The effect of robot-assisted versus standard training on motor function following subacute rehabilitation after ischemic stroke - protocol for a randomised controlled trial nested in a prospective cohort (RoboRehab).

BACKGROUND: Body weight unloaded treadmill training has shown limited efficacy in further improving functional capacity after subacute rehabilitation of ischemic stroke patients. Dynamic robot assisted bodyweight unloading is a novel technology that may provide superior training stimuli and continued functional improvements in individuals with residual impairments in the chronic phase after the ischemic insult. The aim of the present study is to investigate the effect of dynamic robot-assisted versus standard training, initiated 6 months post-stroke, on motor function, physical function, fatigue, and quality of life in stroke-affected individuals still suffering from moderate-to-severe disabilities after subacute rehabilitation. METHODS: Stroke-affected individuals with moderate to severe disabilities will be recruited into a prospective cohort with measurements at 3-, 6-, 12- and 18-months post-stroke. A randomised controlled trial (RCT) will be nested in the prospective cohort with measurements pre-intervention (Pre), post-intervention (Post) and at follow-up 6 months following post-intervention testing. The present RCT will be conducted as a multicentre parallel-group superiority of intervention study with assessor-blinding and a stratified block randomisation design. Following pre-intervention testing, participants in the RCT study will be randomised into robot-assisted training (intervention) or standard training (active control). Participants in both groups will train 1:1 with a physiotherapist two times a week for 6 months (groups are matched for time allocated to training). The primary outcome is the between-group difference in change score of Fugl-Meyer Lower Extremity Assessment from pre-post intervention on the intention-to-treat population. A per-protocol analysis will be conducted analysing the differences in change scores of the participants demonstrating acceptable adherence. A priori sample size calculation allowing the detection of the minimally clinically important between-group difference of 6 points in the primary outcome (standard deviation 6 point, α = 5% and ß = 80%) resulted in 34 study participants. Allowing for dropout the study will include 40 participants in total. DISCUSSION: For stroke-affected individuals still suffering from moderate to severe disabilities following subacute standard rehabilitation, training interventions based on dynamic robot-assisted body weight unloading may facilitate an appropriate intensity, volume and task-specificity in training leading to superior functional recovery compared to training without the use of body weight unloading. TRIAL REGISTRATION: ClinicalTrials.gov. NCT06273475. TRIAL STATUS: Recruiting. Trial identifier: NCT06273475. Registry name: ClinicalTrials.gov. Date of registration on ClinicalTrials.gov: 22/02/2024.

Adaptations to explosive resistance training with partial range of motion are not inferior to full range of motion.

We tested whether explosive resistance training with partial range of motion (ROM) would be as effective as full ROM training using a noninferiority trial design. Fifteen subjects with strength training experience took part in an explosive-concentric only-leg press training program, three times per week for 10 weeks. One leg was randomly assigned to exercise with partial ROM (ie, 9º) and the other leg to full ROM. Before and after training, we assessed leg press performance, isokinetic concentric and isometric knee extension torque, and vastus lateralis muscle architecture. Overall, both training modalities increased maximal strength and rate of force development. Training with partial ROM yielded noninferior results compared to full ROM for leg press peak power (+69 ± 47% vs. +61 ± 64%), isokinetic strength (4-6 ± 6%-12% vs. 1-6 ± 6%-10% at 30, 60, and 180˚s-1 ), and explosive torque after 100 (47 ± 24 vs. 35 ± 22) and 150 ms (57 ± 22% vs. 42 ± 25%). The comparison was inconclusive for other functional parameters (ie, isokinetic peak torque (300˚s-1 ), joint angle at isokinetic peak torque, explosive torque after 50 ms, and electrically evoked torque) and for muscle fascicle length and thickness, although noninferiority was established for pennation angle. However, partial ROM was not found statistically inferior to full ROM for any measured variable. Under the present conditions, the effects of explosive heavy resistance training were independent of joint ROM. Instead, these data suggest that the distinct timing of muscle work in explosive contractions confers more influence to the starting joint angle than ROM on adaptations to this type of training.

