Modulation of visceral function by selective stimulation of the left vagus nerve in dogs

The superficial regions of the left vagus nerves of a dog were selectively stimulated with 39-electrode spiral cuffs having 13 circumferential groups of three electrodes (GTE) to modulate the function of the innervated internal organs and glands. Under general anaesthesia, the cuffs were chronically implanted around the nerve in the neck in two adult Beagle dogs and remained viable for 16 months. The regions were stimulated with biphasic, rectangular current pulses (2 mA, 200 micros, 20 Hz) delivered to the group of GTE lying close to the region innervating the specific internal organs or glands. The results showed that specific electrode configurations had actions on the heart (GTE 9), lungs (GTE 4) and pressure in the urinary bladder (GTE 1). It was also shown that GTE no. 10 significantly modified the endocrine function of the pancreas. The results of this study clearly demonstrate that internal organs and glands can be selectively stimulated via the selective stimulation of innervating superficial regions of the autonomous peripheral nerve.     

Cuff电极长期植入性能初步研究

目的：研究Cuff电极长期植入性能。方法：在狗面神经颧支周围植入Cuff电极，通过这个植入电极，分别在14天、6月后提取自然发生的神经电信号，比较分析其信号的差异性；通过组织形态学的方法，在Cuff电极植人后6个月观察分析周围有电极植入的神经段落的组织形态学变化，以分析植入电极对神经的影响。结果：通过植入的Cuff电极，在电极植入后14天、6月均能提取出能反映眨眼动作发生的神经信号，所提取到的自然睁眼状态下的神经信号在幅值（RMS）和频率（MPF）方面没有明显的变化。长期植入Cuff电极的神经，未发现有明显的损伤性形态改变。结论：Cuff电极适合作为长期植入体使用。可以提取到稳定的神经电信号，对于所附着神经不会产生明显的损伤，提示这种植入电极的应用具有可逆性。     

The production of denervation-like changes in rat muscle by colchicine, without interference with axonal transport or muscle activity.

1. Rat extensor digitorum longus (EDL) muscles were examined after colchicine treatment of the sciatic nerve. Colchicine was applied in one of two ways: (i) a single sub-epineural injection; (ii) a chronically implanted silicone cuff. 2. After the sub-epineural injection, the entire membrane of muscle fibres became sensitive to iontophoretically applied acetylcholine and the muscle action potentials became resistant to tetrodotoxin. However, the majority of these fibres were found to be normally innervated. 3. These effects were not restricted to the EDL muscle of the colchicine injected side but were also found in the EDL muscle of the contralateral side, indicating that the action of colchicine was systemic. 4. In the treated sciatic nerve there was a partial block of axonal transport of 3H-labelled proteins, which correlated with a partial paralysis of the ipsilateral leg. However, axoplasmic transport was found to be normal in the contralateral sciatic nerve and the contralateral limb was not paralysed despite the supersensitivity of the investigated muscle on that side. 5. When colchicine was applied with a silicone cuff, denervation-like changes were confined to the ipsilateral EDL muscle. However, impulse conduction block at the level of the cuff was usually observed. 6. It is concluded that (i) colchicine can produce denervation-like changes in normally active muscle without blocking axoplasmic transport, through an action probably exerted directly on the muscle membrane, and (ii) that colchicine-cuff experiments failed to provide unambiguous evidence in support of the existence of neurotrophic influences on the muscle membrane.     

