Effects of 20 days of bed rest on the viscoelastic properties of tendon structures in lower limb muscles

Neuromuscular mechanisms for the fast decline in rate of force development with muscle disuse - a narrative review

The removal of skeletal muscle tension (unloading or disuse) is followed by many changes in the neuromuscular system, including muscle atrophy and loss of isometric maximal strength (measured by maximal force, Fmax). Explosive strength, i.e. the ability to develop the highest force in the shortest possible time, to maximise rate of force development (RFD), is a fundamental neuromuscular capability, often more functionally relevant than maximal muscle strength. In the present review, we discuss data from studies that looked at the effect of muscle unloading on isometric maximal versus explosive strength. We present evidence that muscle unloading yields a greater decline in explosive relative to maximal strength. The longer the unloading duration, the smaller the difference between the decline in the two measures. Potential mechanisms that may explain the greater decline in measures of RFD relative to Fmax after unloading are higher recruitment thresholds and lower firing rates of motor units, slower twitch kinetics, impaired excitation-contraction coupling, and decreased tendon stiffness. Using a Hill-type force model, we showed that this ensemble of adaptations minimises the loss of force production at submaximal contraction intensities, at the expense of a disproportionately lower RFD. With regard to the high functional relevance of RFD on one hand, and the boosted detrimental effects of inactivity on RFD on the other hand, it seems crucial to implement specific exercises targeting explosive strength in populations that experience muscle disuse over a longer time.

Adaptive responses of skeletal muscle to calcaneal tendon partial injury in rats: insights into remodeling and plasticity

BACKGROUND

Skeletal muscle is a highly adaptive tissue, capable of responding to different physiological and functional demands, even in situations that may cause instability.

OBJECTIVES

To evaluate how partial calcaneal tendon (CT) injuries affect the remodeling and plasticity of the gastrocnemius muscle over time.

METHODS AND RESULTS

The study was carried out with Wistar rats randomly divided into five groups. The control group comprised animals not subjected to partial CT damage. The remaining four groups were subjected to partial CT damage and were further categorized based on the time of euthanasia: 3, 14, 28, and 55 days after injury. The gastrocnemius muscle was collected and used for gene expression analysis, zymography, flow cytometry, and morphology. The calcaneal tendon was analyzed only to verify the presence of the partial injury.

RESULTS

The impact of partial CT injury on the gastrocnemius homeostasis, particularly on gene expression, was more pronounced in the 3-day group compared to the other groups, especially the control group. Cytokine profile and morphologic alterations occurred in the 55 days group when compared to the other groups.

CONCLUSIONS

The data reported here suggest that partial injury can negatively affect intracellular signaling and degradation pathways, disturbing the muscular extracellular matrix regulatory mechanisms and communication with the tendon. However, skeletal muscle seems to mitigate these harmful effects in comparison with lesions that affect muscle and tendon.

Leveraging in vivo animal models of tendon loading to inform tissue engineering approaches

Tendon injuries disrupt successful transmission of force between muscle and bone, resulting in reduced mobility, increased pain, and significantly reduced quality of life for affected patients. There are currently no targeted treatments to improve tendon healing beyond conservative methods such as rest and physical therapy. Tissue engineering approaches hold great promise for designing instructive biomaterials that could improve tendon healing or for generating replacement graft tissue. More recently, engineered microphysiological systems to model tendon injuries have been used to identify therapeutic targets. Despite these advances, current tissue engineering efforts that aim to regenerate, replace, or model injured tendons have largely failed due in large part to a lack of understanding of how the mechanical environment of the tendon influences tissue homeostasis and how altered mechanical loading can promote or prevent disease progression. This review article draws inspiration from what is known about tendon loading from in vivo animal models and identifies key metrics that can be used to benchmark success in tissue engineering applications. Finally, we highlight important challenges and opportunities for the field of tendon tissue engineering that should be taken into consideration in designing engineered platforms to understand or improve tendon healing.

Muscle stiffness indicating mission crew health in space

Muscle function is compromised by gravitational unloading in space affecting overall musculoskeletal health. Astronauts perform daily exercise programmes to mitigate these effects but knowing which muscles to target would optimise effectiveness. Accurate inflight assessment to inform exercise programmes is critical due to lack of technologies suitable for spaceflight. Changes in mechanical properties indicate muscle health status and can be measured rapidly and non-invasively using novel technology. A hand-held MyotonPRO device enabled monitoring of muscle health for the first time in spaceflight (> 180 days). Greater/maintained stiffness indicated countermeasures were effective. Tissue stiffness was preserved in the majority of muscles (neck, shoulder, back, thigh) but Tibialis Anterior (foot lever muscle) stiffness decreased inflight vs. preflight (p < 0.0001; mean difference 149 N/m) in all 12 crewmembers. The calf muscles showed opposing effects, Gastrocnemius increasing in stiffness Soleus decreasing. Selective stiffness decrements indicate lack of preservation despite daily inflight countermeasures. This calls for more targeted exercises for lower leg muscles with vital roles as ankle joint stabilizers and in gait. Muscle stiffness is a digital biomarker for risk monitoring during future planetary explorations (Moon, Mars), for healthcare management in challenging environments or clinical disorders in people on Earth, to enable effective tailored exercise programmes.

Changes in real-world walking speed following 60-day bed-rest

The aim of this work was to explore whether real-world walking speed (RWS) would change as a consequence of 60-day bed-rest. The main hypothesis was that daily RWS would decrease after the bed-rest, with a subsequent recovery during the first days of re-ambulation. Moreover, an exploratory analysis was done in order to understand whether there is an agreement between the loss in RWS after bed-rest and the loss in the maximum oxygen uptake capacity (VO2max), or the loss in maximal vertical jump power (JUMP) respectively. Twenty-four subjects were randomly assigned to one of three groups: a continuous artificial gravity group, an intermittent artificial gravity group, or a control group. The fitted linear mixed effects model showed a significant decrease (p < 0.001) of RWS after the 60-day bed-rest and a subsequent increase (p < 0.001) of RWS during the 14-day recovery period in the study facility. No or little agreement was found between the loss in RWS and the loss in VO2max capacity or the loss in maximal vertical jumping power (RWS vs. VO2max: p = 0.81, RWS vs. JUMP: p = 0.173). Decreased RWS after bed-rest, with a follow-up recovery was observed for all three groups, regardless of the training intervention. This suggests that RWS, also in these settings, was able to reflect a de-conditioning and follow-up recovery process.

