PCR. RESULTS: Our results showed that denervated tendons expressed morphological changes such as hypercellularity; disfigured cells; disorganization of the collagen network; increased production of type III collagen; and increased expression of NK-1R. CONCLUSION: Taken together these data provide new insights into the histopathology of denervated tendons showing that denervation causes somewhat similar changes in the Achilles tendon as does tendinosis in rats.

High resolution real time ultrasonography of the sural nerve after percutaneous repair of the Achilles tendon.

BACKGROUND: Percutaneous Achilles tendon repair has been developed to minimise soft tissue complications following treatment of tendon ruptures. However, there are concerns because of the risk of sural nerve injury. Few studies have investigated the relationship between the Achilles tendon, the sural nerve and its several anatomical course variants. METHODS: We studied 7 cadaveric limbs (7 Achilles tendons) in which a percutaneous repair of the Achilles tendon was performed. On each tendon, high resolution real time ultrasonography examination was performed by an experienced musculoskeletal radiologist before and after the procedure, with the surgeons blind to the results of the scan both before and after surgery. RESULTS: In two instances, high resolution real time ultrasonography examination revealed nerve entrapment at the level of most proximal lateral suture. CONCLUSIONS: Since the sural nerve can be easily visualised using high-frequency high resolution real time ultrasonography, intraoperative ultrasound can be of assistance during percutaneous repair of Achilles tendon rupture. CLINICAL RELEVANCE: The sural nerve can be readily visualised by high-frequency high resolution real time ultrasonography probes. It could be beneficial to use high resolution real time ultrasonography intraoperatively or perioperatively to minimise the risks of sural nerve injury when undertaking percutaneous repair of Achilles tendon tears.

Time-Dependent Changes in the Structure of Calcified Fibrocartilage in the Rat Achilles Tendon-Bone Interface With Sciatic Denervation.

The enthesis transmits a physiological load from soft to hard tissue via fibrocartilage. The histological alterations induced by this physiological loading remain unclear. This study was performed to examine the histomorphological alterations in the collagen fiber bundle alignment and depth of collagen interdigitation between the calcified fibrocartilage and the bone. We examined the Achilles enthesis of rats with sciatic denervation to explore the mechanical effects of structural changes in the enthesis. The parallelism of the collagen fiber bundles was significantly reduced 8 weeks after denervation. However, the depth of collagen interdigitation significantly increased at 2 and 4 weeks after denervation and then significantly decreased 8 weeks after denervation. In conclusion, a lack of muscle loading induced structural alterations in the distal calcified fibrocartilage. These findings suggest that while structural changes in the enthesis are necessary for the development of physiological loading, structural deformities are required in the long term. Anat Rec, 300:2166-2174, 2017. © 2017 Wiley Periodicals, Inc.

Both standing and postural threat decrease Achilles' tendon reflex inhibition from tendon electrical stimulation.

KEY POINTS: Golgi tendon organs (GTOs) and associated Ib reflexes contribute to standing balance, but the potential impacts of threats to standing balance on Ib reflexes are unknown. Tendon electrical stimulation to the Achilles' tendon was used to probe changes in Ib inhibition in medial gastrocnemius with postural orientation (lying prone vs. upright standing; experiment 1) and height-induced postural threat (standing at low and high surface heights; experiment 2). Ib inhibition was reduced while participants stood upright, compared to lying prone (42.2%); and further reduced when standing in the high, compared to low, threat condition (32.4%). These experiments will impact future research because they demonstrate that tendon electrical stimulation can be used to probe Ib reflexes in muscles engaged in standing balance. These results provide novel evidence that human short-latency GTO-Ib reflexes are dependent upon both task, as evidenced by changes with postural orientation, and context, such as height-induced postural threat during standing. ABSTRACT: Golgi tendon organ Ib reflexes are thought to contribute to standing balance control, but it is unknown if they are modulated when people are exposed to a postural threat. We used a novel application of tendon electrical stimulation (TStim) to elicit Ib inhibitory reflexes in the medial gastrocnemius, while actively engaged in upright standing balance, to examine (a) how Ib reflexes to TStim are influenced by upright stance, and (b) the effects of height-induced postural threat on Ib reflexes during standing. TStim evoked short-latency (<47 ms) inhibition apparent in trigger-averaged rectified EMG, which was quantified in terms of area, duration and mean amplitude of inhibition. In order to validate the use of TStim in a standing model, TStim-Ib inhibition was compared from conditions where participants were lying prone vs. standing upright. TStim evoked Ib inhibition in both conditions; however, significant reductions in Ib inhibition area (42.2%) and duration (32.9%) were observed during stance. Postural threat, manipulated by having participants stand at LOW (0.8 m high, 0.6 m from edge) and HIGH (3.2 m, at edge) elevated surfaces, significantly reduced Ib inhibition area (32.4%), duration (16.4%) and amplitude (24.8%) in the HIGH, compared to LOW, threat condition. These results demonstrate TStim is a viable technique for investigating Ib reflexes in standing, and confirm Ib reflexes are modulated with postural orientation. The novel observation of reduced Ib inhibition with elevated postural threat reveals that human Ib reflexes are context dependent, and the human Ib reflex pathways are modulated by threat or emotional processing centres of the CNS.