Assessment of Neuroplasticity With Strength Training.

Including a brief overview of current investigative approaches, the present Perspectives for Progress article offers an overview of potential future experiments in the field of exercise-related neuroplasticity to strength training. It is proposed that the combination of specific experimental approaches and recently developed techniques holds the potential for unraveling spinal and supraspinal mechanisms involved in the adaptation to strength training.

Modulation of muscle-tendon interaction in the human triceps surae during an energy dissipation task.

The compliance of elastic elements allows muscles to dissipate energy safely during eccentric contractions. This buffering function is well documented in animal models but our understanding of its mechanism in humans is confined to non-specific tasks, requiring a subsequent acceleration of the body. The present study aimed to examine the behaviour of the human triceps surae muscle-tendon unit (MTU) during a pure energy dissipation task, under two loading conditions. Thirty-nine subjects performed a single-leg landing task, with and without added mass. Ultrasound measurements were combined with three-dimensional kinematics and kinetics to determine instantaneous length changes of MTUs, muscle fascicles, Achilles tendon and combined elastic elements. Gastrocnemius and soleus MTUs lengthened during landing. After a small concentric action, fascicles contracted eccentrically during most of the task, whereas plantar flexor muscles were activated. Combined elastic elements lengthened until peak ankle moment and recoiled thereafter, whereas no recoil was observed for the Achilles tendon. Adding mass resulted in greater negative work and MTU lengthening, which were accompanied by a greater stretch of tendon and elastic elements and a greater recruitment of the soleus muscle, without any further fascicle strain. Hence, the buffering action of elastic elements delimits the maximal strain and lengthening velocity of active muscle fascicles and is commensurate with loading constraints. In the present task, energy dissipation was modulated via greater MTU excursion and more forceful eccentric contractions. The distinct strain pattern of the Achilles tendon supports the notion that different elastic elements may not systematically fulfil the same function.

Heterogeneous Loading of the Human Achilles Tendon In Vivo.

The free Achilles tendon is considered a homogeneous structure that transmits muscular force in a linear manner. However, the tendon undergoes longitudinal rotation and is separated in mechanically independent segments with distinct mechanical and material tissue properties. The present article examines the hypothesis that the human Achilles tendon is loaded asymmetrically and undergoes heterogeneous deformation during movement.

Acute effects of robot-assisted body weight unloading on biomechanical movement patterns during overground walking.

Body weight unloading (BWU) is used in rehabilitation/training settings to reduce kinetic requirements, however different BWU methods may be unequally capable of preserving biomechanical movement patterns. Biomechanical analysis of both kinetic and kinematic movement trajectories rather than discrete variables has not previously been performed to describe the effect of BWU on gait patterns during horizontal walking. The aim of the present study was to investigate how robot-assisted BWU producing an dynamic unloading force on the body centre of mass, affects kinematic, kinetic, and spatiotemporal gait parameters in healthy young adults by use of time-continuous analysis. Twenty participants walked overground in a 3-D motion-capture lab at 0, 10, 20, 30, 40, and 50 % BWU at a self-selected speed. Vertical and anterior-posterior ground reaction forces (GRFs) and lower limb internal joint moments were obtained during the stance phase, while joint angles were obtained during entire strides. Time-continuous data were analysed using Statistical Parametric Mapping (SPM) and discrete data using conventional statistics to compare different BWU conditions by means of One-Way Repeated Measures Anova. With increasing BWU, corresponding reductions were observed for GRFs, internal joint moments, joint angles, walking speed, stride/step length and cadence. Observed effects were partially caused by decreased walking speed and increased BWU. While amplitude reductions were observed for kinetic and kinematic variables, trajectory shapes were largely preserved. In conclusion, dynamic robot-assisted BWU enables reduced kinetic requirements without distorting biomechanically normal gait patterns during overground walking in young healthy adults.

Mechanical properties of the patellar tendon in elite volleyball players with and without patellar tendinopathy.

