Dynamic behaviour of half-sarcomeres during and after stretch in activated rabbit psoas myofibrils: sarcomere asymmetry but no 'sarcomere popping'.

We examined length changes of individual half-sarcomeres during and after stretch in actively contracting, single rabbit psoas myofibrils containing 10-30 sarcomeres. The myofibrils were fluorescently immunostained so that both Z-lines and M-bands of sarcomeres could be monitored by video microscopy simultaneously with the force measurement. Half-sarcomere lengths were determined by processing of video images and tracking the fluorescent Z-line and M-band signals. Upon Ca2+ activation, during the rise in force, active half-sarcomeres predominantly shorten but to different extents so that an active myofibril consists of half-sarcomeres of different lengths and thus asymmetric sarcomeres, i.e. shifted A-bands, indicating different amounts of filament overlap in the two halves. When force reached a plateau, the myofibril was stretched by 15-20% resting length (L0) at a velocity of approximately 0.2 L0 s(-1). The myofibril force response to a ramp stretch is similar to that reported from muscle fibres. Despite the approximately 2.5-fold increase in force due to the stretch, the variability in half-sarcomere length remained almost constant during the stretch and A-band shifts did not progress further, independent of whether half-sarcomeres shortened or lengthened during the initial Ca2+ activation. Moreover, albeit half-sarcomeres lengthened to different extents during a stretch, rapid elongation of individual sarcomeres beyond filament overlap ('popping') was not observed. Thus, in contrast to predictions of the 'popping sarcomere' hypothesis, a stretch rather stabilizes the uniformity of half-sarcomere lengths and sarcomere symmetry. In general, the half-sarcomere length changes (dynamics) before and after stretch were slow and the dynamics after stretch were not readily predictable on the basis of the steady-state force-sarcomere length relation.

Effect of electrical stimulation-induced cycling on bone mineral density in spinal cord-injured patients.

BACKGROUND: Bone atrophy in spinal cord-injured people (SCI) is, among other factors, caused by immobilization and is initiated shortly after the injury. The present study measured the effect of an functional electrical stimulation (FES)-cycling intervention on bone mineral density (BMD) of the tibia in recently injured SCI people. METHODS: As soon as possible after the injury (mean 4.5 weeks), para- and tetraplegic patients were recruited into an intervention and control group comparable with regard to gender, age, and lesion level. The intervention consisted of 30-min functional electrical stimulation-cycling three times a week for the duration of their primary rehabilitation (mean = 6 months). Computed tomography (CT) scans of the right tibia diaphysis were taken at the beginning and at the end of the intervention. Bone mineral density of cortical bone was calculated from the CT scans. RESULTS: A total of 38 subjects (19 in each group) were included in the study. Both groups showed a reduction in tibial cortical BMD of 0-10% of initial values within 3-10 months. The mean decrease in BMD was 0.3% (+/- 0.6) per month in the intervention group and 0.7% (+/- 0.8) in the control group. This difference did not reach statistical significance. Decrease of BMD was linearly correlated to initial BMD and age in the pooled data of both groups; subjects who had a high initial BMD and/or were older lost more bone. In neither group was bone loss associated with duration of immobilization nor lesion level. CONCLUSIONS: Functional electrical stimulation-cycling applied shortly after SCI did not significantly attenuate bone loss.

Inter-sarcomere dynamics in muscle fibres. A neglected subject?

The sarcomere is the functional unit of muscle, and all sarcomeres are connected in series in myofibrils within a muscle fibre. From this point of view of the structure a single model consisting of a contractile, a series and a parallel element can not account for the description of a real muscle fibre. Additionally, the titin protein filament needs to be considered as a passive visco-elastic element in parallel with the contractile apparatus. Therefore, the structure of a single muscle fibre is complex due mechanical elements ("motors") operating in series and in parallel. Moreover, variability does exist in the mechanical properties along a fibre and hence a multi-segmental model is more realistic and would give rise to many new insights. By attributing a segment model to each half-sarcomere, a fibre can be constructed through rigorous coupling of these units in series and parallel. The dynamics of such a multi-segmental model is much more complex, but it can explain a variety of effects reported in standard classical mechanics experiments. With a relatively simple mechanistic description we can show that the dynamics of such multi-sarcomere systems exhibit a variety of effects (relaxation phenomena, permanent extra-tension, biphasic force-velocity relation) and should therefore not be neglected in muscle fibre modelling. We have observed in single skinned fibre experiments that non-uniformities in sarcomere length changes are prominent during activation and relaxation.