Nerve distributions in insertional Achilles tendinopathy - a comparison of bone, bursae and tendon.

BACKGROUND/AIM: In a condition of pain in the Achilles tendon insertion there are multiple structures involved, such as the Achilles tendon itself, the retrocalcaneal bursa and a bony protrusion at the calcaneal tuberosity called Haglund's deformity. The innervation patterns of these structures are scarcely described, and the subcutaneous calcaneal bursa is traditionally not considered to be involved in the pathology. This study aimed at describing the innervation patterns of the four structures described above to provide a better understanding of possible origins of pain at the Achilles tendon insertion. METHODS: Biopsies were taken from 10 patients with insertional Achilles tendinopathy, which had pathological changes in the subcutaneous and retrocalcaneal bursae, a Haglund deformity and Achilles tendon tendinopathy as verified by ultrasound. The biopsies were stained using immunohistochemistry in order to delineate the innervation patterns in the structures involved in insertional Achilles tendinopathy. RESULTS: Immunohistochemical examinations found that the subcutaneous bursa scored the highest using a semi-quantitative evaluation of the degree of innervation when compared to the retrocalcaneal bursa, the Achilles tendon, and the calcaneal bone. CONCLUSIONS: These findings suggest that the subcutaneous bursa, which is traditionally not included in surgical treatment, may be a clinically important factor in insertional Achilles tendinopathy.

Medial gastrocnemius specific force of adult men with spastic cerebral palsy.

INTRODUCTION: Muscle weakness determines functional impairment in spastic cerebral palsy (SCP). Measurement of specific force (SF) allows for strength comparison with unimpaired populations (controls) accounting for neural (activation and coactivation), architectural (fascicle length and pennation angle), and structural differences (moment arm length). METHODS: Medial gastrocnemius (MG) SF (and its determinants) was assessed in both paretic and non-paretic legs of 11 men with SCP and 11 age-matched controls during plantarflexion maximal voluntary isometric contraction (MVIC). RESULTS: SCP fascicles were 28% longer than control fascicles (P < 0.05). Pennation angle of SCP patients was 41% smaller than in controls. The physiological cross-sectional area of SCP MG patients was 47% smaller than in controls (P < 0.05). There was no difference in SF between controls and SCP patients. CONCLUSIONS: Weakness in SCP is primarily attributable to deficits in agonist activation and muscle size; consequently, SF measured in the MG is similar between SCP and controls. Muscle Nerve 56: 298-306, 2017.

Anatomy of the sural nerve related to calcaneal tendon, intermalleolar line and small saphenous vein / Anatomía del nervio sural en relación con el tendón calcáneo, la línea intermaleolar y la vena safena parva