Simulated spaceflight-induced cardiac remodeling is modulated by gut microbial-derived trimethylamine N-oxide

Spaceflight is physically demanding and can negatively affect astronauts' health. It has been shown that the human gut microbiota and cardiac function are affected by spaceflight and simulated spaceflight. This study investigated the effects of the gut microbiota on simulated spaceflight-induced cardiac remodeling using 10° of head-down bed rest (HDBR) in rhesus macaques and 30° of hindlimb unloading (HU) in mice. The gut microbiota, fecal metabolites, and cardiac remodeling were markedly affected by HDBR in macaques and HU in mice, cardiac remodeling in control mice was affected by the gut microbiota of HU mice and that of HU mice was protected by the gut microbiota of control mice, and there was a correlation between cardiac remodeling and the gut microbial-derived metabolite trimethylamine N-oxide. These findings suggest that spaceflight can affect cardiac remodeling by modulating the gut microbiota and fecal metabolites.

Multimodal and multiscale characterization reveals how tendon structure and mechanical response are altered by reduced loading

Tendons are collagen-based connective tissues where the composition, structure and mechanics respond and adapt to the local mechanical environment. Adaptation to prolonged inactivity can result in stiffer tendons that are more prone to injury. However, the complex relation between reduced loading, structure, and mechanical performance is still not fully understood. This study combines mechanical testing with high-resolution synchrotron X-ray imaging, scattering techniques and histology to elucidate how reduced loading affects the structural properties and mechanical response of rat Achilles tendons on multiple length scales. The results show that reduced in vivo loading leads to more crimped and less organized fibers and this structural inhomogeneity could be the reason for the altered mechanical response. Unloading also seems to change the fibril response, possibly by altering the strain partitioning between hierarchical levels, and to reduce cell density. This study elucidates the relation between in vivo loading, the Achilles tendon nano-, meso‑structure and mechanical response. The results provide fundamental insights into the mechanoregulatory mechanisms guiding the intricate biomechanics, tissue structural organization, and performance of complex collagen-based tissues. STATEMENT OF SIGNIFICANCE: Achilles tendon properties allow a dynamic interaction between muscles and tendon and influence force transmission during locomotion. Lack of physiological loading can have dramatic effects on tendon structure and mechanical properties. We have combined the use of cutting-edge high-resolution synchrotron techniques with mechanical testing to show how reduced loading affects the tendon on multiple hierarchical levels (from nanoscale up to whole organ) clarifying the relation between structural changes and mechanical performance. Our findings set the first step to address a significant healthcare challenge, such as the design of tailored rehabilitations that take into consideration structural changes after tendon immobilization.

Effects of 60 days of 6° head-down bed rest on the composition and function of the human gut microbiota

Spaceflight is rigorous and dangerous environment which can negatively affect astronauts' health and the entire mission. The 60 days of 6° head-down bed rest (HDBR) experiment provided us with an opportunity to trace the change of gut microbiota under simulated microgravity. The gut microbiota of volunteers was analyzed and characterized by 16S rRNA gene sequencing and metagenomic sequencing. Our results showed that the composition and function of the volunteers' gut microbiota were markedly was affected by 60 days of 6° HDBR. We further confirmed the species and diversity fluctuations. Resistance and virulence genes in the gut microbiota were also affected by 60 days of 6° HDBR, but the species attributions remained stable. The human gut microbiota affected by 60 days of 6° HDBR which was partially consistent with the effect of spaceflight, this implied that HDBR was a simulation of how spaceflight affects the human gut microbiota.

Muscle-tendon unit design and tuning for power enhancement, power attenuation, and reduction of metabolic cost

The contractile elements in skeletal muscle fibers operate in series with elastic elements, tendons and potentially aponeuroses, in muscle-tendon units (MTUs). Elastic strain energy (ESE), arising from either work done by muscle fibers or the energy of the body, can be stored in these series elastic elements (SEEs). MTUs vary considerably in their design in terms of the relative lengths and stiffnesses of the muscle fibers and SEEs, and the force and work generating capacities of the muscle fibers. However, within an MTU it is thought that contractile and series elastic elements can be matched or tuned to maximize ESE storage. The use of ESE is thought to improve locomotor performance by enhancing contractile element power during activities such as jumping, attenuating contractile element power during activities such as landing, and reducing the metabolic cost of movement during steady-state activities such as walking and running. The effectiveness of MTUs in these potential roles is contingent on factors such as the source of mechanical energy, the control of the flow of energy, and characteristics of SEE recoil. Hence, we suggest that MTUs specialized for ESE storage may vary considerably in the structural, mechanical, and physiological properties of their components depending on their functional role and required versatility.

Stimuli for Adaptations in Muscle Length and the Length Range of Active Force Exertion-A Narrative Review

Treatment strategies and training regimens, which induce longitudinal muscle growth and increase the muscles’ length range of active force exertion, are important to improve muscle function and to reduce muscle strain injuries in clinical populations and in athletes with limited muscle extensibility. Animal studies have shown several specific loading strategies resulting in longitudinal muscle fiber growth by addition of sarcomeres in series. Currently, such strategies are also applied to humans in order to induce similar adaptations. However, there is no clear scientific evidence that specific strategies result in longitudinal growth of human muscles. Therefore, the question remains what triggers longitudinal muscle growth in humans. The aim of this review was to identify strategies that induce longitudinal human muscle growth. For this purpose, literature was reviewed and summarized with regard to the following topics: (1) Key determinants of typical muscle length and the length range of active force exertion; (2) Information on typical muscle growth and the effects of mechanical loading on growth and adaptation of muscle and tendinous tissues in healthy animals and humans; (3) The current knowledge and research gaps on the regulation of longitudinal muscle growth; and (4) Potential strategies to induce longitudinal muscle growth. The following potential strategies and important aspects that may positively affect longitudinal muscle growth were deduced: (1) Muscle length at which the loading is performed seems to be decisive, i.e., greater elongations after active or passive mechanical loading at long muscle length are expected; (2) Concentric, isometric and eccentric exercises may induce longitudinal muscle growth by stimulating different muscular adaptations (i.e., increases in fiber cross-sectional area and/or fiber length). Mechanical loading intensity also plays an important role. All three training strategies may increase tendon stiffness, but whether and how these changes may influence muscle growth remains to be elucidated. (3) The approach to combine stretching with activation seems promising (e.g., static stretching and electrical stimulation, loaded inter-set stretching) and warrants further research. Finally, our work shows the need for detailed investigation of the mechanisms of growth of pennate muscles, as those may longitudinally grow by both trophy and addition of sarcomeres in series.

Commentary: The impact of COVID-19 Delta variant home confinement on student physical health literacy—a call for awareness in physical education

During the coronavirus disease 2019 (COVID-19) Delta variant wave of the pandemic, governments implemented health orders and policies that included citizens in lockdowns and confined to the home. These health orders and home confinement reduced children’s time for engagement in physical activity and access to supervised, structured physical education (PE) lessons. To understand the potential impact on the physiological aspects of children’s physical health literacy, one must understand the adverse consequences of detraining in relation to the reduction in physical activity and supervised structured PE lessons. The aim of this paper is to give a brief overview of how PE students were affected during the current COVID-19 pandemic. It will also discuss the impact of confinement on physiological adaptions. The article ends with a short discussion on the resumption of face-to-face PE lessons after enforced confinement and PE programming considerations. In such an uncertain environment, PE teachers need to show caution and consider the effects of detraining when programming PE lessons to ensure their safety and strategies for reconditioning.