BACKGROUND: Although differences in mechanical properties between symptomatic and healthy tendons have been observed for the Achilles tendon, the impact of tendinopathy on patellar tendon mechanics is not fully documented. The aim of the present case-control study was to assess the mechanical properties of the tendon and jump performance in elite athletes with and without patellar tendinopathy. METHODS: We identified 17 male volleyball players with patellar tendinopathy and 18 healthy matched controls from a 5-year prospective cohort study on junior elite volleyball players. Outcome variables included three measures of maximal vertical jump performance and ultrasound-based assessments of patellar tendon cross-sectional area, stiffness and Young's modulus. RESULTS: The proximal cross-sectional area of the patellar tendon was significantly larger in the tendinopathic group (133 ± 11 vs 112 ± 9 mm(2), respectively; p < 0.001). Pathological tendons presented lower stiffness (2254 ± 280 vs 2826 ± 603 N/mm, respectively; p = 0.006) and Young's modulus (0.99 ± 0.16 vs 1.17 ± 0.25 GPa, respectively; p = 0.04) than healthy tendons. However, the difference between the countermovement jump height and the squat jump height (3.4 ± 2.2 vs 1.2 ± 1.5 cm, p = 0.005) was significantly higher in the tendinopathic group compared with the control group. CONCLUSIONS: Patellar tendinopathy is associated with a decrease in the mechanical and material properties of the tendon in elite athletes subjected to a high volume of jumping activity. However, compared with their healthy counterparts, tendinopathic volleyball players have a better ability to utilise the stretch-shortening cycle when jumping.

Linking muscle architecture and function in vivo: conceptual or methodological limitations?

Background: Despite the clear theoretical link between sarcomere arrangement and force production, the relationship between muscle architecture and function remain ambiguous in vivo. Methods: We used two frequently used ultrasound-based approaches to assess the relationships between vastus lateralis architecture parameters obtained in three common conditions of muscle lengths and contractile states, and the mechanical output of the muscle in twenty-one healthy subjects. The relationship between outcomes obtained in different conditions were also examined. Muscle architecture was analysed in panoramic ultrasound scans at rest with the knee fully extended and in regular scans at an angle close to maximum force (60°), at rest and under maximum contraction. Isokinetic and isometric strength tests were used to estimate muscle force production at various fascicle velocities. Results: Measurements of fascicle length, pennation angle and thickness obtained under different experimental conditions correlated moderately with each other (r = 0.40-.74). Fascicle length measured at 60° at rest correlated with force during high-velocity knee extension (r = 0.46 at 400° s-1) and joint work during isokinetic knee extension (r = 0.44 at 200° s-1 and r = 0.57 at 100° s-1). Muscle thickness was related to maximum force for all measurement methods (r = 0.44-0.73). However, we found no significant correlations between fascicle length or pennation angle and any measures of muscle force or work. Most correlations between architecture and force were stronger when architecture was measured at rest close to optimal length. Conclusion: These findings reflect methodological limitations of current approaches to measure fascicle length and pennation angle in vivo. They also highlight the limited value of static architecture measurements when reported in isolation or without direct experimental context.

Sequenced response of extracellular matrix deadhesion and fibrotic regulators after muscle damage is involved in protection against future injury in human skeletal muscle.

The purpose of this study was to test the hypothesis that remodeling of skeletal muscle extracellular matrix (ECM) is involved in protecting human muscle against injury. Biopsies were obtained from medial gastrocnemius muscles after a single bout of electrical stimulation (B) or a repeated bout (RB) 30 d later, or 30 d after a single stimulation bout (RBc). A muscle biopsy was collected from the control leg for comparison with the stimulated leg. Satellite cell content, tenascin C, and muscle regeneration were assessed by immunohistochemistry; real-time PCR was used to measure mRNA levels of collagens, laminins, heat-shock proteins (HSPs), inflammation, and related growth factors. The large responses of HSPs, CCL2, and tenascin C detected 48 h after a single bout were attenuated in the RB trial, indicative of protection against injury. Satellite cell content and 12 target genes, including IGF-1, were elevated 30 d after a single bout. Among those displaying the greatest difference vs. control muscle, ECM laminin-ß1 and collagen types I and III were elevated ∼6- to 9-fold (P<0.001). The findings indicate that the sequenced events of load-induced early deadhesion and later strengthening of skeletal muscle ECM play a role in protecting human muscle against future injury.