Sural nerve (SN) courses from the posterior aspect of leg to the lateral side of ankle and foot. Anatomy of the SN is of clinical importance due to its involvement in nerve biopsy, nerve graft harvesting including injuries during calcaneal tendon repair. Despite substantial knowledge in the literature, more easily located landmarks and data regarding the symmetry are still needed. Ninety-eight lower extremities from 31 males and 18 females were dissected in this study. The SN originated from the union between the medial and lateral sural cutaneous nerves in 52.0 % of the legs. The distance from the union point to the intermalleolar line (IML) was 16.3±8.4 (SD) cm. The ratio of this distance to the fibular length (apex of head to lateral malleolus or LM) was 0.47±0.23 cm. In 84.7 % of the legs, the SN initially coursed medial to and then crossed the lateral border of calcaneal tendon at the distance of 8.4±2.1 cm above the IML. The ratio to the fibular length was 0.25±0.06 cm. At the level of IML, the SN was posterior to the most prominent part of the LM in 95.9 % of the legs with the distance of 2.6±0.5 cm. At the closest point, the SN was lateral to the small saphenous vein in 74.5 % of the legs and the distance from this point to the IML was 10.6±4.7 cm. The ratio to the fibular length was 0.31±0.14 cm. Side difference or asymmetry was observed in a substantial number of specimens. These data are crucial for not only localizing the SN during biopsy and graft harvesting but also avoiding the nerve injury during relevant surgical procedures.

El nervio sural (NS) transcurre por la región posterior de la pierna, el tobillo y el pie. La anatomía del NS es de importancia clínica durante la realización de una biopsia, el desarrollo de injerto de nervio, incluyendo lesiones del nervio durante la reparación del tendón calcáneo. A pesar del conocimiento sustancial en la literatura, aún se necesitan puntos de referencia más fáciles de localizar e información sobre la simetría. Noventa y ocho miembros inferiores de 31 hombres y 18 mujeres fueron disecados. El NS se originó a partir de la unión entre los nervios cutáneo sural medial y cutáneo sural lateral en el 52,0 % de las piernas. La distancia desde el punto de unión a la línea intermaleolar (IML) fue de 16,3±8,4 (SD) cm. La relación de esta distancia a la longitud de la fíbula (vértice de la cabeza al maléolo lateral o LM) fue de 0,47±0,23 cm. En el 84,7 % de las piernas, el NS se ubicó inicialmente medial y luego cruzó el margen lateral del tendón calcáneo a una distancia de 8,4±2,1 cm proximal a la IML. La relación de longitud de la fíbula fue de 0,25±0,06 cm. A nivel de la IML, el NS se localizaba posterior a la parte más prominente de la LM en el 95,9 % de las piernas, con una distancia de 2,6±0,5 cm. En el punto más cercano, el NS se localizaba lateral a la vena safena parva en 74,5 % de las piernas y la distancia desde este punto hasta la IML fue de 10,6±4,7 cm. La relación de longitud de la fíbula fue 0,31±0,14 cm. La diferencia entre los lados o asimetría se observó en un número considerable de ejemplares. Estos datos son esenciales no sólo para localizar el NS durante la biopsia y la realización del injerto sino también para evitar la lesión del nervio durante procedimientos quirúrgicos relevantes.

The mechano-gated channel inhibitor GsMTx4 reduces the exercise pressor reflex in rats with ligated femoral arteries.

Mechanical and metabolic stimuli arising from contracting muscles evoke the exercise pressor reflex. This reflex is greater in a rat model of simulated peripheral arterial disease in which a femoral artery is chronically ligated than it is in rats with freely perfused femoral arteries. The role played by the mechanically sensitive component of the exaggerated exercise pressor reflex in ligated rats is unknown. We tested the hypothesis that the mechano-gated channel inhibitor GsMTx4, a relatively selective inhibitor of mechano-gated Piezo channels, reduces the exercise pressor reflex in decerebrate rats with ligated femoral arteries. Injection of 10 µg of GsMTx4 into the arterial supply of the hindlimb reduced the pressor response to Achilles tendon stretch (a purely mechanical stimulus) but had no effect on the pressor responses to intra-arterial injection of α,ß-methylene ATP or lactic acid (purely metabolic stimuli). Moreover, injection of 10 µg of GsMTx4 into the arterial supply of the hindlimb reduced both the integrated pressor area (control 535 ± 21, GsMTx4 218 ± 24 mmHg·s; P < 0.01), peak pressor (control 29 ± 2, GsMTx4 14 ± 3 mmHg; P < 0.01), and renal sympathetic nerve responses to electrically induced intermittent hindlimb muscle contraction (a mixed mechanical and metabolic stimulus). The reduction of the integrated pressor area during contraction caused by GsMTx4 was greater in rats with ligated femoral arteries than it was in rats with freely perfused femoral arteries. We conclude that the mechanically sensitive component of the reflex contributes to the exaggerated exercise pressor reflex during intermittent hindlimb muscle contractions in rats with ligated femoral arteries.

