Muscle Activation During Gravity-Independent Resistance Exercise Compared to Common Exercises.

INTRODUCTION: The aim was to study quadriceps muscle activation during resistance exercise using a flywheel device, developed as a gravity-independent resistance exercise device to be used during spaceflight, compared with traditional strength training exercises.METHODS: Eight healthy men experienced in resistance exercise performed the following exercises in random order: flywheel leg press (FW), knee extension isokinetic dynamometry (ID), barbell front squat (FS), weight stack leg press (LP), and weight stack knee extension (KE). They accomplished eight repetitions of coupled concentric and eccentric actions with simultaneous recordings of surface electromyography (EMG) from the three superficial quadriceps muscles and knee angles using electrogoniometry. Maximal voluntary contraction (MVC) in knee extension was performed before and after these measurements.RESULTS: EMG averaged across muscles and angles and normalized to MVC was 99/76% in FW, 48/41% FS, 65/47% LP, 81/52% KE, and 93/84% ID in concentric/eccentric phases, respectively. FW and ID showed higher mean EMG activity than LP and FS concentrically and higher than all other exercises eccentrically. No difference in activity between FW and ID was found. Pre- and post-MVC torque was comparable.DISCUSSION: Quadriceps muscle activation was superior in FW and ID exercises compared to the other exercises. The difference was most pronounced in the eccentric phase, but even concentric activation was lower in traditional closed chain exercises. This data supports that FW is an effective training tool and should be considered when designing strength training programs for spaceflights and on Earth.Alkner BA, Bring DK-I. Muscle activation during gravity-independent resistance exercise compared to common exercises. Aerosp Med Hum Perform. 2019; 90(6):506-512.

Residual substance P levels after capsaicin treatment correlate with tendon repair.

The aim of the study was to assess healing after capsaicin-induced substance P (SP) depletion during rat Achilles tendon repair by biomechanical testing. Capsaicin treatment reduced the concentrations of SP by ∼60% and calcitonin gene-related peptide by ∼40% as compared with the control group, as assessed by radioimmunoassay in the dorsal root ganglia, at 1 and 4 weeks post-tendon rupture. Also, the peripheral neuronal presence of SP and calcitonin gene-related peptide, as assessed by immunohistochemistry, was lower at both weeks 1 and 4. The decreased peripheral neuronal presence of SP at week 1 correlated with the corresponding levels in the dorsal root ganglia (r = 0.54, p = 0.018). The reduced presence of SP/calcitonin gene-related peptide after capsaicin treatment was verified by a decreased sensitivity to painful mechanical and thermal stimuli (p < 0.05). Correlation analyses between individual residual SP levels and biomechanical tissue properties were performed because of differences in failure mode between the groups and high individual variations in the SP levels after capsaicin treatment. Thus, the residual SP levels in the dorsal root ganglia correlated with transverse area, ultimate tensile strength, and stress at failure (r = 0.39, p = 0.036; r = 0.53, p = 0.005; and r = 0.43, p = 0.023, respectively). Furthermore, individual pain sensitivity at week 2 correlated with peripheral occurrence of SP and was correlated with tensile strength and stress at failure (r = 0.89, p = 0.006 and r = 0.78, p = 0.015) at week 4. In conclusion, rats with higher residual SP levels after capsaicin-induced neuropathy develop improved tensile strength and stress at failure in the healing of Achilles tendon.

Joint immobilization reduces the expression of sensory neuropeptide receptors and impairs healing after tendon rupture in a rat model.

Healing after mobilization versus immobilization was assessed in a model of rat Achilles tendon rupture, by RT-PCR at 8 and 17 days and by histological analyses at 14 and 28 days postrupture. The expression of mRNA for extracellular matrix (ECM) molecules (collagen type I and type III, versican, decorin, and biglycan), and the subjective histological maturation of the healing area were analyzed. Effects of immobilization on healing were related to changes in the peripheral expression of substance P (NK(1))- and calcitonin gene-related peptide (CRLR and RAMP-1)- receptors. At 8 days postinjury, mRNA levels for ECM molecules were equal in both groups. However, by day 17, the ECM mRNA expression in the mobilized group had increased up to approximately 14x that of the immobilized group, which were comparable to intact tendon values. Histological analysis confirmed a higher regenerating activity in the mobilized group, with an increased amount of blood vessels, fibroblasts, and new collagen. The expression of sensory neuropeptide receptors in the mobilized group exhibited a significant increase from 8 to 17 days postinjury similar to the increased ECM mRNA expression, whereas the immobilized group at 17 days exhibited levels comparable to the intact tendon values. Therefore, immobilization postrupture appears to hamper tendon healing, a process which may prove to be directly linked to a downregulated peripheral sensitivity to sensory neuropeptide stimulation.

Intermittent pneumatic compression enhances neurovascular ingrowth and tissue proliferation during connective tissue healing: a study in the rat.