Muscle morphological and strength adaptations to endurance vs. resistance training.

Fascicle angle (FA) is suggested to increase as a result of fiber hypertrophy and furthermore to serve as the explanatory link in the discrepancy in the relative adaptations in the anatomical cross-sectional area (CSA) and fiber CSA after resistance training (RT). In contrast to RT, the effects of endurance training on FA are unclear. The purpose of this study was therefore to investigate and compare the longitudinal effects of either progressive endurance training (END, n = 7) or RT (n = 7) in young untrained men on FA, anatomical CSA, and fiber CSA. Muscle morphological measures included the assessment of vastus lateralis FA obtained by ultrasonography and anatomical CSA by magnetic resonance imaging of the thigh and fiber CSA deduced from histochemical analyses of biopsy samples from m. vastus lateralis. Functional performance measures included VO2max and maximal voluntary contraction (MVC). The RT produced increases in FA by 23 ± 8% (p < 0.01), anatomical CSA of the knee extensor muscles by 9 ± 3% (p = 0.001), and fiber CSA by 19 ± 7% (p < 0.05). RT increased knee extensor MVC by 20 ± 5% (p < 0.001). END increased VO2max by 10 ± 2% but did not evoke changes in FA, anatomical CSA, or in fiber CSA. In conclusion, the morphological changes induced by 10 weeks of RT support that FA does indeed serve as the explanatory link in the observed discrepancy between the changes in anatomical and fiber CSA. Contrarily, 10 weeks of endurance training did not induce changes in FA, but the lack of morphological changes from END indirectly support the fact that fiber hypertrophy and FA are interrelated.

Rate of force development relationships to muscle architecture and contractile behavior in the human vastus lateralis.

In this study, we tested the hypotheses that (i) rate of force development (RFD) is correlated to muscle architecture and dynamics and that (ii) force-length-velocity properties limit knee extensor RFD. Twenty-one healthy participants were tested using ultrasonography and dynamometry. Vastus lateralis optimal fascicle length, fascicle velocity, change in pennation angle, change in muscle length, architectural gear ratio, and force were measured during rapid fixed-end contractions at 60° knee angle to determine RFD. Isokinetic and isometric tests were used to estimate individual force-length-velocity properties, to evaluate force production relative to maximal potential. Correlation analyses were performed between force and muscle parameters for the first three 50 ms intervals. RFD was not related to optimal fascicle length for any measured time interval, but RFD was positively correlated to fascicle shortening velocity during all intervals (r = 0.49-0.69). Except for the first interval, RFD was also related to trigonometry-based changes in muscle length and pennation angle (r = 0.45-0.63) but not to architectural gear ratio. Participants reached their individual vastus lateralis force-length-velocity potential (i.e. their theoretical maximal force at a given length and shortening velocity) after 62 ± 24 ms. Our results confirm the theoretical importance of fascicle shortening velocity and force-length-velocity properties for rapid force production and suggest a role of fascicle rotation.

Supervised, Heavy Resistance Training Is Tolerated and Potentially Beneficial in Women with Knee Pain and Knee Joint Hypermobility: A Case Series.