Reliability of the Achilles tendon tap reflex evoked during stance using a pendulum hammer.

The tendon tap reflex (T-reflex) is often evoked in relaxed muscles to assess spinal reflex circuitry. Factors contributing to reflex excitability are modulated to accommodate specific postural demands. Thus, there is a need to be able to assess this reflex in a state where spinal reflex circuitry is engaged in maintaining posture. The aim of this study was to determine whether a pendulum hammer could provide controlled stimuli to the Achilles tendon and evoke reliable muscle responses during normal stance. A second aim was to establish appropriate stimulus parameters for experimental use. Fifteen healthy young adults stood on a forceplate while taps were applied to the Achilles tendon under conditions in which postural sway was constrained (by providing centre of pressure feedback) or unconstrained (no feedback) from an invariant release angle (50°). Twelve participants repeated this testing approximately six months later. Within one experimental session, tap force and T-reflex amplitude were found to be reliable regardless of whether postural sway was constrained (tap force ICC=0.982; T-reflex ICC=0.979) or unconstrained (tap force ICC=0.968; T-reflex ICC=0.964). T-reflex amplitude was also reliable between experimental sessions (constrained ICC=0.894; unconstrained ICC=0.890). When a T-reflex recruitment curve was constructed, optimal mid-range responses were observed using a 50° release angle. These results demonstrate that reliable Achilles T-reflexes can be evoked in standing participants without the need to constrain posture. The pendulum hammer provides a simple method to allow researchers and clinicians to gather information about reflex circuitry in a state where it is involved in postural control.

Marked innervation but also signs of nerve degeneration in between the Achilles and plantaris tendons and presence of innervation within the plantaris tendon in midportion Achilles tendinopathy.

OBJECTIVES: The plantaris tendon is increasingly recognised as an important factor in midportion Achilles tendinopathy. Its innervation pattern is completely unknown. METHODS: Plantaris tendons (n=56) and associated peritendinous tissue from 46 patients with midportion Achilles tendinopathy and where the plantaris tendon was closely related to the Achilles tendon were evaluated. Morphological evaluations and stainings for nerve markers [general (PGP9.5), sensory (CGRP), sympathetic (TH)], glutamate NMDA receptor and Schwann cells (S-100ß) were made. RESULTS: A marked innervation, as evidenced by evaluation for PGP9.5 reactions, occurred in the peritendinous tissue located between the plantaris and Achilles tendons. It contained sensory and to some extent sympathetic and NMDAR1-positive axons. There was also an innervation in the zones of connective tissue within the plantaris tendons. Interestingly, some of the nerve fascicles showed a partial lack of axonal reactions. CONCLUSION: New information on the innervation patterns for the plantaris tendon in situations with midportion Achilles tendinopathy has here been obtained. The peritendinous tissue was found to be markedly innervated and there was also innervation within the plantaris tendon. Furthermore, axonal degeneration is likely to occur. Both features should be further taken into account when considering the relationship between the nervous system and tendinopathy.

Modulation of soleus corticospinal excitability during Achilles tendon vibration.

Soleus (SOL) corticospinal excitability has been reported to increase during Achilles tendon vibration. The aim of the present study was to further investigate SOL corticospinal excitability and elucidate the changes to intracortical mechanisms during Achilles tendon vibration. Motor-evoked potentials (MEPs) were elicited in the SOL by transcranial magnetic stimulation (TMS) of the corresponding motor cortical area of the leg with and without 50-Hz Achilles tendon vibration. SOL input-output curves were determined. Paired-pulse protocols were also performed to investigate short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF) by conditioning test TMS pulses with sub-threshold TMS pulses at inter-stimulus intervals of 3 and 13 ms, respectively. During Achilles tendon vibration, motor threshold was lower than in the control condition (43 ± 13 vs. 49 ± 11 % of maximal stimulator output; p = 0.008). Input-output curves were also influenced by vibration, i.e. there was increased maximal MEP amplitude (0.694 ± 0.347 vs. 0.268 ± 0.167 mV; p < 0.001), decreased TMS intensity to elicit a MEP of half the maximal MEP amplitude (100 ± 13 vs. 109 ± 9 % motor threshold; p = 0.009) and a strong tendency for decreased slope constant (0.076 ± 0.04 vs. 0.117 ± 0.04; p = 0.068). Vibration reduced ICF (98 ± 61 vs. 170 ± 105 % of test MEP amplitude; p = 0.05), but had no effect on SICI (53 ± 26 vs. 48 ± 22 % of test MEP amplitude; p = 0.68). The present results further document the increased vibration-induced corticospinal excitability in the soleus muscle and suggest that this increase is not mediated by changes in SICI or ICF.