Impact of chronic obstructive pulmonary disease on passive viscoelastic components of the musculoarticular system

Chronic obstructive pulmonary disease (COPD) produces skeletal muscle atrophy and weakness, leading to impairments of exercise performance. The mechanical work needed for movement execution is also provided by the passive tension developed by musculoarticular connective tissue. To verify whether COPD affects this component, the passive viscoelastic properties of the knee joint were evaluated in 11 patients with COPD and in 11 healthy individuals. The levels of stiffness and viscosity were assessed by means of the pendulum test, consisting in a series of passive leg oscillations. In addition, to explore the contribution of passive tension in the mechanical output of a simple motor task, voluntary leg flexion-extension movements were performed. Patients with COPD showed a statistically significant reduction in stiffness and viscosity compared to controls. Voluntary execution of flexion-extension movements revealed that the electromyographic activity of the Rectus Femoris and Biceps Femoris was lower in patients than in controls, and the low viscoelastic tension in the patients conditioned the performance of active movements. These results provide novel insights on the mechanism responsible for the movement impairments associated with COPD.

Peripheral nerve adaptations to 10 days of horizontal bed rest in healthy young adult males

Space analogs, such as bed rest, are used to reproduce microgravity-induced morphological and physiological changes and can be used as clinical models of prolonged inactivity. Nevertheless, nonuniform decreases in muscle mass and function have been frequently reported, and peripheral nerve adaptations have been poorly studied, although some of these mechanisms may be explained. Ten young healthy males (18-33 yr) underwent 10 days of horizontal bed rest. Peripheral neurophysiological assessments were performed bilaterally for the dominant (DL) and nondominant upper and lower limbs (N-DL) on the 1st and 10th day of bed rest, including ultrasound of the median, deep peroneal nerve (DPN), and common fibular nerve (CFN) , as well as a complete nerve conduction study (NCS) of the upper and lower limbs. Consistently, reduced F waves, suggesting peripheral nerve dysfunction, of both the peroneal (DL: P = 0.005, N-DL: P = 0.013) and tibial nerves (DL: P = 0.037, N-DL: P = 0.005) were found bilaterally, whereas no changes were observed in nerve ultrasound or other parameters of the NCS of both the upper and lower limbs. In these young healthy males, only the F waves, known to respond to postural changes, were significantly affected by short-term bed rest. These preliminary results suggest that during simulated microgravity, most changes occur at the muscle or central nervous system level. Since the assessment of F waves is common in clinical neurophysiological examinations, caution should be used when testing individuals after prolonged immobility.

Nonuniform loss of muscle strength and atrophy during bed rest: a systematic review

Muscle atrophy and decline in muscle strength appear very rapidly with prolonged disuse or mechanical unloading after acute hospitalization or experimental bed rest. The current study analyzed data from short-, medium-, and long-term bed rest (5-120 days) in a pooled sample of 318 healthy adults and modeled the mathematical relationship between muscle strength decline and atrophy. The results show a logarithmic disuse-induced loss of strength and muscle atrophy of the weight-bearing knee extensor muscles. The greatest rate of muscle strength decline and atrophy occurred in the earliest stages of bed rest, plateauing later, and likely contributed to the rapid neuromuscular loss of function in the early period. In addition, during the first 2 wk of bed rest, muscle strength decline is much faster than muscle atrophy: on day 5, the ratio of muscle atrophy to strength decline as a function of bed rest duration is 4.2, falls to 2.4 on day 14, and stabilizes to a value of 1.9 after ∼35 days of bed rest. Positive regression revealed that ∼79% of the muscle strength loss may be explained by muscle atrophy, while the remaining is most likely due to alterations in single fiber mechanical properties, excitation-contraction coupling, fiber architecture, tendon stiffness, muscle denervation, neuromuscular junction damage, and supraspinal changes. Future studies should focus on neural factors as well as muscular factors independent of atrophy (single fiber excitability and mechanical properties, architectural factors) and on the role of extracellular matrix changes. Bed rest results in nonuniform loss of isometric muscle strength and atrophy over time, where the magnitude of change was greater for muscle strength than for atrophy. Future research should focus on the loss of muscle function and the underlying mechanisms, which will aid in the development of countermeasures to mitigate or prevent the decline in neuromuscular efficiency.NEW & NOTEWORTHY Our study contributes to the characterization of muscle loss and weakness processes reflected by a logarithmic decline in muscle strength induced by chronic bed rest. Acute short-term hospitalization (≤5 days) associated with periods of disuse/immobilization/prolonged time in the supine position in the hospital bed is sufficient to significantly decrease muscle mass and size and induce functional changes related to weakness in maximal muscle strength. By bringing together integrated evaluation of muscle structure and function, this work identifies that 79% of the loss in muscle strength can be explained by muscle atrophy, leaving 21% of the functional loss unexplained. The outcomes of this study should be considered in the development of daily countermeasures for preserving neuromuscular integrity as well as preconditioning interventions to be implemented before clinical bed rest or chronic gravitational unloading (e.g., spaceflights).

Sarcomere length of the vastus intermedius with the knee joint angle change

The force-length relation of the skeletal muscles is an important factor influencing the joint torque at a given joint angle. We aimed to clarify the relationship between the resting sarcomere length and knee joint angle in the vastus intermedius (VI) and to compare it with that of the vastus lateralis (VL). The left and right legs were fixed at knee joint angles of 0° and 90°, respectively, in seven cadavers (age at the time of death: 70-91 years). Muscle tissues were dissected by necropsy of the VL and the VI, and electron microscopy images were obtained to calculate the sarcomere length. At knee joint angles of 0° and 90°, the VL sarcomere length was 2.28 ± 0.49 μm and 2.30 ± 0.48 μm, respectively, and the VI sarcomere length was 2.19 ± 0.35 μm and 2.46 ± 0.53 μm, respectively, with a significant difference between the two (p = 0.028). The magnitude of sarcomere length changes with knee joint angle changes was significantly greater for the VI (0.27 ± 0.20 μm) than for the VL (0.02 ± 0.09 μm) (p = 0.009). Thus, knee joint angle changes may affect the passive and active tension produced by the VI more than those produced by the VL.