Factors determining segmental reflex action in normal and decerebrate cats

1. In the companion paper the gain of the stretch reflex in the ankle extensor muscles of normal cats was shown to increase after decerebration. The objectives of this study were 1) to identify the origin of the increased reflex and 2) to evaluate the contribution from afferents other than ankle extensor muscle afferents to the short-latency reflex. 2. Six cats were trained to stand unaided on four pedestals. Three cats were also trained to control the force exerted with the left hindlimb. The left soleus (SOL) and lateral gastrocnemius (LG) electromyogram (EMG), length, force, and temperature were recorded by chronically implanted electrodes and transducers. Measurements were taken before and after decerebration at the premammillary level. After decerebration limb temperature was returned to its normal range by the use of radiant heat. 3. Reproducible ramp-and-hold stretches and releases of the ankle extensor muscles were produced by a servo-controlled motor that rotated the left rear pedestal about the ankle joint. The length of the ankle extensor muscles changed by 2-3 mm within 30-35 ms after the onset of a ramp perturbation. Reflex responses before and after decerebration were compared at matched background values of muscle length and force. 4. In both the SOL and LG muscles, a short-latency EMG burst appeared 8-12 ms after stretch onset and lasted approximately 20 ms. After decerebration the onset of the rectified and smoothed EMG burst remained unchanged, but its area was increased by 36-89%. 5. The lateral gastrocnemius-soleus (LG-S) electroneurogram (ENG) was chronically recorded in two cats with a nerve cuff recording electrode implanted on the LG-S nerve. LG-S ENG activity started to increase soon after stretch onset and remained high during the entire ramp phase. The stretch-evoked LG-S ENG burst started approximately 8 ms earlier than the short-latency SOL and LG EMG bursts. It was interpreted to reflect mainly an increase in the activity of Group Ia and Ib muscle afferents, caused by increases in both muscle length and muscle force during the stretch. After the cats were decerebrated, for matched postural conditions, the area of the stretch-evoked LG-S ENG burst was increased by 29-35%. Because the length and force changes sensed by the muscle receptors before and after decerebration were similar, this suggests that the sensitivity of muscle spindles was increased as a consequence of altered activity in fusimotor neurons after decerebration.(ABSTRACT TRUNCATED AT 400 WORDS)     

Short-latency crossed inhibitory responses in extensor muscles during locomotion in the cat

During locomotion, contacting an obstacle generates a coordinated response involving flexion of the stimulated leg and activation of extensors contralaterally to ensure adequate support and forward progression. Activation of motoneurons innervating contralateral muscles (i.e., crossed extensor reflex) has always been described as an excitation, but the present paper shows that excitatory responses during locomotion are almost always preceded by a short period of inhibition. Data from seven cats chronically implanted with bipolar electrodes to record electromyography (EMG) of several hindlimb muscles bilaterally were used. A stimulating cuff electrode placed around the left tibial and left superficial peroneal nerves at the level of the ankle in five and two cats, respectively, evoked cutaneous reflexes during locomotion. During locomotion, short-latency ( approximately 13 ms) inhibitory responses were frequently observed in extensors of the right leg (i.e., contralateral to the stimulation), such as gluteus medius and triceps surae muscles, which were followed by excitatory responses ( approximately 25 ms). Burst durations of the left sartorius (Srt), a hip flexor, and ankle extensors of the right leg increased concomitantly in the mid- to late-flexion phases of locomotion with nerve stimulation. Moreover, the onset and offset of Srt and ankle extensor bursts bilaterally were altered in specific phases of the step cycle. Short-latency crossed inhibition in ankle extensors appears to be an integral component of cutaneous reflex pathways in intact cats during locomotion, which could be important in synchronizing EMG bursts in muscles of both legs.     

神经电信号中眼轮匝肌功能状态的识别

目的:提取能反映眼轮匝肌功能活动的狗面神经颧支的神经电信号(ENG),通过对ENG信号的分析,识别出眼轮匝肌的功能状态。方法:利用植入到狗面神经颧支周围的Cuff电极提取有闭眼动作发生期间的神经电信号,采用幅度阈值法,通过对ENG信号的分析,识别出闭眼动作发生时眼轮匝肌的收缩动作。结果:研究中我们提取到了能反映闭眼动作发生的ENG信号,并且通过对信号的分析,识别出了眼轮匝肌收缩动作的发生。结论:可以通过对眼轮匝肌支配神经上ENG信号的分析,监控眼轮匝肌的功能状态。     

Multielectrode spiral cuff for selective stimulation of nerve fibres.

