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Notes: The present study measured passive resistance to stretch in the hamstring muscles during a standardized stretch maneuver to estimate tensile forces and energy of the individual hamstring muscles in 7 flexible and 6 inflexible persons defined according to joint range of motion. Using a dynamometer, knee joint moment was measured during slow passive knee extension to a maximal angle (dynamic phase) followed by a 90-s static phase. Cross-sectional areas (CSA) of the separate hamstring muscles were obtained with magnetic resonance (MR) imaging. Mathematical modeling was used to calculate instantaneous muscle length and joint moment arm for each muscle. Subsequently, passive muscle tension (N/cm2) was calculated based on moment arm lengths, knee joint moments, and CSA. Maximal tolerated joint angle was greater in flexible (Δ1.30±0.06 rad) than inflexible (Δ0.84±0.06 rad) subjects, P〈0.01. The peak tension at maximal angle was greater in flexible (81.8±12.5 N/cm2) than inflexible subjects (29.3±4.1 N/cm2), P〈0.001. For the separate muscles the overall change in muscle length (Δ cm) and moment arm (Δ cm) differed between groups, P〈0.01. Similarly, muscle stiffness (Δ tension/Δ muscle length) was greater in flexible than inflexible subjects in the final 3 cm, P〈0.01, and in the final 20% of length change, P〈0.01. Absorbed energy (mJ/cm2)was greater in flexible than inflexible subjects in the final 40% of length change, P〈0.05. These data show that flexible persons can attain a greater angle of stretch with an accompanying greater tensile stress and energy than inflexible persons due to an apparant greater tolerance to the externally applied load, and larger change in moment arm. The obtained stress data appear to be in the toe region of a ‘classical’ stress–strain curve, and energy rather than stiffness may therefore be more appropriate to analyze during the stretch procedure.

Notes: Because anterior cruciate ligament (ACL) injuries are common in European handball the present study assessed knee joint shear forces to estimate ACL loading in six elite female handball players during a side-cutting maneuver. A pilot investigation in three dimensions showed that peak moments occurred in the sagittal plane at a high velocity. Therefore, analysis of the movement was performed in two dimensions using high-speed cinematography, ground reaction forces, and electromyography (EMG). Film and force plate data allowed for calculation of net joint moments (inverse dynamics), estimates of instantaneous muscle-tendon lengths, contraction velocities, and peak loading of the ACL. During the breaking phase of the maneuver the peak knee joint moment was 239 Nm (99–309), which yielded an ACL-load of 520 N (215–673). The corresponding peak EMG amplitudes for the hamstring muscles were 34–39% of maximum EMG. During the breaking phase the quadriceps muscle contracted eccentrically with a velocity of 216–253% fiber length/s. In constrast, the hamstring muscles contracted concentrically with a velocity of 222–427% fiber length/s. These results suggest that a side-cutting maneuver produces loads that are insufficient to rupture the ACL. Furthermore, the rapid concentric hamstring contraction suggests that even during maximal activation, the ability of the hamstrings to reduce the ACL load is marginal.

Notes: The aim of the present study was to quantify the amount of antagonist coactivation and the resultant moment of force generated by the hamstring muscles during maximal quadriceps contraction in slow isokinetic knee extension. The net joint moment at the knee joint and electromyographic (EMG) signals of the vastus medialis, vastus lateralis, rectus femoris muscles (quadriceps) and the biceps femoris caput longum and semitendinosus muscles (hamstrings) were obtained in 16 male subjects during maximal isokinetic knee joint extension (KinCom, ROM 90–10°, 30° · s−1). Two types of extension were performed: [1] maximal concentric quadriceps contractions and [2] maximal eccentric hamstring contractions. Hamstring antagonist EMG in [1] were converted into antagonist moment based on the EMG-moment relationships determined in [2] and vice versa. Since antagonist muscle coactivation was present in both [1] and [2] a set of related equations was constructed to yield the moment/EMG relationships for the hamstring and quadriceps muscles, respectively. The equations were solved separately for every 0.05° knee joint angle in the 90–10° range of excursion (0°=full extension) ensuring that the specificity of muscle length and internal muscle lever arms were incorporated into the moment/EMG relationships established. Substantial hamstring coactivation was observed during quadriceps agonist contraction. This resulted in a constant level of antagonist hamstring moment of about 30 Nm throughout the range of motion. In the range of 30–10° from full knee extension this antagonist hamstring moment corresponded to 30–75% of the measured knee extensor moment. The level of antagonist coactivation was 3-fold higher for the lateral (Bfcl) compared to medial (ST) hamstring muscles. The amount of EMG crosstalk between agonist–antagonist muscle pairs was negligible (RXY2〈0.02–0.06). The present data show that substantial antagonist coactivation of the hamstring muscles may be present during slow isokinetic knee extension. In consequence substantial antagonist flexor moments are generated. The antagonist hamstring moments potentially counteract the anterior tibial shear and excessive internal tibial rotation induced by the contractile forces of the quadriceps near full knee extension. In doing so the hamstring coactivation is suggested to assist the mechanical and neurosensory functions of the anterior cruciate ligament (ACL).