Mechanotransduction of stem cells for tendon repair

Tendon is a mechanosensitive tissue that transmits force from muscle to bone. Physiological loading contributes to maintaining the homeostasis and adaptation of tendon, but aberrant loading may lead to injury or failed repair. It is shown that stem cells respond to mechanical loading and play an essential role in both acute and chronic injuries, as well as in tendon repair. In the process of mechanotransduction, mechanical loading is detected by mechanosensors that regulate cell differentiation and proliferation via several signaling pathways. In order to better understand the stem-cell response to mechanical stimulation and the potential mechanism of the tendon repair process, in this review, we summarize the source and role of endogenous and exogenous stem cells active in tendon repair, describe the mechanical response of stem cells, and finally, highlight the mechanotransduction process and underlying signaling pathways.

Lower Limb Tendinopathy Tissue Changes Assessed through Ultrasound: A Narrative Review

Tendinopathy is a common disease that affects athletes, causing pain and dysfunction to the afflicted tendon. A clinical diagnose is usually combined with imaging and, among all the existing techniques, ultrasound is widely adopted. The aim of this review is to sum up the existing evidence on ultrasound as an imaging tool and guide for treatments in lower limbs tendinopathy. Using three different databases—PubMed, MEDLINE and CENTRAL—a literature search has been performed in May 2020 combining MeSH terms and free terms with Boolean operators. Authors independently selected studies, conducted quality assessment, and extracted results. Ultrasound imaging has a good reliability in the differentiation between healthy and abnormal tendon tissue, while there are difficulties in the identification of tendinopathy stages. The main parameters considered by ultrasound imaging are tendon thickness, hypoechogenicity of tendon structure and neovascularization of the tendon bound tissue. Ultrasound-guide is also used in many tendinopathy treatments and the available studies gave encouraging results, even if further studies are needed in this field.

Mechanical and Material Tendon Properties in Patients With Proximal Patellar Tendinopathy

Introduction

The effect of chronic patellar tendinopathy on tissue function and integrity is currently unclear and underinvestigated. The aim of this cohort comparison was to examine morphological, material, and mechanical properties of the patellar tendon and to extend earlier findings by measuring the ability to store and return elastic energy in symptomatic tendons.

Methods

Seventeen patients with chronic (>3 months, VISA-P < 80), inferior pole patellar tendinopathy (24 ± 4 years; male = 12, female = 5) were carefully matched to controls (25 ± 3 years) for training status, pattern, and history of loading of the patellar tendon. Individual knee extension force, patellar tendon stiffness, stress, strain, Young’s modulus, hysteresis, and energy storage capacity, were obtained with combined dynamometry, ultrasonography, magnetic resonance imaging, and electromyography.

Results

Anthropometric parameters did not differ between groups. VISA-P scores ranged from 28 to 78 points, and symptoms had lasted from 10 to 120 months before testing. Tendon proximal cross-sectional area was 61% larger in the patellar tendinopathy group than in the control group. There were no differences between groups in maximal voluntary isometric knee extension torque (p = 0.216; d < −0.31) nor in tensile tendon force produced during isometric ramp contractions (p = 0.185; d < −0.34). Similarly, tendon strain (p = 0.634; d < 0.12), hysteresis (p = 0.461; d < 0.18), and strain energy storage (p = 0.656; d < 0.36) did not differ between groups. However, patellar tendon stiffness (−19%; p = 0.007; d < −0.74), stress (−27%; p< 0.002; d < −0.90) and Young’s modulus (−32%; p = 0.001; d < −0.94) were significantly lower in tendinopathic patients compared to healthy controls.

Discussion

In this study, we observed lower stiffness in affected tendons. However, despite the substantial structural and histological changes occurring with tendinopathy, the tendon capacity to store and dissipate energy did not differ significantly.

Objective assessment of stiffness in the gastrocnemius muscle in patients with symptomatic Achilles tendons

OBJECTIVES

To establish quantitative values of stiffness for the gastrocnemius muscle in patients diagnosed with Achilles tendinopathy.

DESIGN

Case-control study with a single (cross sectional) time point of patients diagnosed with unilateral Achilles tendinopathy and an asymptomatic group.

METHODS

Sample of 50 participants: 25 participants with symptomatic unilateral Achilles tendinopathy (symptomatic group) with the asymptomatic side of this group used as a control (control group 1). A third group of 25 participants with asymptomatic Achilles tendons (control group 2). Mechanical stiffness parameters, including natural oscillation frequency (F), dynamic stiffness (S), mechanical stress relaxation time (R), logarithmic decrement (D) and creep (C), were assessed using the MyotonPRO. Measurements at the medial and lateral gastrocnemii were taken as non-weight bearing (NWB) and weight bearing (WB).

RESULTS

Significant (p<0.05) differences were observed between the mean symptomatic group and control groups 1 and 2 in the WB and NWB conditions for natural oscillation frequency (F), dynamic stiffness (S), and creep (C). There was a significant reduction in S WB and conversely a significant increase in S of the symptomatic group NWB. Creep (C) and mechanical stress relaxation time (R) were significantly (p<0.05) less in the symptomatic group NWB. Significant differences were observed between the NWB and WB data sets for logarithmic decrement (D). NWB D demonstrated significantly (p<0.05) higher scores than the WB condition.

CONCLUSION

This study demonstrated that there was a significant reduction in stiffness of the gastrocnemius muscle in participants with Achilles tendinopathy compared with the two control groups. The WB MyotonPRO measures for the symptomatic groups of F and S were significantly lower, indicating a decrease in the tonal properties (F) and dynamic stiffness (S) of the muscle. The application of MyotonPRO technology to measure changes in stiffness in the gastrocnemius muscle may be useful to measure as part of a rehabilitation programme where specific attention could be placed to increase the tonal properties of the muscle. Further studies are required to develop a robust clinical application of this technology, particularly with regard to Achilles tendinopathy.

Effects of the strain rate on mechanical properties of tendon structures in knee extensors and plantar flexors in vivo

The purpose of present study was to investigate site differences in the effects of strain rate on tendon properties between knee extensors and plantar flexors. Elongation of tendon structures (L) in vastus lateralis and medial gastrocnemius muscles was measured by ultrasonography while participants performed ramp and ballistic contractions to their voluntary maximum, followed by ramp and sudden relaxation. The relationship between muscle force (Fm) and L was fit to linear regression, the slope of which was defined as stiffness of tendon structures. Hysteresis of tendon structures was calculated as ratio of area within Fm-L loop to area beneath loading portion of curve. In knee extensors and plantar flexors, L values at all force levels were significantly lower during ballistic than ramp contractions. However, no significant differences were observed in stiffness of tendon structures between two contractions at both measured sites. Hysteresis of tendon structures was significantly higher during ballistic than ramp contractions for knee extensors and plantar flexors. In conclusion, elongation of tendon structures was lower and hysteresis was greater during ballistic than ramp contractions. Furthermore, site differences in the effects of strain rate on tendon properties were not found between knee extensors and plantar flexors.