Achilles tendon vibration-induced changes in plantar flexor corticospinal excitability.

Daily Achilles tendon vibration has been shown to increase muscle force, likely via corticospinal neural adaptations. The aim of the present study was to determine the extent by which corticospinal excitability is influenced during direct Achilles tendon vibration. Motor-evoked potentials (MEPs) were elicited in the soleus (SOL), gastrocnemius medialis (GM) and tibialis anterior (TA) by transcranial magnetic stimulation of the motor cortical area of the leg with and without Achilles tendon vibration at various frequencies (50, 80 and 110 Hz). Contralateral homologues were also investigated. SOL and GM MEP amplitude significantly increased by 226 ± 188 and 66 ± 39%, respectively, during Achilles tendon vibration, without any difference between the tested frequencies. No MEP changes were reported for TA or contralateral homologues. Increased SOL and GM MEP amplitude suggests increased vibration-induced corticospinal excitability independent of vibration frequency.

Reduced plantar sole sensitivity induces balance control modifications to compensate ankle tendon vibration and vision deprivation.

The aim of this study was to investigate if sensory reweighting occurred to control balance when the sensitivity of the plantar sole is reduced using cooling. To address this question, visual information was manipulated and/or ankle proprioception was altered by Achilles tendon vibration. It was expected that Achilles tendon vibration and vision deprivation would induce greater center of pressure (CoP) excursions and/or increase of electromyographic (EMG) activity of the ankle muscles (triceps surea and tibialis anterior) with than without cooling of the plantar sole. To verify these hypotheses, the CoP and EMG activity of the ankle muscles were simultaneously recorded during quiet standing trials of 30s before and after feet cooling procedure. Results showed that plantar sole sensitivity alteration did not lead to larger CoP excursions even during Achilles tendon vibration in absence of vision. This could be explained by an increase in the EMG activity of the triceps surae after the cooling procedure without modification of tibialis anterior EMG activity. This study suggests that to compensate alteration in plantar sole sensitivity, the central nervous system increased the muscular activity of the triceps surae to limit CoP excursions.

Can passive stretch inhibit motoneuron facilitation in the human plantar flexors?

The purpose of the present study was to examine the possible inhibitory effect of passive plantar flexor muscle stretching on the motoneuron facilitatory system. Achilles tendon vibration (70 Hz) and triceps surae electrical stimulation (20 Hz) were imposed simultaneously in 11 subjects to elicit contraction through reflexive pathways in two experiments. In experiment 1, a vibration-stimulation protocol was implemented with the ankle joint plantar flexed (+10°), neutral (0°), and dorsiflexed (-10°). In experiment 2, the vibration-stimulation protocol was performed twice before (control), then immediately, 5, 10, and 15 min after a 5-min intermittent muscle stretch protocol. Plantar flexor torque and medial and lateral gastrocnemius and soleus (EMGSol) EMG amplitudes measured during and after (i.e., self-sustained motor unit firing) the vibration protocol were used as an indicator of this facilitatory pathway. In experiment 1, vibration torque, self-sustained torque and EMGSol were higher with the ankle at -10° compared with 0° and +10°, suggesting that this method is valid to assess motoneuronal facilitation. In experiment 2, torque during vibration was reduced by ∼ 60% immediately after stretch and remained depressed by ∼ 35% at 5 min after stretch (P < 0.05). Self-sustained torque was also reduced by ∼ 65% immediately after stretch (P < 0.05) but recovered by 5 min. Similarly, medial gastrocnemius EMG during vibration was reduced by ∼ 40% immediately after stretch (P < 0.05), and EMGSol during the self-sustained torque period was reduced by 44% immediately after stretch (P < 0.05). In conclusion, passive stretch negatively affected the motoneuronal amplification for at least 5 min, suggesting that motoneuron disfacilitation is a possible mechanism influencing the stretch-induced torque loss.