In this paper we present the modelling, design, and initial experimental testing of a nerve cuff multielectrode system for selective stimulation of fibres in superficial peripheral nerve trunk regions which is capable of making a selective activation of multiple muscles. The developed multielectrode nerve cuff consists of 14 platinum stimulating electrodes embedded within a self-curling sheet of biocompatible insulation, exhibiting a spiral transverse cross-section. The spiral shape of the system is such that the number of stimulating electrodes which can be utilized depends on the diameter of the stimulated nerve. Nerves with a greater diameter automatically make use of more electrodes than thin ones. The development was based on results obtained by a histological examination of the peripheral nerves which were planned to be stimulated, and on models of excitation of myelinated nerve fibres. The modelling objectives were to determine the electric field that would be generated within a nerve trunk by a specific electrode. Moreover, the extent of initial excitation of the nerve fibres within the superficial region of the dog sciatic nerve elicited by a certain discrete stimulating electrode was predicted. For this purpose a calculation of activating function for six positions where the nerve fibres were supposed to lie within the longitudinally dissected sciatic nerve was performed. In two acute experiments on the sciatic nerve of the dog the objective was to characterize the effectiveness of the multielectrode system in monopolar selective stimulation of the superficial regions, innervating the gastrocnemius and tibialis anterior muscle. A selectivity preliminary tested by measuring the myoelectric activity of the gastrocnemius and tibialis anterior muscle after 2 months showed good results in both animals.     

Recruitment plasticity of neuromuscular compartments in exercised tibialis anterior using echo-planar magnetic resonance imaging in humans

We investigated the recruitment plasticity of the superficial tibialis anterior (TA-s) and deep tibialis anterior (TA-d) regions of neuromuscular compartments (NMCs) in the m. tibialis anterior (TA) during exercise using echo-planar imaging (EPI). Six healthy men performed dorsiflexion exercise at 60% of maximum voluntary contraction at a frequency of 10 contractions/min inside the magnetic resonance imaging. Transaxial EPIs of the right leg were acquired every 6 s at rest (0.5 min), during exercise (2.5 min) and recovery (5 min). In TA-s, significantly higher signal intensities (SIs) were shown than those in TA-d from immediately after starting the exercise to recovery. It has been demonstrated that SI reflects the degree of recruitment in the activated muscle, thus our result suggest that preferential firing of motor neurons in the superficial region of the NMC occurs during exercise in human TA muscle.     

Effects of chronic electrical stimulation on long-term denervated muscles of the rabbit hind limb

We investigated the extent to which activity induced by chronic electrical stimulation could restore the mass and contractile function of rabbit tibialis anterior (TA) muscles that had undergone atrophy as a result of prolonged denervation. Denervation was carried out by selectively interrupting the motor nerve branches to the ankle dorsiflexors in one hind limb. Stimulators were implanted, with electrodes on the superficial and deep surfaces of the denervated TA muscle. Ten weeks later, the mass and mid-belly cross-sectional area (CSA) of TA muscles subjected to denervation alone had fallen to approximately 40% of normal. At this stage, stimulators in the other rabbits were activated for 1 h/day to deliver 20-ms rectangular bipolar constant-current pulses of 4 mA amplitude at 20 Hz with a duty cycle of 1s ON/2s OFF, a total of 24,000 impulses/day. The animals were examined after a further 2, 6 or 10 weeks. Stimulation restored the wet weight of the denervated muscles to values not significantly different to those of normal, innervated controls. It increased CSA from 39% to 66% of normal, and there was a commensurate increase in maximum isometric tetanic force from 27% to 50% of normal. Light and electron microscopic examination revealed a marked improvement in the size, packing, and internal organization of the stimulated-denervated muscle fibres, suggestive of an ongoing process of restoration. Excitability, contractile speed, power, and fatigue resistance had not, however, been restored to normal levels after 10 weeks of stimulation. Similar results were found for muscles that had been denervated for 39 weeks and then stimulated for 12 weeks. The study demonstrates worthwhile benefits of long-term electrical stimulation in the treatment of established denervation atrophy.     

