Is High-frequency stiffness a measure for the number of attached cross-bridges?

Muscle stiffness is an important property for movement control. Stiffness is a measure for the resistance against mechanical disturbances in muscular-skeletal systems. In general muscle stiffness is assumed to depend on the number of attached cross-bridges. It is not possible to measure this number in vivo or vitro. In experiments, high frequency perturbations are used to obtain a measurement of stiffness. In this paper a simulation study is presented concerning the correlation between the number of attached cross-bridges and high-frequency stiffness. A model based on the sliding-filament theory was used for the simulation of dynamic contractions. It is concluded that these two methods of muscle stiffness determination do not yield compatible results during lengthenin

Analisi della struttura spaziale e pianificazione del paesaggio agro-forestale: prospettive d’integrazione

L’ecologia del paesaggio offre un ampio spettro di tecniche d’analisi applicabili a cartografie di uso e copertura del suolo per derivare informazioni quantitative su una varietà di aspetti della configurazione spaziale del mosaico paesistico; tra gli altri, misure relative alla estensione, forma, e livello di frammentazione spaziale delle superfici forestali (o di altre categorie di habitat naturali e seminaturali) sono ritenute utili a rispondere a requisiti informativi tipicamente formulati nel contesto di politiche di conservazione della biodiversità. Analisi della struttura del paesaggio possono anche supportare l’individuazione di ambiti territoriali omogenei per valore paesaggistico da individuare nel piano paesaggistico regionale, rispetto ai quali pianificare obiettivi di qualità paesaggistica e prescrizioni per la tutela e l'uso del territorio (vd. D. Lgs. 42/2004, Codice Urbani). Le metodologie di analisi e il monitoraggio della struttura spaziale del paesaggio agro-forestale sono ormai numerose e consolidate; tuttavia, solo di recente i risultati di questi studi hanno cominciato a supportare e orientare politiche di pianificazione del territorio per la formulazione di strategie di sviluppo territoriale differenziate in rapporto alle esigenze di conservazione degli habitat e della biodiversità e di salvaguardia del paesaggio calibrate in base alle specificità dei singoli territori. In particolare, il tema delle reti ecologiche, è divenuto oggetto specifico di pianificazione all’interno degli strumenti di area vasta. In questa prospettiva il presente contributo intende delineare alcune riflessioni su come raccordare le conoscenze dell’ecologia del paesaggio alla formulazione di obiettivi di salvaguardia e valorizzazione del paesaggio, tenendo conto della configurazione attuale del sistema delle pianificazioni aventi competenza in materia di governo del paesaggio. Focalizzando l’attenzione su le problematiche di governo del paesaggio più tipiche del paesaggio agro-forestale, vengono inoltre prospettate alcune considerazioni sul possibile ruolo della gestione forestale come strumento d’attuazione delle previsioni inerenti la tutela e la riqualificazione del paesaggio espresse dagli strumenti di pianificazione di area vasta

Remodeling of Rat M. Gastrocnemius Medialis During Recovery From Aponeurotomy

Aponeurotomy is a surgical intervention by which the aponeurosis is transsected perpendicularly to its longitudinal direction, halfway along its length. This surgical principle of aponeurotomy has been applied also to intramuscular lengthening and fibrotomia. In clinics, this intervention is performed in patients with cerebral palsy in order to lengthen or weaken spastic and/or short muscles. If the aponeurotomy is performed on the proximal aponeurosis, as is the case in the present study, muscle fibers located distally from the aponeurosis gap that develops lose their myotendinous connection to the origin. During recovery from this intervention, new connective (scar) tissue repairs the gap in the aponeurosis, as well as within the muscle belly. As a consequence, the aponeurosis is longer during and after recovery. In addition, the new connective tissue is more compliant than regular aponeurosis material. The aim of this study was to investigate changes in muscle geometry and adaptation of the number of sarcomeres in series after recovery from aponeurotomy of the proximal gastrocnemius medialis (GM) aponeurosis, as well as to relate these results to possible changes in the muscle length-force characteristics. Aponeurotomy was performed on the proximal aponeurosis of rat muscle GM and followed by 6 weeks of recovery. Results were compared to muscles of a control group and those of a sham-operated group. After recovery from aponeurotomy, proximal and distal muscle fiber lengths were similar to that of the control group. The mean sarcomere length from fibers located proximally relative to the aponeurosis gap remained unchanged. In contrast, fibers located distally showed 16–20% lower mean sarcomere lengths at different muscle lengths. The number of sarcomeres in series within the proximal as well as distal muscle fibers was unchanged. After recovery, muscle length-force characteristics were similar to those of the control group. A reversal of proximal-distal difference of fibers mean sarcomere lengths within muscles during recovery from aponeurotomy is hypothesized to be responsible for the lack of an effect. These results indicate that after recovery from aponeurotomy, geometrical adaptations preserved the muscle function. Moreover, it seems that the generally accepted rules of adaptation of serial sarcomere numbers are not applicable in this situation