Tensiomyography detects early hallmarks of bed-rest-induced atrophy before changes in muscle architecture

In young and older people, skeletal muscle mass is reduced after as little as 7 days of disuse. The declines in muscle mass after such short periods are of high clinical relevance, particularly in older people who show a higher atrophy rate and a slower or even a complete lack of muscle mass recovery after disuse. Ten men (24.3 yr; SD 2.6) underwent 35 days of 6° head-down tilt bed rest, followed by 30 days of recovery. During bed rest, a neutral energy balance was maintained, with three weekly passive physiotherapy sessions to minimize muscle soreness and joint stiffness. All measurements were performed in a hospital at days 1-10, 16, 28, and 35 of bed rest (BR1-BR10, BR16, BR28, and BR35, respectively) and days 1, 3, and 30 after reambulation (R + 1, R + 3, and R + 30, respectively). Vastus medialis obliquus (VMO), vastus medialis longus (VML), and biceps femoris (BF) thickness (d) and pennation angle (Θ) were assessed by ultrasonography, whereas twitch muscle belly displacement (Dm) and contraction time (Tc) were assessed with tensiomyography (TMG). After bed rest, d and Θ decreased by 13-17% in all muscles ( P < 0.001) and had recovered at R + 30. Dm was increased by 42.3-84.4% ( P < 0.001) at BR35 and preceded the decrease in d by 7, 5, and 3 days in VMO, VML, and BF, respectively. Tc increased only in BF (32.1%; P < 0.001) and was not recovered at R + 30. TMG can detect early bed-rest-induced changes in muscle with higher sensitivity before overt architectural changes, and atrophy can be detected. NEW & NOTEWORTHY Detection of early atrophic processes and irreversible adaptation to disuse are of high clinical relevance. With the use of tensiomyography (TMG), we detected early atrophic processes before overt architectural changes, and atrophy can be detected using imaging technique. Furthermore, TMG detected irreversible changes of biceps femoris contraction time.

Hypoxia Aggravates Inactivity-Related Muscle Wasting

Poor musculoskeletal state is commonly observed in numerous clinical populations such as chronic obstructive pulmonary disease (COPD) and heart failure patients. It, however, remains unresolved whether systemic hypoxemia, typically associated with such clinical conditions, directly contributes to muscle deterioration. We aimed to experimentally elucidate the effects of systemic environmental hypoxia upon inactivity-related muscle wasting. For this purpose, fourteen healthy, male participants underwent three 21-day long interventions in a randomized, cross-over designed manner: (i) bed rest in normoxia (NBR; P_iO₂ = 133.1 ± 0.3 mmHg), (ii) bed rest in normobaric hypoxia (HBR; P_iO₂ = 90.0 ± 0.4 mmHg) and ambulatory confinement in normobaric hypoxia (HAmb; P_iO₂ = 90.0 ± 0.4 mmHg). Peripheral quantitative computed tomography and vastus lateralis muscle biopsies were performed before and after the interventions to obtain thigh and calf muscle cross-sectional areas and muscle fiber phenotype changes, respectively. A significant reduction of thigh muscle size following NBR (-6.9%, SE 0.8%; P < 0.001) was further aggravated following HBR (-9.7%, SE 1.2%; P = 0.027). Bed rest-induced muscle wasting in the calf was, by contrast, not exacerbated by hypoxic conditions (P = 0.47). Reductions in both thigh (-2.7%, SE 1.1%, P = 0.017) and calf (-3.3%, SE 0.7%, P < 0.001) muscle size were noted following HAmb. A significant and comparable increase in type 2× fiber percentage of the vastus lateralis muscle was noted following both bed rest interventions (NBR = +3.1%, SE 2.6%, HBR = +3.9%, SE 2.7%, P < 0.05). Collectively, these data indicate that hypoxia can exacerbate inactivity-related muscle wasting in healthy active participants and moreover suggest that the combination of both, hypoxemia and lack of activity, as seen in COPD patients, might be particularly harmful for muscle tissue.

Lower tendon stiffness in very old compared with old individuals is unaffected by short-term resistance training of skeletal muscle

Aging negatively affects collagen-rich tissue, like tendons, but in vivo tendon mechanical properties and the influence of physical activity after the 8th decade of life remain to be determined. This study aimed to compare in vivo patellar tendon mechanical properties in moderately old (old) and very old adults and the effect of short-term resistance training. Twenty old (9 women, 11 men, >65 yr) and 30 very old (11 women, 19 men, >83 yr) adults were randomly allocated to heavy resistance training (HRT) or no training (CON) and underwent testing of in vivo patellar tendon (PT) mechanical properties and PT dimensions before and after a 3-mo intervention. Previous measurements of muscle properties, blood parameters, and physical activity level were included in the analysis. Data from 9 old HRT, 10 old CON, 14 very old CON, and 12 old HRT adults were analyzed. In addition to lower quadriceps muscle strength and cross-sectional area (CSA), we found lower PT stiffness and Young's modulus ( P < 0.001) and a trend toward the lower mid-portion PT-CSA ( P = 0.09) in very old compared with old subjects. Daily step count was also lower in very old subjects ( P < 0.001). Resistance training improved muscle strength and cross-sectional area equally in old and very old subjects ( P < 0.05) but did not affect PT mechanical properties or dimension. We conclude that PT material properties are reduced in very old age, and this may likely be explained by reduced physical activity. Three months of resistance training however, could not alter PT mechanical properties in very old individuals. NEW & NOTEWORTHY This research is the first to quantify in vivo tendon mechanical properties in a group of very old adults in their eighties. Patellar tendon stiffness was lower in very old (87 yr on average) compared with moderately old (68 yr on average) individuals. Reduced physical activity with aging may explain some of the loss in tendon stiffness, but regular heavy resistance training for 3 mo was not sufficient to change tendon mechanical properties.

Effects of 14 days of bed rest and following physical training on metabolic cost, mechanical work, and efficiency during walking in older and young healthy males

In this study, we investigated: i) the effects of bed rest and a subsequent physical training program on metabolic cost (Cw), mechanical work and efficiency during walking in older and young men; ii) the mechanisms underlying the higher Cw observed in older than young men.Twenty-three healthy male subjects (N = 16 older adults, age 59.6±3.4 years; N = 7 young, age: 23.1±2.9 years) participated in this study. The subjects underwent 14 days of bed rest followed by two weeks of physical training (6 sessions). Cw, mechanical work, efficiency, and co-contraction time of proximal muscles (vastus lateralis and biceps femoris) and distal muscles (gastrocnemius medialis and tibialis anterior) were measured during walking at 0.83, 1.11, 1.39, 1.67 m·s-1 before bed rest (pre-BR), after bed rest (post-BR) and after physical training (post-PT).No effects of bed rest and physical training were observed on the analysed parameters in either group. Older men showed higher Cw and lower efficiency at each speed (average +25.1 and -20.5%, P<0.001, respectively) compared to young. Co-contraction time of proximal and distal muscles were higher in older than in young men across the different walking speeds (average +30.0 and +110.3%, P<0.05, respectively).The lack of bed rest and physical training effects on the parameters analyzed in this study may be explained by the healthy status of both young and older men, which could have mitigated the effects of these interventions on walking motor function. On the other hand, the fact that older adults showed greater Cw, overall higher co-contraction time of antagonist lower limb muscles, and lower efficiency compared to the young cohort throughout a wide range of walking speed may suggest that older adults sacrificed economy of walking to improve stability.