Introduction: Adults with generalised joint hypermobility including knee joint hypermobility (GJHk) report more knee joint symptoms when compared to adults without GJHk. There is no consensus on best practice for symptom management. For instance, controversy exists regarding the appropriateness and safety of heavy resistance training as an intervention for this specific group. This case series aims to describe a supervised, progressive heavy resistance training program in adults with GJHk and knee pain, the tolerability of the intervention, and the outcomes of knee pain, knee-related quality of life, muscle strength, proprioception, and patellar tendon stiffness through a 12-week period. Materials and Methods: Adults with GJHk and knee pain were recruited to perform supervised, progressive heavy resistance training twice a week for 12 weeks. The main outcome was the tolerability of the intervention. Secondary outcomes were knee pain during a self-nominated activity (VASNA); Knee injury and Osteoarthritis Outcome Score (KOOS); Tampa Scale of Kinesiophobia (TSK); maximal quadriceps voluntary isometric contraction and rate of torque development; 5 repetition maximum strength in five different leg exercises; single leg hop for distance; knee proprioception and patellar tendon stiffness. Results: In total, 16 women (24.2 years, SD 2.5) completed at least 21/24 training sessions. No major adverse events were observed. On average, VASNA decreased by 32.5 mm (95% CI 21.4-43.6), in addition to improvements in KOOS and TSK scores. These improvements were supported by an increase in all measures of lower extremity muscle strength, knee proprioception, and patellar tendon stiffness. Conclusion: Supervised heavy resistance training seems to be well tolerated and potentially beneficial in young women with GJHk and knee pain.

Altered Triceps Surae Muscle-Tendon Unit Properties after 6 Months of Static Stretching.

INTRODUCTION: This study examined the effects of 24 wk of daily static stretching of the plantarflexors (unilateral 4 × 60-s stretching, whereas the contralateral leg served as a control; n = 26) on joint range of motion (ROM), muscle-tendon unit morphological and mechanical properties, neural activation, and contractile function. METHODS: Torque-angle/velocity was obtained in passive and active conditions using isokinetic dynamometry, whereas muscle-tendon morphology and mechanical properties were examined using ultrasonography. RESULTS: After the intervention, ROM increased (stretching, +11° ± 7°; control, 4° ± 8°), and passive torque (stretching, -10 ± 11 N·m; control, -7 ± 10 N·m) and normalized EMG amplitude (stretching, -3% ± 6%; control, -3% ± 4%) at a standardized dorsiflexion angle decreased. Increases were seen in passive tendon elongation at a standardized force (stretching, +1.3 ± 1.6 mm; control, +1.4 ± 2.1 mm) and in maximal passive muscle and tendon elongation. Angle of peak torque shifted toward dorsiflexion. No changes were seen in tendon stiffness, resting tendon length, or gastrocnemius medialis fascicle length. Conformable changes in ROM, passive dorsiflexion variables, tendon elongation, and angle of peak torque were observed in the nonstretched leg. CONCLUSIONS: The present findings indicate that habitual stretching increases ROM and decreases passive torque, altering muscle-tendon behavior with the potential to modify contractile function.

Mechanical properties, physiological behavior, and function of aponeurosis and tendon.

During human movement, the muscle and tendinous structures interact as a mechanical system in which forces are generated and transmitted to the bone and energy is stored and released to optimize function and economy of movement and/or to reduce risk of injury. The present review addresses certain aspects of how the anatomical design and mechanical and material properties of the force-transmitting tissues contribute to the function of the muscle-tendon unit and thus overall human function. The force-bearing tissues are examined from a structural macroscopic point of view down to the nanoscale level of the collagen fibril. In recent years, the understanding of in vivo mechanical function of the force-bearing tissues has increased, and it has become clear that these tissues adapt to loading and unloading and furthermore that force transmission mechanics is more complex than previously thought. Future investigations of the force-transmitting tissues in three dimensions will enable a greater understanding of the complex functional interplay between muscle and tendon, with relevance for performance, injury mechanisms, and rehabilitation strategies.

Distinct muscle-tendon interaction during running at different speeds and in different loading conditions.