Recovery of electromyographic activity after transection and surgical repair of the rat sciatic nerve

The recovery of soleus (SOL), gastrocnemius (GAS), and tibialis anterior (TA) electromyographic activity (EMG) after transection and surgical repair of the sciatic nerve was studied in Sprague-Dawley rats using chronically implanted stimulation and recording electrodes. Spontaneous EMG activity in SOL and GAS and direct muscle (M) responses to posterior tibial nerve stimulation persisted for < or =2 days after sciatic nerve transection, but SOL and GAS H-reflexes disappeared immediately. Spontaneous EMG activity began to return 2-3 wk after transection, rose nearly to pretransection levels by 60 days, and persisted for the duration of the study period (120 days). Recovery of stimulus-evoked EMG responses began about 30 days after sciatic nerve transection as multiple small responses with a wide range of latencies. Over time, the latencies of these fractionated responses shortened, their amplitudes increased, and they merged into a distinct short-latency component (the putative M response) and a distinct long-latency component (the putative H-reflex). The extent of recovery of stimulation-evoked EMG was modest: even 100 days after sciatic nerve transection, the responses were still much smaller than those before transection. Similar gradual development of responses to posterior tibial nerve stimulation was also seen in TA, suggesting that some regenerating fibers sent branches into both tibial and common peroneal nerves.     

Surface electromyogram power spectrum changes in human leg muscles following 4 weeks of simulated microgravity

Surface electromyogram (EMG) spectrum changes in human tibialis anterior (TA) and gastroenemius medialis (GM) muscles were studied to investigate the effect of 4-week bed rest (BR) on muscle fatigability. An exhausting isometric test at 50% of the maximal voluntary contraction was performed by 12 clinically healthy men before and after BR. During this test, mean power frequency (MPF) calculated from surface EMG decreased linearly for TA and GM. When changes in MPF were expressed in terms of rate of decrease a significant difference appeared between TA and GM. Furthermore, as a result of BR, the shift in MPF increased significantly for GM (6.1% vs 10.4%) whereas it was not significantly changed for TA (28.6% vs 20.95%). Alterations in maximal torque were also observed with a more pronounced decrease for plantar-flexor (20.5%) compared with dorsiflexor (15.1%) muscles. These results would seem to indicate that simulated microgravity preferentially affects muscles having an antigravity function.     

The role of blood flow and/or muscle hypoxia in capillary growth in chronically stimulated fast muscles

Capillary supply, the proportion of oxidative fibres and blood flow were studied in fast rat muscles (tibialis anterior, TA, and extensor digitorum, EDL) made ischaemic by ligation of the common iliac artery, in chronically stimulated muscles and in ischaemic chronically stimulated muscles. Stimulation was carried out for 6 h/day at 10 Hz (three periods of 2 h with 90–120-min intervals between stimulations) for 10–12 days using electrodes implanted in the vicinity of the lateral popliteal nerve. Blood flow (measured by radioactive microspheres) was 3.62±0.52 ml · 100 g^–1 · min^–1 at rest and 78.4±14.6 ml · 100 g^–1 · min^–1 (mean ± SEM) during isometric contractions at 4 Hz. Ischaemic muscles had significantly lower blood flow at rest as well as during contractions (72±14% and 25±4% of the values in contralateral muscles respectively). Stimulated muscles had significantly higher flow than contralateral control muscles during contractions; stimulted ischaemic muscles had normal blood flow at rest, but the increase in flow during contractions was limited to a similar extent to that in ischaemic muscles alone. Of all anatomically present capillaries (staining for alkaline phosphatase in frozen sections) the capillary/fibre ratio increased by 36% in stimulated tibialis anterior, but was not significantly different from control muscles in stimulated ischaemic TA and was even lower than in control muscles in stimulated ischaemic EDL. The proportion of fast oxidative fibres (estimated on the basis of histochemical staining for myosin ATPase and succinate dehydrogenase) increased from 53.2±3.2% in normal EDL to 82.0±2.3% in chronically stimulated EDL and to 100% in chronically stimulted ischaemic muscles. Chronic stimulation of ischaemic muscles resulted in muscle fibre damage and consequently a loss of muscle weight (34.7±7.3% in EDL and 45.4±4% in TA). Thus capillary growth occurring in chronically stimulated muscles with normal blood flow was prevented when blood supply was limited, although the ischemic stimulated muscles showed a higher oxidative capacity. This finding emphasizes the role of blood flow in the initiation of capillary proliferation.     