Skeletal Muscle Contraction Time and Tone Decrease After 8 Weeks of Plyometric Training

The aim of the study was to examine whether an improvement in jumping performance after 8 weeks of plyometric training (PT) runs in parallel with changes in lower-limb skeletal muscle contractile properties. Using noninvasive tensiomyography (TMG), we assessed contraction time (Tc) and the maximal amplitude of radial displacement (Dm) in 20 subjects (50% men; age 22.4 ± 4.7 years of age), randomly divided in PT group (N = 10; PLYO) and a control group (N = 10; CTRL). The PLYO performed 8 weeks of PT. Tensiomyography was measured in 5 leg skeletal muscles: vastus lateralis (VL), biceps femoris (BF), tibialis anterior (TA), gastrocnemius medialis (GM), and gastrocnemius lateralis (GL). Additionally, we evaluated countermovement jump (CMJ) height improvement on a ground force plate. Assessments were repeated before and after PT. After 8 weeks of PT, CMJ height increased by 12.2% in PLYO (p = 0.015), but not in CRTL. Contraction time, which is related to myosin heavy-chain type 1 (MHC-1) proportion, decreased in VL (-8.7%; p < 0.001), BF (-26.7%; p = 0.032), TA (-32.9%; p = 0.004), and GL (-25.8%; p = 0.044), but not in GM (-8.1%; p = 0.158). The estimated VL MHC-1 proportion decreased by -8.2% (p = 0.041). The maximal amplitude of radial displacement, inversely related to muscle tone, decreased in BF (-26.5%; p = 0.032), GM (-14.9%; p = 0.017), GL (-31.5%; p = 0.017), but not in TA (-16.8%; p = 0.113) and VL (-6.0%; p = 0.654). After PT, jumping performance increased, which was paralleled by decreased Tc and decreased muscle tone. Additionally, adaptations to contractile properties were muscle specific, which is important for future studies. It seems that adjustments were dose dependent, being higher in muscles with lower habitual load.

Tendon collagen synthesis declines with immobilization in elderly humans: no effect of anti-inflammatory medication

Nonsteroidal anti-inflammatory drugs (NSAIDs) are used as pain killers during periods of unloading caused by traumatic occurrences or diseases. However, it is unknown how tendon protein turnover and mechanical properties respond to unloading and subsequent reloading in elderly humans, and whether NSAID treatment would affect the tendon adaptations during such periods. Thus we studied human patellar tendon protein synthesis and mechanical properties during immobilization and subsequent rehabilitating resistance training and the influence of NSAIDs upon these parameters. Nineteen men (range 60–80 yr) were randomly assigned to NSAIDs (ibuprofen 1,200 mg/day; Ibu) or placebo (Plc). One lower limb was immobilized in a cast for 2 wk and retrained for 6 wk. Tendon collagen protein synthesis, mechanical properties, size, expression of genes related to collagen turnover and remodeling, and signal intensity (from magnetic resonance imaging) were investigated. Tendon collagen synthesis decreased ( P < 0.001), whereas tendon mechanical properties and size were generally unchanged with immobilization, and NSAIDs did not influence this. Matrix metalloproteinase-2 mRNA tended to increase ( P < 0.1) after immobilization in both groups, whereas scleraxis mRNA decreased with inactivity in the Plc group only ( P < 0.05). In elderly human tendons, collagen protein synthesis decreased after 2 wk of immobilization, whereas tendon stiffness and modulus were only marginally reduced, and NSAIDs had no influence upon this. This indicates an importance of mechanical loading for maintenance of tendon collagen turnover. However, reduced collagen production induced by short-term unloading may only marginally affect tendon mechanical properties in elderly individuals.

NEW & NOTEWORTHY In elderly humans, 2 wk of inactivity reduces tendon collagen protein synthesis, while tendon stiffness and modulus are only marginally reduced, and NSAID treatment does not affect this. This indicates that mechanical loading is important for maintenance of tendon collagen turnover and that changes in collagen turnover induced by short-term immobilization may only have minor impact on the internal structures that are essential for mechanical properties in elderly tendons.

New Imaging Methods for Non-invasive Assessment of Mechanical, Structural, and Biochemical Properties of Human Achilles Tendon: A Mini Review

The mechanical properties of tendon play a fundamental role to passively transmit forces from muscle to bone, withstand sudden stretches, and act as a mechanical buffer allowing the muscle to work more efficiently. The use of non-invasive imaging methods for the assessment of human tendon's mechanical, structural, and biochemical properties in vivo is relatively young in sports medicine, clinical practice, and basic science. Non-invasive assessment of the tendon properties may enhance the diagnosis of tendon injury and the characterization of recovery treatments. While ultrasonographic imaging is the most popular tool to assess the tendon's structural and indirectly, mechanical properties, ultrasonographic elastography, and ultra-high field magnetic resonance imaging (UHF MRI) have recently emerged as potentially powerful techniques to explore tendon tissues. This paper highlights some methodological cautions associated with conventional ultrasonography and perspectives for in vivo human Achilles tendon assessment using ultrasonographic elastography and UHF MRI.

Muscle and tendon connective tissue adaptation to unloading, exercise and NSAID

The extracellular matrix network of skeletal muscle and tendon connective tissue is primarily composed of collagen and connects the muscle contractile protein to the bones in the human body. The mechanical properties of the connective tissue are important for the effectiveness of which the muscle force is transformed into movement. Periods of unloading and exercise affect the synthesis rate of connective tissue collagen protein, whereas only sparse information exits regarding collagen protein degradation. It is likely, though, that changes in both collagen protein synthesis and degradation are required for remodeling of the connective tissue internal structure that ultimately results in altered mechanical properties of the connective tissue. Both unloading and exercise lead to increased production of growth factors and inflammatory mediators that are involved in connective tissue remodeling. Despite the fact that non-steroidal anti-inflammatory drugs seem to inhibit the healing process of connective tissue and the stimulating effect of exercise on connective tissue protein synthesis, these drugs are often consumed in relation to connective tissue injury and soreness. However, the potential effect of non-steroidal anti-inflammatory drugs on connective tissue needs further investigation.