The interaction between the Achilles tendon and the triceps surae muscles seems to be modulated differently with various task configurations. Here we tested the hypothesis that the increased forces and ankle joint work during running under contrasting conditions (altered speed or load) would be met by different, time-dependent adjustments at the muscle-tendon level. Ultrasonography, electromyography, kinematics, and ground reaction force measurements were used to examine Achilles tendon, gastrocnemius, and soleus muscle mechanics in 16 runners in four different running conditions, consisting of a combination of two different speeds (preferred and +20% of preferred speed) and two loading conditions (unloaded and +20% of body mass). Positive ankle joint work increased similarly (+13%) with speed and load. Gastrocnemius and soleus muscle fascicle length and peak velocity were not altered by either condition, suggesting that contractile conditions are mostly preserved despite the constraints imposed in this experimental design. However, at higher running speed, tendon length changes were unaltered but mean muscle electromyographic activity increased in gastrocnemius (+10%, P < 0.01) and soleus (+14%, P < 0.01). Conversely, when loading was increased, mean muscle activity remained similar to unloaded conditions but the mean velocity of gastrocnemius fascicles was reduced and tendon recoil increased (+29%, P < 0.01). Collectively, these results suggest that the neuromuscular system meets increased mechanical demands by favoring economical force production when enough time is available. NEW & NOTEWORTHY We demonstrate that muscle-tendon mechanics are adjusted differently when running under constraints imposed by speed or load, despite comparable increases in work. The neuromuscular system likely modulates the way force is produced as a function of availability of time and potential energy.

Training-induced increase in Achilles tendon stiffness affects tendon strain pattern during running.

BACKGROUND: During the stance phase of running, the elasticity of the Achilles tendon enables the utilisation of elastic energy and allows beneficial contractile conditions for the triceps surae muscles. However, the effect of changes in tendon mechanical properties induced by chronic loading is still poorly understood. We tested the hypothesis that a training-induced increase in Achilles tendon stiffness would result in reduced tendon strain during the stance phase of running, which would reduce fascicle strains in the triceps surae muscles, particularly in the mono-articular soleus. METHODS: Eleven subjects were assigned to a training group performing isometric single-leg plantarflexion contractions three times per week for ten weeks, and another ten subjects formed a control group. Before and after the training period, Achilles tendon stiffness was estimated, and muscle-tendon mechanics were assessed during running at preferred speed using ultrasonography, kinematics and kinetics. RESULTS: Achilles tendon stiffness increased by 18% (P < 0.01) in the training group, but the associated reduction in strain seen during isometric contractions was not statistically significant. Tendon elongation during the stance phase of running was similar after training, but tendon recoil was reduced by 30% (P < 0.01), while estimated tendon force remained unchanged. Neither gastrocnemius medialis nor soleus fascicle shortening during stance was affected by training. DISCUSSION: These results show that a training-induced increase in Achilles tendon stiffness altered tendon behaviour during running. Despite training-induced changes in tendon mechanical properties and recoil behaviour, the data suggest that fascicle shortening patterns were preserved for the running speed that we examined. The asymmetrical changes in tendon strain patterns supports the notion that simple in-series models do not fully explain the mechanical output of the muscle-tendon unit during a complex task like running.

Lateral force transmission between human tendon fascicles.

Whether adjacent collagen fascicles transmit force in parallel is unknown. The purpose of the present study was to examine the magnitude of lateral force transmission between adjacent collagen fascicles from the human patellar and Achilles tendon. From each sample two adjacent strands of fascicles (phi 300-530 mum) enclosed in a fascicular membrane were dissected. The specimen was deformed to approximately 3% strain in three independent load-displacement cycles in a small-scale tensile testing device. Cycle 1: the fascicles and the fascicular membrane were intact. Cycle 2: one fascicle was transversally cut while the other fascicle and the fascicular membrane were kept intact. Cycle 3: both fascicles were cut in opposite ends while the fascicular membrane was left intact. A decline in peak force of 45% and 55% from cycle 1 to cycle 2, and 93% and 92% from cycle 2 to cycle 3 was observed in the patellar and Achilles tendon fascicles, respectively. A decline in stiffness of 39% and 60% from cycle 1 to cycle 2, and of 93% and 100% from cycle 2 to cycle 3 was observed in the patellar and Achilles tendon fascicles, respectively. The present data demonstrate that lateral force transmission between adjacent collagen fascicles in human tendons is small or negligible, suggesting that tendon fascicles largely act as independent structures and that force transmission principally takes place within the individual fascicles.

Evidence of skeletal muscle damage following electrically stimulated isometric muscle contractions in humans.