Intermittent pneumatic compression (IPC) is a treatment method to decrease venous stasis and stimulate blood flow. Recently, it was hypothesized that IPC may exert positive effects on tissue healing, a process highly dependent upon adequate circulation. In this study, we investigated the effects of daily 1-h IPC treatment during 2 and 4 weeks post-rat Achilles tendon rupture. The tendons were subjectively and semiquantitatively analyzed for collagen organization, fibroblast density, angiogenesis, and the occurrence of sensory neuropeptides, substance P (SP) and calcitonine gene related peptide (CGRP), as well as for a nerve regeneration marker, growth associated protein 43 (GAP-43). After 2 weeks of treatment, fibroblast density increased by 53% (p = 0.0004), vessel density by 64% (p = 0.022), and the occurrence of SP by 110% (p = 0.047) and CGRP by 47% (p = 0.0163) compared to untreated controls. Following 4 weeks of treatment, both the occurrence of sensory neuropeptides and the vessel density remained significantly higher (p < 0.05), whereas fibroblast density returned to normal. However, at 4 weeks the treated tendons displayed a higher degree of organized parallel collagen fibers, a sign of increased maturation. Daily IPC treatment improves neurovascular ingrowth and fibroblast proliferation in the healing tendon and may accelerate the repair process.

Physical activity modulates nerve plasticity and stimulates repair after Achilles tendon rupture.

In a rat model of tendon rupture using semiquantitative methodology, healing was assessed according to the diameter of newly organized collagen and the occurrence of the sensory neuropeptides (SP, CGRP) in relation to different levels of physical activity. Normally, innervation of the Achilles tendon is confined to the paratenon. After rupture new nerve fibers grow into the tendon proper, but disappear after healing. In a first experiment to establish peak tissue and nerve regeneration after rupture, tendon tissues from freely moving rats were collected consecutively over 16 weeks. A peak increase in organized collagen and nerve ingrowth was observed between week 2 to 4 post rupture. Therefore, in a second experiment week 4 was chosen to assess the effect of physical activity on tendon healing in three groups of rats, that is, wheel running, plaster treated, and freely moving (controls). In the wheel-running group, the diameter of newly organized collagen was 94% ( p = 0.001) greater than that in the plaster-treated group and 48% ( p = 0.02) greater than that in the controls. Inversely, the neuronal occurrence of CGRP in the tendon proper was 57% ( p = 0.02) lower in the wheel-running group than that in the plaster-treated group and 53% ( p = 0.02) lower than that in the controls, suggesting an earlier neuronal in-growth and disappearance in the more active group. Physical activity speeds up tendon healing, which may prove to be linked to accelerated neuronal plasticity.

Increase in sensory neuropeptides surrounding the Achilles tendon in rats with adjuvant arthritis.

The Achilles tendon in rats with adjuvant arthritis was analyzed by radioimmunoassay (RIA) and semi-quantitative immunohistochemistry for the occurrence of two sensory neuropeptides, substance P (SP) and calcitonin gene related peptide (CGRP), and a sensory modulating peptide, galanin (GAL). The tissue concentration of SP and CGRP in the Achilles tendon and its envelope, i.e. the paratenon and bony insertion, as assessed by RIA was increased by 22% and 71%, respectively, compared to normal controls, whereas the level of GAL was unchanged. Semi-quantitative immunohistochemistry applied to different regions of the tendon in arthritic rats disclosed an increased occurrence of SP and CGRP positive nerve fibers in the paratenon and bone tendinous junction, whereas GAL fibers were only increased at the bone tendinous junction. Notably, neither neuropeptides nor inflammatory cells were seen in the tendon proper. The increased occurrence of SP and CGRP in the tendon envelope presumably reflects inflammatory actions, whereas that of GAL implies an endogenous anti-inflammatory response. The observed SP and CGRP upregulation in the paratenon and bony insertion suggests a pathophysiological role in paratenonitis and enthesitis often seen in patients with rheumatoid arthritis. Presumably Achillodynia originates in the tendon envelope rather than the tendon proper. The observations could be used to define new pharmacological targets for mitigating symptoms from tendons in rheumatoid arthritis and possibly also in other disorders. Whether a neuronal pathogenic mechanism underlies tendon overuse disorders in non-arthritic tendinopathies and the development of degeneration, i.e. tendinosis, remains to be studied.

Effects of 17-day spaceflight on knee extensor muscle function and size.

It is generally held that space travelers experience muscle dysfunction and atrophy during exposure to microgravity. However, observations are scarce and reports somewhat inconsistent with regard to the time course, specificity and magnitude of such changes. Hence, we examined four male astronauts (group mean approximately 43 years, 86 kg and 183 cm) before and after a 17-day spaceflight (Space Transport System-78). Knee extensor muscle function was measured during maximal bilateral voluntary isometric and iso-inertial concentric, and eccentric actions. Cross-sectional area (CSA) of the knee extensor and flexor, and gluteal muscle groups was assessed by means of magnetic resonance imaging. The decrease in strength (P<0.05) across different muscle actions after spaceflight amounted to 10%. Eight ambulatory men, examined on two occasions 20 days apart, showed unchanged (P>0.05) muscle strength. CSA of the knee extensor and gluteal muscles, each decreased (P<0.05) by 8%. Knee flexor muscle CSA showed no significant (P>0.05) change. The magnitude of these changes concord with earlier results from ground-based studies of similar duration. The results of this study, however, do contrast with the findings of no decrease in maximal voluntary ankle plantar flexor force previously reported in the same crew.