Acute and chronic implantation of coiled wire intraneural electrodes during cyclical electrical stimulation

The posterior tibial nerves of 18 rabbits were intraneurally implanted with coiled wire electrodes for up to 9 weeks to evaluate their usefulness for neuromuscular electrical stimulation. In one group an electrode was implanted and removed in one leg while the other leg was chronically implanted. A second group was chronically implanted without electrical stimulation in one leg and implanted with cyclical electrical stimulation applied through the electrode in the other leg. No significant changes in nerve conduction velocities between the time of implantation and up to 9 weeks post-implantation were observed in either the stimulated or the non-stimulated nerves. Little change in motor current threshold was observed beyond 10 days postimplantation. The nerves showed little or no histologic demyelination or denervation in most specimens, although in about 40% of the nerves, a bulbous formation of connective tissue was observed at electrode entry and exit sites with some demyelination in these regions. The spinal cords showed no histologic abnormalities in either group. The gastrocenemius and soleus muscles showed only occasional signs of denervation. One cat was implanted in both the posterior tibial and peroneal nerves of each leg for a 4-year period. Threshold current showed very little change during the implantation period. The nerves showed minimal focal demyelination at the electrode site and the muscles showed normal fibers.     

Effect of 5 weeks horizontal bed rest on human muscle thickness and architecture of weight bearing and non-weight bearing muscles

The aim of the present study was to investigate the changes in thickness, fascicle length (L (f)) and pennation angle (theta) of the antigravity gastrocnemius medialis (GM) and vastus lateralis (VL) muscles, and the non-antigravity tibialis anterior (TA) and biceps brachii (BB) muscles measured by ultrasonography in ten healthy males (aged 22.3 +/- 2.2 years) in response to 5 weeks of horizontal bed rest (BR). After BR, muscle thickness decreased by 12.2 +/- 8.8% (P < 0.05) and 8.0 +/- 9.1% (P < 0.005) in the GM and VL, respectively. No changes were observed in the TA and BB muscles. L (f) and theta decreased by 4.8 +/- 5.0% (P < 0.05) and 14.3 +/- 6.8% (P < 0.005) in the GM and by 5.9 +/- 5.3% (P < 0.05) and 13.5 +/- 16.2% (P < 0.005) in the VL, again without any changes in the TA and BB muscles. The finding that amongst the antigravity muscles of the lower limbs, the GM deteriorated to a greater extent than the VL is possibly related to the differences in relative load that this muscle normally experiences during daily loading. The dissimilar response in antigravity and non-antigravity muscles to unloading likely reflects differences in loading under normal conditions. The significant structural alterations of the GM and VL muscles highlight the rapid remodelling of muscle architecture occurring with disuse.     

Corticospinal input in human gait: modulation of magnetically evoked motor responses