It is unknown whether muscle damage at the level of the sarcomere can be induced without lengthening contractions. To investigate this, we designed a study where seven young, healthy men underwent 30 min of repeated electrical stimulated contraction of m. gastrocnemius medialis, with the ankle and leg locked in a fixed position. Two muscle biopsies were collected 48 h later: one from the stimulated muscle and one from the contralateral leg as a control. The biopsies were analyzed immunohistochemically for inflammatory cell infiltration and intermediate filament disruption. Ultrastructural changes at the level of the z-lines were investigated by transmission electron microscopy. Blood samples were collected for measurement of creatine kinase activity, and muscle soreness was assessed in the days following stimulation. The biopsies from the stimulated muscle revealed macrophage infiltration and desmin-negative staining in a small percentage of myofibers in five and four individuals, respectively. z-Line disruption was evident at varying magnitudes in all subjects and displayed a trend toward a positive correlation (r = 0.73, P = 0.0663) with the force produced by stimulation. Increased muscle soreness in all subjects, combined with a significant increase in creatine kinase activity (P < 0.05), is indirectly suggestive of muscle damage, and the novel findings of the present study, i.e., 1) macrophages infiltration, 2) lack of desmin staining, and 3) z-line disruption, provide direct evidence of damage at the myofiber and sarcomere levels. These data support the hypothesis that muscle damage at the level of the sarcomere can be induced without lengthening muscle contractions.

Effect of Training-Induced Changes in Achilles Tendon Stiffness on Muscle-Tendon Behavior During Landing.

During rapid deceleration of the body, tendons buffer part of the elongation of the muscle-tendon unit (MTU), enabling safe energy dissipation via eccentric muscle contraction. Yet, the influence of changes in tendon stiffness within the physiological range upon these lengthening contractions is unknown. This study aimed to examine the effect of training-induced stiffening of the Achilles tendon on triceps surae muscle-tendon behavior during a landing task. Twenty-one male subjects were assigned to either a 10-week resistance-training program consisting of single-leg isometric plantarflexion (n = 11) or to a non-training control group (n = 10). Before and after the training period, plantarflexion force, peak Achilles tendon strain and stiffness were measured during isometric contractions, using a combination of dynamometry, ultrasound and kinematics data. Additionally, testing included a step-landing task, during which joint mechanics and lengths of gastrocnemius and soleus fascicles, Achilles tendon, and MTU were determined using synchronized ultrasound, kinematics and kinetics data collection. After training, plantarflexion strength and Achilles tendon stiffness increased (15 and 18%, respectively), and tendon strain during landing remained similar. Likewise, lengthening and negative work produced by the gastrocnemius MTU did not change detectably. However, in the training group, gastrocnemius fascicle length was offset (8%) to a longer length at touch down and, surprisingly, fascicle lengthening and velocity were reduced by 27 and 21%, respectively. These changes were not observed for soleus fascicles when accounting for variation in task execution between tests. These results indicate that a training-induced increase in tendon stiffness does not noticeably affect the buffering action of the tendon when the MTU is rapidly stretched. Reductions in gastrocnemius fascicle lengthening and lengthening velocity during landing occurred independently from tendon strain. Future studies are required to provide insight into the mechanisms underpinning these observations and their influence on energy dissipation.

Contraction-induced [18F]-fluoro-deoxy-glucose uptake can be measured in human calf muscle using high-resolution PET.

Contraction-induced glucose uptake can be imaged and quantified by the use of positron emission tomography (PET). In the human extremities, such data may reveal important information regarding the in vivo mechanical function of e.g. the force transmitting tissues such as tendons. However, to investigate structures of limited size, a PET scanner with high resolution is required. We tested the potential of the recently developed high-resolution brain PET scanner (ECAT HRRT) for imaging of human lower extremities. [18F]-fluoro-deoxy-glucose uptake following voluntary and stimulated isometric muscle contractions was studied in a 30-year-old male. The results showed that the activated muscle or muscles are clearly delineated in the high-resolution PET images. Furthermore, the load-induced gain in tendon uptake was clearly visualized. In conclusion, the HRRT scanner is an appropriate tool for investigating physiological processes within the human extremities, and the very high resolution yields a potential for more accurate conclusions when target tissues are limited in size.