Extramuscular myofascial force transmission within the rat anterior tibial compartment: Proximo-distal differences in muscle force

Intramuscular connective tissues are continuous to extramuscular connective tissues. If force is transmitted there, differences should be present between force at proximal and distal attachments of muscles. Extensor digitorum longus (EDL), tibialis anterior (TA), and extensor hallucis longus muscles (EHL) were excited simultaneously and maximally. Only EDL length was changed, exclusively by moving the position of its proximal tendon. Distal force exerted by TA + EHL complex was not affected significantly. Proximal and distal EDL isometric force were not equal for most EDL lengths:

Effects of distribution of muscle fiber length on active length-force characteristics of rat gastrocnemius medialis

The length-force curve of mammalian skeletal muscle is often wider than expected on basis of the optimum length of the muscle fibers. Two important effects may explain this discrepancy: muscle pennation and distribution of fiber lengths in the muscle. In the present study the effects of a Gaussian distribution of fiber lengths on muscle length-force characteristics were investigated in rat gastrocnemius medialis.Fiber length-force characteristics and parameter values of the Gaussian distribution were derived from literature data (Stephenson et al., 1989, J. Physiol., 410:351-366; Heslinga and Huijing, 1990, J. Morph. 206:119-132; Zuurbier and Huijing, 1993, J. Morphol., 218:167-180). Three different constructions of the distribution model were compared with experimental data. Pennation effects were incorporated in the model.Two constructions gave reasonably good results: 1) the model with a fiber optimum distribution, in which fibers acted at the same absolute length at a given muscle length; 2) the model in which fiber optimum length was uniform but absolute length at a given muscle length was distributed.In rat gastrocnemius medialis, the magnitude of the effects of these distributions is similar to pennation effects. The current results help to explain the relative wide working range of skeletal muscles in human movement and the differences in specific muscle tension as affected by muscle type, species, and age

Myofascial force transmission is increasingly important at lower forces: firing frequency-related length-force characteristics of rat extensor digitorum longus

Aim: Effects of submaximal stimulation frequencies on myofascial force transmission were investigated for rat anterior crural muscles with all motor units activated. -\ud Methods: Tibialis anterior and extensor hallucis longus (TAEHL) muscles were kept at constant muscle-tendon complex length, but extensor digitorum longus muscle (EDL) was lengthened distally. All muscles were activated simultaneously at 10, 20, 30 and 100 Hz within an intact anterior crural compartment. - \ud Results: At lower frequencies, significant proximo-distal EDL force differences exist. Absolute EDL proximo-distal active force differences were highest at 100 Hz (ΔFdist-prox = 0.4 N). However, the normalized difference was highest at 10 Hz (ΔFdist-prox = 30%Fdist). Firing-frequency dependent shifts of the ascending limb of the EDL length-force curve to higher lengths were confirmed for a muscle within an intact compartment, although effects of firing frequency assessed at proximal and distal EDL tendons differed quantitatively. As EDL was lengthened distally, TAEHL distal isometric active force decreased progressively. The absolute decrease was highest for 100 Hz (ΔFfrom initial = -0.25 N). However, the highest normalized decrease was found for 10 Hz stimulation (ΔFfrom initial = -40%). - \ud Conclusions: At submaximal stimulation frequencies, myofascial force transmission is present and the fraction of force transmitted myofascially increases with progressively lower firing frequencies. Evidently, the stiffness of epimuscular myofascial paths of force transmission decreases less than the stiffness of serial sarcomeres and myotendinous pathways. It is concluded that low firing frequencies as encountered in vivo enhance the relative importance of epimuscular myofascial force transmission with respect to myotendinous force transmission