 Transcranial magnetic stimulation (TMS) of the motor cortex was applied during locomotion to investigate the significance of corticospinal input upon the gait pattern. Evoked motor responses (EMR) were studied in the electromyogram (EMG) of tibialis anterior (TA), gastrocnemius (GM) and, for reference, abductor digiti minimi (AD) muscles by applying below-threshold magnetic stimuli during treadmill walking in healthy adults. Averages of 15 stimuli introduced randomly at each of 16 phases of the stride cycle were analysed. Phase-dependent amplitude modulation of EMR was present in TA and GM which did not always parallel the gait-associated modulation of the EMG activity. No variation of onset latency of the EMR was observed. The net modulatory response was calculated by comparing EMR amplitudes during gait with EMR amplitudes obtained (at corresponding background EMG activities) during tonic voluntary muscle contraction. Large net responses in both muscles occurred prior to or during phasic changes of EMG activity in the locomotor pattern. This facilitation of EMR was significantly higher in leg flexor than extensor muscles, with maxima in TA prior to and during late swing phase. A comparison of this facilitation of TA EMR prior to swing phase and prior to a phasic voluntary foot dorsiflexion revealed a similar onset but an increased amount of early facilitation in the gait condition. The modulated facilitation of EMR during locomotion could in part be explained by spinal effects which are different under dynamic and static motor conditions. However, we suggest that changes in corticospinal excitability during gait are also reflected in this facilitation. This suggestion is based on: (1) the similar onset yet dissimilar size of facilitatory effects in TA EMR prior to the swing phase of the stride cycle and during a voluntary dynamic activation, (2) the inverse variation of EMR and EMG amplitudes during this phase, and (3) the occurrence of this inversion at stimulation strengths below motor threshold (motor threshold was determined during weak tonic contraction and EMR were facilitated during gait). It is hypothesized that the facilitation is phase linked to ensure postural stability and is most effective during the phases prior to and during rhythmical activation of the leg muscles resulting in anticipatory adjustment of the locomotor pattern. Received: 17 May 1996 / Accepted: 29 November 1996     

Functional coupling of motor units is modulated during walking in human subjects

Time- and frequency-domain analysis of the coupling between pairs of electromyograms (EMG) recorded from leg muscles was investigated during walking in healthy human subjects. For two independent surface EMG signals from the tibialis anterior (TA) muscle, coupling estimated from coherence measurements was observed at frequencies 相似文献   

Voluntary control of human gait: conditioning of magnetically evoked motor responses in a precision stepping task

 The aim of this study was to investigate visuomotor control during human gait. It was assumed that visual input should modulate transcranially evoked motor potentials (EMPs) during walking. The effect of transcranial magnetic stimulation (TMS) in a visually guided precision stepping task was compared with that during normal gait. EMPs were studied in tibialis anterior (TA), gastrocnemius (GM), and abductor digiti minimi (AD) muscles during treadmill walking. In both stepping tasks, a facilitation of EMPs was observed prior to activation of the respective leg muscle. EMP facilitation proved to be modulated throughout the stride cycle when normalising EMP with respect to the underlying electromyogram (EMG). Facilitation was strongest in TA prior to the swing phase. Significant differences of EMP facilitation between the visual and control tasks were present. In the visual task, maximal facilitation of TA EMPs prior to and during the swing phase was decreased compared to the control task. Conversely, there was increased facilitation of GM EMPs during swing phase of the visual task, prior to the heel strike and prior to the plantarflexion, which was the moment when the target was hit. Thus, the effect of visual input upon EMPs in TA and GM was differential and reciprocal according to the respective functional state. The results support the hypothesis of a conditioning effect of visual or, alternatively volitional, drive on EMPs during stepping. Received: 24 June 1998 / Accepted: 4 February 1999     

Models of Metabolic Utilization Predict Limiting Conditions for Sustained Power from Conditioned Skeletal Muscle

Application and development of muscle powered cardiac assist devices is limited by the ability to predict the sustainable power output of in situ conditioned muscle under the expected loading conditions and geometrical constraints. Empirical definition of the sustained power limits and representative models of the bounding conditions where continuous power can be obtained are needed for device design and optimization. The latissimus dorsi muscles of four goats were chronically conditioned for 11 weeks with an implanted myostimulator. The ability to sustain power under isotonic conditions was evaluated across a range of contraction durations (100–600 ms) and rates (10–120 contractions/min). Muscles were characterized both biomechanically and myothermically to develop and evaluate three increasingly complex empirically-based models of metabolic utilization per contraction based on (1) the duty cycle, (2) a linear function of activation time, and (3) a multivariate-derived function of contraction duration, muscle load, and shortening distance. A clearly defined boundary for sustainable stimulation conditions was observed and was best represented by the linear metabolic model. These data provide both an empirical measure of chronically sustainable muscle power and predictive metabolic models that may be used to optimize the power harnessed for skeletal muscle actuated devices.     