Tendon mineralization is accelerated bilaterally and creep of contralateral tendons is increased after unilateral needle injury of murine achilles tendons

Heterotopic mineralization may result in tendon weakness, but effects on other biomechanical responses have not been reported. We used a needle injury, which accelerates spontaneous mineralization of murine Achilles tendons, to test two hypotheses: that injured tendons would demonstrate altered biomechanical responses; and that unilateral injury would accelerate mineralization bilaterally. Mice underwent left hind (LH) injury (I; n = 11) and were euthanized after 20 weeks along with non-injured controls (C; n = 9). All hind limbs were examined by micro computed tomography followed by biomechanical testing (I = 7 and C = 6). No differences were found in the biomechanical responses of injured tendons compared with controls. However, the right hind (RH) tendons contralateral to the LH injury exhibited greater static creep strain and total creep strain compared with those LH tendons (p ≤ 0.045) and RH tendons from controls (p ≤ 0.043). RH limb lesions of injured mice were three times larger compared with controls (p = 0.030). Therefore, despite extensive mineralization, changes to the responses we measured were limited or absent 20 weeks postinjury. These results also suggest that bilateral occurrence should be considered where tendon mineralization is identified clinically. This experimental system may be useful to study the mechanisms of bilateral new bone formation in tendinopathy and other conditions.

Ultrasound-based subject-specific parameters improve fascicle behaviour estimation in Hill-type muscle model

The estimation of muscle fascicle behaviour is decisive in a Hill-type model as they are related to muscle force by the force-length-velocity relationship and the tendon force-strain relationship. This study was aimed at investigating the influence of subject-specific tendon force-strain relationship and initial fascicle geometry (IFG) on the estimation of muscle forces and fascicle behaviour during isometric contractions. Ultrasonography was used to estimate the in vivo muscle fascicle behaviour and compare the muscle fascicle length and pennation angle estimated from the Hill-type model. The calibration-prediction process of the electromyography-driven model was performed using generic or subject-specific tendon definition with or without IFG as constraint. The combination of subject-specific tendon definition and IFG led to muscle fascicle behaviour closer to ultrasound data and significant lower forces of the ankle dorsiflexor and plantarflexor muscles compared to the other conditions. Thus, subject-specific ultrasound measurements improve the accuracy of Hill-type models on muscle fascicle behaviour.

The effects of immobilization on the mechanical properties of the patellar tendon in younger and older men

BACKGROUND

It remains unknown if inactivity changes the mechanical properties of the human patellar tendon in younger and older healthy persons. The purpose was to examine the effects of short-term unilateral immobilization on the structural and mechanical properties of the patellar tendon in older men and younger men, in vivo.

METHODS

Eight older men and eight younger men underwent 14 days of unilateral immobilization. All individuals were assessed on both sides before and after the intervention. MRI was used to assess whole patellar tendon dimensions. The mechanical properties of the patellar tendon were assessed using simultaneous force and ultrasonographic measurements during isometric ramp contractions.

FINDINGS

In older men, tendon stiffness [Pre: mean 2949 (SD 799) vs. Post: mean 2366 (SD 774) N mm(-1), P<0.01] and Young's Modulus [Pre: mean 1.2 (SD 0.3) vs. Post: mean 1.0 (SD 0.3) GPa, P<0.05] declined with immobilization on the immobilized side. On the control side, tendon stiffness [Pre: mean 3340 (SD 1209) vs. Post: mean 2230 (SD 503), P<0.01] and Young's Modulus [Pre: mean 1.5 (SD 0.4) vs. Post: mean 0.9 (SD 0.3) GPa, P<0.05] also decreased with immobilization. In younger men, tendon stiffness [Pre: 3622 (SD 1760) vs. Post: mean 2910 (SD 1528) N mm(-1), P<0.01] and Young's Modulus [Pre: mean 1.7 (SD 1.1) vs. Post: mean 1.4 (SD 0.8) GPa, P<0.05] decreased only on the immobilized side.

INTERPRETATION

Short-term immobilization led to impaired mechanical properties of the patellar tendon on the immobilized side in both younger men and older men, which can influence the function of the muscle-tendon complex.

Effects of training on muscle and tendon in knee extensors and plantar flexors in vivo

The purpose of this study was to compare the effects of resistance training on muscle and tendon properties between knee extensors and plantar flexors in vivo. Twenty healthy young men voluntarily participated in this study. The subjects were randomly divided into two training groups: knee extension group (n=10) and plantar flexion group (n=10). They performed five sets of exercises with a 1-min rest between sets, which consisted of unilateral knee extension for the knee extension group and plantar flexion for the plantar flexion group at 80% of 1 repetition maximum with 10 repetitions per set (4 days/wk, 12 wk). Before and after training, muscle strength, neural activation level (by interpolated twitch), muscle volume (by magnetic resonance imaging), and tendon stiffness (by ultrasonography) were measured. There were no differences in the training-induced increases in muscle strength, activation level, muscle volume, and tendon stiffness between knee extensors and plantar flexors. These results suggested that if the used protocol of training (i.e., intensity, repetition, etc.) were the same, there were no differences in the training-induced changes in muscle and tendon properties between knee extensors and plantar flexors.

Panoramic ultrasound to measure in vivo tendo Achilles strain

BACKGROUND

The tendo Achilles (TA) is a tendon very susceptible to injury. Biomechanical methodologies for exploring tendon strain are limited, however, as they are typically performed in vitro or by indirectly measuring the displacement of reference markers. By using panoramic ultrasound, this study reports a novel approach to acquire direct, in vivo tendon extension data.

MATERIALS AND METHODS

Sonographic scans were acquired between two in vivo landmarks following a consistent pathway along the long axis of the distal TA. Ten subjects were scanned with an unloaded tendon and then when resisting a range of loads. Tendon cross-sectional area was measured following coronal scans of the TA in three subjects, allowing for calculation and plotting of stress versus strain. Coefficients of variation were also calculated to measure the reliability and robustness of the protocol.

RESULTS

Data from all ten subjects were found to fit with classic tendon force versus extension trend. The stress versus strain plot indicated that the wavy collagen fibers were fully straightened at 4% to 6% strain, which is comparable to results reported in other studies. The Young's modulus of 0.5 to 2.1 GPa also compared favorably to published data.

CONCLUSION

Coefficients of variation indicated that the protocol was repeatable, although the technique for measuring cross-sectional area could be improved.

CLINICAL RELEVANCE

As the reported data is comparable to previous invasive and in vitro studies, we believe sports medicine specialists and orthopaedic surgeons can utilize panoramic ultrasound to directly measure in vivo tendon strain.

Plyometric training effects on Achilles tendon stiffness and dissipative properties

The aim of this study was to determine the effects of 14 wk of plyometric training on mechanical properties of the Achilles tendon. Nineteen subjects were randomly assigned to trained or control group. Cross-sectional area (CSA), stiffness, and dissipation coefficient of the Achilles tendon were measured before and after the training period. In the trained group, a decrease in dissipation coefficient (−35.0%; P < 0.05) and an upward trend in stiffness (+24.1%) of the Achilles tendon was found, without any changes in Achilles tendon CSA ( P > 0.05). Plyometric training enhances the muscular tension transmission mainly through a reduction in energy dissipated by the tendon. The lack of changes in the Achilles tendon CSA indicates that changes in mechanical properties would mainly result from a qualitative change in tendinous tissues rather than from changes in the geometry of the Achilles tendon.

