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Objective. Alterations in the morphologic, biochemical, and mechanical properties of cartilage occur after unloading and immobilization in animals. However, the findings have been inconsistent and it is unclear whether such changes also take place in humans. This study tested the hypothesis that progressive thinning of knee joint cartilage is observed after spinal cord injury.Methods. In this in vivo study, knee cartilage was assessed in patients with complete, traumatic spinal cord injury at 6 (n ‫؍‬ 9), 12 (n ‫؍‬ 11), and 24 months (n ‫؍‬ 6) after injury. Morphologic parameters of the knee cartilage (mean and maximum thickness as well as surface area) were computed from magnetic resonance imaging (MRI) data, and results were compared with those in young, healthy volunteers (n ‫؍‬ 9).Results. After 6 months of injury, the mean articular-cartilage thickness was significantly less in the patella and medial tibia (decrease of 10% and 16%, respectively; P < 0.05), but not in the lateral tibia (decrease of 10%), compared with the MRI findings in healthy volunteers. After 12 and 24 months of injury, the differences amounted to a reduction of 21% and 23%, respectively, in the patella, 24% and 25%, respectively, in the medial tibia, and 16% and 19%, respectively, in the lateral tibia. The changes were significant in all 3 surfaces of the spinal cord-injured joint cartilage (P < 0.05-0.01).Conclusion. Our data show, for the first time, that progressive thinning (atrophy) of human cartilage occurs in the absence of normal joint loading and movement. This may have important implications for patient management, in particular for spinal cord-injured patients and patients who are immobilized after surgery.

The cardiac regulation effects of a mental task added to regular office work are described. More insight into the time evolution during the different tasks is created by using time-frequency analysis (TFA). Continuous wavelet transformation was applied to create time series of instantaneous power and frequency in specified frequency bands (LF 0.04-0.15 Hz; HF 0.15-0.4 Hz), in addition to the traditional linear heart rate variability (HRV) parameters. In a laboratory environment, 43 subjects underwent a protocol with three active conditions: a clicking task with low mental load and a clicking task with high mental load (mental arithmetic) performed twice, each followed by a rest condition. The heart rate and measures related to vagal modulation could differentiate the active conditions from the rest condition, meaning that HRV is sensitive to any change in mental or physical state. Differences between physical and mental stress were observed and a higher load in the combined task was observed. Mental stress decreased HF power and caused a shift toward a higher instantaneous frequency in the HF band. TFA revealed habituation to the mental load within the task (after 3 min) and between the two tasks with mental load. In conclusion, the use of TFA in this type of analysis is important as it reveals extra information. The addition of a mental load to a physical task elicited further effect on HRV parameters related to autonomic cardiac modulation.

Objective. A previous cross-sectional study indicated that the morphology of patellar and tibial cartilage is subject to change after spinal cord injury (SCI). The aim of this study was to perform a longitudinal analysis of cartilage atrophy in all knee compartments, including the femoral condyles, in SCI patients over 12 months.Methods. The right knees of 9 patients with complete, traumatic SCI were examined shortly after the injury (mean ؎ SD 9 ؎ 4 weeks) and at 6 and 12 months postinjury. Three-dimensional morphology of the patellar, tibial, and femoral cartilage (mean and maximum thickness, volume, and surface area) was determined from coronal and transversal magnetic resonance images (fat-suppressed gradient-echo sequences) using validated postprocessing techniques.Results. The mean thickness of knee joint cartilage decreased significantly during the first 6 months after injury (range 5-7%; P < 0.05). The mean change at 12 months was 9% in the patella, 11% in the medial tibia, 11% in the medial femoral condyle, 13% in the lateral tibia, and 10% in the lateral femoral condyle (P < 0.05 for all compartments).Conclusion. This is the first report of a longitudinal analysis of cartilage atrophy in patients with SCI. These data show that human cartilage atrophies in the absence of normal joint loading and movement after SCI, with a rate of change that is higher than that observed in osteoarthritis (OA). A potential clinical implication is that cartilage thinning after SCI may affect the stress distribution in the joint and render it vulnerable to OA. Future studies should focus on whether specific exercise protocols and rehabilitation programs can prevent cartilage thinning.Integrity of the morphologic and mechanical properties of articular cartilage is a prerequisite for appropriate function of diarthrodial joints. As suggested by various animal studies, a certain amount of mechanical loading and movement of the joint is required in order to maintain normal cartilage morphology, biochemical composition, and biomechanical properties (1). However, the results of these animal studies have been highly variable and inconsistent. Jurvelin et al (2), for example, observed a 9% decrease in cartilage thickness in the canine knee after 11 weeks of rigid immobilization. Haapala et al (3) reported a large decrease (ϳ20%) in cartilage of the medial femur, but no changes in cartilage of the lateral tibia and lateral femur (4), of canine knee joints. In contrast, Leroux et al (5) found that a shorter period of nonrigid immobilization (4 weeks) did not change the knee cartilage thickness of the dogs in their study.Spinal cord injury (SCI) causes unloading and restricted movement of the lower limb joints for substantial periods of time. As a consequence, patients with SCI encounter secondary complications, such as decreases in muscle mass, cardiovascular fitness, and bone density (6,7). Radiographic evidence of narrowing of the hip joint space by more than 50% in 25 of 200 patients with flaccid paralysis was identified in one st...

Recommendations on backpack loading advice restricting the load to 10% of body weight and carrying the load high on the spine. The effects of increasing load (0%-5%-10%-15% of body weight) and changing the placement of the load on the spine, thoracic vs. lumbar placement, during standing and gait were analysed in 20 college-aged students by studying physiological, biomechanical and subjective data. Significant changes were: (1) increased thorax flexion; (2) reduced activity of M. erector spinae vs. increased activation of abdominals; (3) increased heart rate and Borg scores for the heaviest loads. A trend towards increased spinal flexion, reduced pelvic anteversion and rectus abdominis muscle activity was observed for the lumbar placement. The subjective scores indicate a preference for the lumbar placement. These findings suggest that carrying loads of 10% of body weight and above should be avoided, since these loads induce significant changes in electromyography, kinematics and subjective scores. Conclusions on the benefits of the thoracic placement for backpack loads could not be drawn based on the parameter set studied.

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