Assessing muscle-specific potassium concentrations in human lower leg using potassium magnetic resonance imaging

Noninvasively assessing tissue potassium concentrations (TPCs) using potassium magnetic resonance imaging (³⁹K MRI) could give valuable information on physiological processes connected to various pathologies. However, because of inherently low ³⁹K MR image resolution and strong signal blurring, a reliable measurement of the TPC is challenging. The aim of this work was to investigate the feasibility of a muscle-specific TPC determination with a focus on the influence of a varying residual quadrupolar interaction in human lower leg muscles. The quantification accuracy of a muscle-specific TPC determination was first assessed using simulated ³⁹K MRI data. In vivo ³⁹K and corresponding sodium (²³Na) MRI data of healthy lower leg muscles (n = 14, seven females) were acquired on a 7-T MR system using a double-resonant ²³Na/³⁹K birdcage Tx/Rx RF coil. Additional ¹H MR images were acquired on a 3-T MR system and used for tissue segmentation. Quantification of TPC was performed after a region-based partial volume correction (PVC) using five external reference phantoms. Simulations not only underlined the importance of PVC for correctly assessing muscle-specific TPC values, but also revealed the strong impact of a varying residual quadrupolar interaction between different muscle regions on the measured TPC. Using ³⁹K T₂^* decay curves, we found significantly higher residual quadrupolar interaction in tibialis anterior muscle (TA; ω_q = 194 ± 28 Hz) compared with gastrocnemius muscle (medial/lateral head, GM/GL; ω_q = 151 ± 25 Hz) and soleus muscle (SOL; ω_q = 102 ± 32 Hz). If considered in the PVC, TPC in individual muscles was similar (TPC = 98 ± 11/96 ± 14/99 ± 8/100 ± 12 mM in GM/GL/SOL/TA). Comparison with tissue sodium concentrations suggested that residual quadrupolar interactions might also influence the ²³Na MRI signal of lower leg muscles. A TPC determination of individual lower leg muscles is feasible and can therefore be applied in future studies. Considering a varying residual quadrupolar interaction for PVC of ³⁹K MRI data is essential to reliably assess potassium concentrations in individual muscles.     

Neuromuscular and biomechanical coupling in human cycling: modulation of cutaneous reflex responses to sural nerve stimulation

This study tested the hypothesis that the modulation of cutaneous reflexes during human cycling would be dependent on muscle biomechanical function and phase of leg movement. The coupling between neuromuscular (electromyographic, EMG), kinetic and kinematic responses to brief innocuous (75% of the pain threshold PnT) and noxious (125% PnT) sural nerve stimulation were studied. Stimuli were delivered pseudorandomly at eight equidistant (45°) positions of the crank cycle. Peak ipsilateral middle latency EMG reflex responses were calculated between 70 and 130 ms post stimulus in Biceps Femoris (BF), Rectus Femoris (RF), Tibialis Anterior (TA) and Soleus (SOL). Peak torque, knee and ankle joint angle changes were calculated between 140 and 220 ms post stimulus to quantify net kinetic and kinematic reflex modulation. Reflex responses were predominately suppressive during early activation of all muscles and facilitatory during BF and TA muscle inactivation. EMG reflex responses in monoarticular lower leg muscles TA and SOL were well correlated with ankle angle in dorsi/plantaflexion, whereas the correlation between reflex modulation in biarticular upper leg muscles (BF and RF) and knee angle changes in flexion/extension was weaker. Stimulation provoked significant ankle eversion over the whole crank cycle for both stimulus intensities, which was correlated with TA and BF EMG reflex responses. Torque modulation followed EMG and kinematic changes in a movement phase-dependent manner. Reflex magnitude was stimulation intensity-dependent. Supplementary nociceptive activation may contribute for this increase. We conclude that sural nerve stimulation during human cycling evokes distinct reflex responses in muscles operating around the knee (BF and RF) and the ankle (TA and SOL). These reflexes are modulated in a phase-dependent manner depending on muscle biomechanical function to generate energy for limb and crank propulsion during a specific region in the cycle. This modulation contributed to a specific adaptation of joint motion and force production in order to maintain task performance.