Mechanical properties of the achilles tendon aponeurosis are altered in athletes with achilles tendinopathy

BACKGROUND

Achilles tendinopathy is a considerable problem for active people. The degenerative processes associated with tendinopathy may be associated with changes in the inherent mechanical properties of the musculotendinous unit.

PURPOSE

The purpose of this study was to compare Achilles tendon-aponeurosis strain between male athletes with and without Achilles tendinopathy.

STUDY DESIGN

Cross-sectional study; Level of evidence, 3.

METHODS

Fifteen healthy men (age, 35 +/- 9 years; height, 1.78 +/- 0.05 m; mass, 79 +/- 11 kg) and 14 men with midportion Achilles tendinopathy (age, 40 +/- 8 years; height, 1.77 +/- 0.06 m; mass, 80 +/- 9 kg) who were all running over 20 km per week participated in the study. Each participant was tested in a single session that involved maximal isometric plantar flexion efforts being performed on a calf-raise apparatus while synchronous real-time ultrasonography of the triceps surae aponeurosis was recorded. Achilles tendon-aponeurosis strain (%) was calculated by dividing tendon displacement during plantar flexion by resting tendon length (intrarater reliability: intraclass correlation coefficient = .92).

RESULTS

Participants in the Achilles tendinopathy group (5.2% +/- 2.6%) had significantly (P = .039) higher Achilles tendon-aponeurosis strain compared with the control group (3.4% +/- 1.8%). In contrast, there were no significant between-group differences for maximal isometric plantar flexion force.

CONCLUSION

Achilles tendon-aponeurosis strain is higher in male athletes with tendinopathy than those without. The results of this study provide a rationale for current clinical approaches to management of Achilles tendinopathy, whereby repetitive mechanical loading may impart a positive benefit through reduced compliance of the musculotendinous unit.

Disuse of the musculo-skeletal system in space and on earth

Muscle mass and strength are well known to decline in response to actual and simulated microgravity exposure. However, despite the considerable knowledge gained on the physiological changes induced by spaceflight, the mechanisms of muscle atrophy and the effectiveness of in-flight countermeasures still need to be fully elucidated. The present review examines the effects and mechanisms of actual and simulated microgravity on single fibre and whole muscle structural and functional properties, protein metabolism, tendon mechanical properties, neural drive and reflex excitability. The effects of inflight countermeasures are also discussed in the light of recent advances in resistive loading techniques, in combined physical, pharmacological and nutritional interventions as well as in the development of artificial gravity systems. Emphasis has been given to the pioneering work of Pietro Enrico di Prampero in the development of artificial gravity systems and in the progress of knowledge on the limits of human muscular performance in space.

Bed rest in health and critical illness: a body systems approach

Bed rest is a common intervention for critically ill adults. Associated with both benefits and adverse effects, bed rest is undergoing increasing scrutiny as a therapeutic option in the intensive care unit. Bed rest has molecular and systemic effects, ultimately affecting functional outcomes in healthy individuals as well as in those with acute and critical illnesses. Using empirical sources, the purpose of this article was to describe the consequences of bed rest and immobility, especially consequences with implications for critically ill adults in the intensive care unit. This review uses body systems to cluster classic and current results of bed rest studies, beginning with cardiovascular and including pulmonary, renal, skin, nervous, immune, gastrointestinal/ metabolic, and skeletal systems. It concludes with effects on muscles, a system profoundly affected by immobility and bed rest.

Influence of aging on the in vivo properties of human patellar tendon

Tendons are important for optimal muscle force transfer to bone and play a key role in functional ability. Changes in tendon properties with aging could contribute to declines in physical function commonly associated with aging. We investigated the in vivo mechanical properties of the patellar tendon in 37 men and women [11 young (27 +/- 1 yr) and 26 old (65 +/- 1 yr)] using ultrasonography and magnetic resonance imaging (MRI). Patella displacement relative to the tibia was monitored with ultrasonography during ramped isometric contractions of the knee extensors, and MRI was used to determine tendon cross-sectional area (CSA) and signal intensity. At peak force, patellar tendon deformation, stress, and strain were 13 (P = 0.05), 19, and 12% less in old compared with young (P < 0.05). Additionally, deformation, stiffness, stress, CSA, and length were 18, 35, 41, 28, and 11% greater (P < 0.05), respectively, in men compared with women. After normalization of mechanical properties to a common force, no age differences were apparent; however, stress and strain were 26 and 22% higher, respectively, in women compared with men (P < 0.05). CSA and signal intensity decreased 12 and 24%, respectively, with aging (P < 0.05) in the midregion of the tendon. These data suggest that differences in patellar tendon in vivo mechanical properties with aging are more related to force output rather than an age effect. In contrast, the decrease in signal intensity indirectly suggests that the internal milieu of the tendon is altered with aging; however, the physiological and functional consequence of this finding requires further study.

Effect of chronic unloading and rehabilitation on human Achilles tendon properties: a velocity-encoded phase-contrast MRI study

The objective of this study was to measure and monitor changes in Achilles tendon mechanical properties and force production capability of triceps surae muscles after 4 wk of limb suspension and 6 wk of physical rehabilitation. Five healthy volunteers underwent unilateral lower limb suspension followed by weekly physiotherapy. A velocity-encoded, phase-contrast magnetic resonance imaging (VE-PC-MRI) technique was used to estimate the tendon strain as a function of force produced during the submaximal isometric contractions. After limb suspension, triceps surae muscle strength decreased to 53.2 +/- 15.6% (mean +/- SD) of the presuspension level (P < 0.05). Young's modulus, estimated from the slope of the tendon stress-strain relationship, decreased by 17.1% (from 140.50 +/- 29.33 to 119.95 +/- 36.07 MPa, P < 0.05), while the tendon transition point, reflecting the "toe region," increased by 55.7% (from 2.2 +/- 1.0% to 3.4 +/- 1.24%). Muscle strength, tendon stiffness, and transition point recovered to presuspension levels by the end of 6 wk of rehabilitation. Calcaneus movement was significant during the "isometric" contraction, accounting for 52.13 +/- 7.63% of the tendon displacement. Tendon cross-sectional area determined from anatomic magnetic resonance axial images remained unchanged, suggesting that the altered tendon elastic modulus and transition point were largely due to material deterioration. The increase in the transition point following chronic unloading as measured by the VE-PC-MRI technique has not been previously reported and offers new insights into the biomechanical changes that may occur in the tendon crimp structure.