Notes: The load-displacement and stress–strain characteristics of the human triceps surae tendon and aponeurosis, in vivo, was examined during graded maximal voluntary plantarflexion efforts in runners who trained 80 km/ week or more and age-matched non-runners. Synchronous real-time ultrasonography of triceps surae tendon and aponeurosis displacement, electromyography of the gastrocnemius, soleus and dorsiflexor muscles, and joint angular rotation were obtained. Tendon cross-sectional area and ankle joint moment arm were obtained from magnetic resonance imaging. Tensile tendon force was calculated from the joint moments and tendon moment arm and stress was obtained by dividing force by cross-sectional area. Strain was obtained from the displacements normalized to tendon length. Antagonist coactivation and small amounts of ankle joint rotation significantly affected tensile tendon force and aponeurosis and tendon displacement, respectively (P 〈 0.01). Plantarflexion moment was similar in runners (138 ± 27 Nm, mean ± SEM) and non-runners (142 ± 17 Nm). Tendon moment arm was alike in non-runner (58.3 ± 0.2 mm) and runners (55.1 ± 0.1 mm). Similarly, there was no difference in tendon tensile force between runners (2633 ± 465 N) and non-runners (2556 ± 401 N). The cross-sectional area of the Achilles tendon was larger in runners (95 ± 3 mm2) than non-runners (73 ± 3 mm2) (P 〈 0.01). The load-deformation data yielded similar stiffness (runners 306 ± 61 N/mm, non-runners 319 ± 42 N/mm). The maximal strain and stress was 4.9 ± 0.8% and 38.2 ± 9.8 MPa in non-runners and 4.1 ± 0.8% and 26.3 ± 5.1 MPa in runners. The larger tendon cross-sectional area in trained runners suggests that chronic exposure to repetitive loading has resulted in a tissue adaptation.

Notes: The mechanical properties of the human vastus lateralis (VL) tendon-aponeurosis complex were investigated in eight male subjects. Knee extensor force, knee joint angle, and corresponding longitudinal VL aponeurosis displacement were monitored synchronously during graded (10-s) maximal isometric knee extension contractions. Displacement observed during isometric conditions may be regarded as an expression of deformation in the tissues distal to the measurement site. Furthermore, aponeurosis displacement was measured during passive knee extension (90–75°°), and used to correct displacement values obtained during active contraction for joint angular motion. The passive trial yielded a highly linear relationship between aponeurosis displacement and joint angular motion(r2 = 0.998 ± 0.002) with a mean correction factor of 0.41 ± 0.10 mm/degree. Maximal knee extensor force was 5834 ± 1341 N with a corresponding VL aponeurosis displacement of 12.7 ± 2.5 mm, while correcting for joint angular motion reduced maximal displacement ∼9% (to 11.6 ± 2.5 mm, P 〈 0.005) (data presented as means ± SD). Two separate graded contraction trials were performed, and no between-trial differences were observed in either maximal force or maximal displacement. Between trial coefficient of determination and CV for maximal force and maximal displacement were r2 = 0.97, CV = 2.9% and r2 = 0.92, CV = 4.6%, respectively, indicating intra-day reproducibility of measurements. These data demonstrate that when applying the newly established ultrasound-based method of investigating quadriceps connective tissue mechanical properties, maximal isometric contraction is inevitably associated with some joint angular motion that significantly influences the calculations.

Notes: Movement is caused by force transmission from contracting muscles to bone via tendon. The collagen structure of tendon is organized in a very hierarchical manner. The collagen fibril is considered the basic force-transmitting unit of tendon, and it is embedded in a hydrophilic extracellular matrix of proteoglycans, glycoproteins and glycosaminoglycans. It has recently been shown in human peritendinous tissue is more metabolically active in response to activity than previously thought, although it remains to be established, if the level of activity influences affects fibril diameter and/or total tendon cross-sectional area. Moreover, it cannot be unequivocally concluded that tendon adaptation to physical activity is one of a quantitative and/or qualitative nature. The currently available information is almost exclusively obtained from animal data, however, techniques such as microdialysis for tendon metabolism and ultrasound combined with MRI for tendon mechanical properties has already provided information on human tendon behavior, and is likely to further add to our understanding of how tendon adapt to physical activity. This review will address the structure and function of tendon, and the current knowledge of how tendons respond to activity with respect to biomechanical properties.

Notes: The oligodeoxyribonucleotide (ODN) 5'-TTTTCAATTCGAAGATGAAT-3' (ODN H), identified in systemic lupus erythematosus (SLE) serum, induced the production of interferon (IFN)-α in human peripheral blood mononuclear cells (PBMC) when combined with lipofectin. Flow cytometric analysis with staining for surface antigens and intracellular IFN-α, showed that the IFN-α-producing cells (IPC) were the natural IPC, also termed type 2 dendritic cell precursors (pDC2) or plasmacytoid monocytes. The importance of unmethylated CpG dinucleotides for the interferogenic activity of ODN was studied. Methylation of CpG impaired the activity of single-stranded (ss) ODN H, but increased that of the complementary ssODN I. Furthermore, CpG-methylated double-stranded (ds) ODN Hmet-Imet lost, but hemimethylated dsODN H-Imet retained interferogenic activity. Inversion of the CpG to GpC had no effect on the interferogenic activity of ssODN H, increased that of ssODN I, however abolished the activity of dsODN H-I. Alteration of the CpG in ODN H to ApG and in the ODN I to CpT destroyed their activity. The induction of IFN-α is therefore sequence-specific, but unmethylated CpGs are not always required, especially not in ssODNs. Interferogenic DNA sequences could therefore be more frequent in eukaryotic genomes than previously thought and their capacity to activate natural IPC may have implications for immune responses to microbial antigens and nuclear autoantigens.