Postural leaning direction challenges the manifestation of tendon vibration responses at the ankle joint.

In this study, we examined the interaction between central and peripheral proprioceptive afferent pathways by applying ankle tendon vibration during postural leaning in different directions. Twenty young participants stood for 60s over the midline of two adjacent force platforms in (a) neutral stance distributing Body Weight (BW) equally between the platforms, (b) forward leaning transferring 80% of BW to the front platform and (c) backward leaning transferring 80% of BW to the rear platform. Participants controlled the degree of leaning by receiving on-line visual feedback of BW distribution matched to a target line. Vibration (80Hz, 1.5-1.8mm) was applied over the Achilles or tibialis anterior tendon during the middle 20s of standing. This induced a postural shift towards the vibration side and an increase in the variability of the BW distribution that was greater in backward compared to forward leaning. EMG responses to tendon vibration were independent of the leaning direction. Antagonistic activity also increased in response to vibration, the amplitude of this increase however was direction dependent. These results favor the hypothesis about the central co-modulation of the vibration evoked proprioceptive inflow based on postural and visual feedback rather than muscle tension constraints.

Effects of interleukin-1 receptor antagonist on collagen and matrix metalloproteinases in stress-shielded achilles tendons of rats.

Based on previous studies showing that interleukin-1 (IL-1) significantly increased after stress shielding, this article reports further research into the possible therapeutic applications of IL-1 receptor antagonist (IL-1Ra). Forty rats whose left Achilles tendons were denervated and completely stress shielded were divided into 5 groups: 2-week phosphate-buffered saline (PBS); 4-week PBS; 2-week IL-1Ra; 4-week IL-1Ra; and normal control. The Achilles tendons were tested morphologically, and the changes in collagen I and III, matrix metalloproteinases (MMP)-1 and -3, and tissue inhibitors of metalloproteinase (TIMP)-1 were determined. The collagen fibrils in the IL-1Ra groups were morphologically more similar to those in the control group than to those in the PBS groups. The collagen I levels increased in the 2-week groups. Significant differences existed between the PBS and IL-1Ra groups at 4 weeks. The MMP-1 level increased dramatically after stress shielding and increased less in the 2-week IL-1Ra group than in the 2-week PBS group. The degree of decrease of MMP-3 in the IL-1Ra groups was significantly less than that in the PBS groups. The collagen III and TIMP-1 levels continued to increase, and no difference was found between the PBS and IL-1Ra groups. Interleukin-1 receptor antagonist prevented morphological deterioration and collagen metabolism of the denervated Achilles tendons after stress shielding, likely by inhibiting the decline of MMP-3 and increasing MMP-1 levels at an early stage.

Acute effects of Achilles tendon vibration on soleus and tibialis anterior spinal and cortical excitability.

Prolonged vibration is known to alter muscle performance. Attenuation of Ia afferent efficacy is the main mechanism suggested. However, changes in motor cortex excitability could also be hypothesized. The purpose of the present study was therefore to analyze the acute and outlasting effects of 1 h of Achilles tendon vibration (frequency, 50 Hz) on the soleus (SOL) and tibialis anterior (TA) neuromuscular excitability. Spinal excitability was investigated by means of H-reflexes and F-waves while cortical excitability was characterized by motor evoked potentials (MEPs) obtained by transcranial magnetic stimulation. Twelve subjects performed the experimental procedures 3 times: at the beginning of the testing session (PRE), immediately after 1 h of Achilles tendon vibration (POST), and 1 h after the end of vibration (POST-1H). Prolonged vibration led to acute reduced H-reflex amplitudes for SOL only (46.9% ± 7.7% vs. 32.8% ± 7%; p = 0.006). Mainly presynaptic inhibition mechanisms were thought to be involved because of unchanged F-wave persistence and amplitude mean values, suggesting unaffected motoneuronal excitability. While no acute effects were reported for SOL and TA cortical excitability, both muscles were characterized by an outlasting increase in their MEP amplitude (0.64 ± 0.2 mV vs. 0.43 ± 0.18 mV and 2.17 ± 0.56 mV vs. 1.26 ± 0.36 mV, respectively; p < 0.05). The high modulation of Ia afferent input by vibration led to changes in motor cortex excitability that could contribute to the enhancement in muscular activation capacities reported after chronic use of tendon vibration.