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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: The aim of the present study was to describe the isokinetic strength profile and its relation to hiking performance in male (Sm, n=15) and fmale (Sf, n=6) elite sailors compared to a group of male control subjects (Cm, n=8) similar in age, anthropometry and level of fitness. Eccentric knee extension strength was higher in Sm compared to Cm. (P〈0.01). Furthermore, Sm were stronger during trunk extension (P 〈 0.05), but not during trunk flexion compared to CM. Overall muscle strength was lower in SF compared to SM (P 〈 0.01) and CM (P 〈 0.05), except for eccentric knee extension strength, where SF and CM did not differ (P 〉 0.05). Hiking performance correlated to maximal eccentric and isometric knee extensor strength in SF (rs= 0.83–0.88, P 〈 0.05) and in CM (rs= 0.73-0.77, P 〈 0.05) and to maximal eccentric knee extensor strength at high velocity in SM (rs= 0.46-0.54, P 〈 0.05). For a subgroup of hikers in SM (n= 8), hiking peformance correlated to maximal isometric-eccentric knee extensor strength (rs=0.67-0.74, P〈0.05), whereas no correlations emerged for the non-hikers (n=7). Few correlations were observed between hiking performance and maximal concentric trunk flexor strength (rs=0.69-0.92, P 〈 0.05). Unexpectedly, in SM correlations also were observed between hiking performance and maximal strength of the trunk extensors (rs=0.46-0.53, hike subgroup: rs=0.64-0.67, P 〈 0.05). In conclusion, notably high levels of maximal eccentric knee extesor strength were observed for the male and female elite sailors examined in the present study. Furthermore, the present results suggest that hiking performance depends in part on maximal isometric-eccentric knee extensor strength. The maximal strength of the trunk extensors, which potentially stabilizes the lower back and spine, also seems to have some importance for the hiking performance of top-level sailors.

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 present study sought to investigate the role of EMG activity during passive static stretch. EMG and passive resistance were measured during static stretching of human skeletal muscle in eight neurologically intact control subjects and six spinal cord-injured (SCI) subjects with complete motor loss. Resistance to stretch offered by the hamstring muscles during passive knee extension was defined as passive torque (Nm). The knee was passively extended at 5o/s to a predetermined final position, where it remained stationary for 90 s (static phase) while force and integrated EMG of the hamstring muscle were recorded. EMG was sampled for frequency domain analysis in a second stretch maneuver in five control and three SCI subjects. There was a decline in passive torque in the 90-s static phase for both control and SCI subjects, P〈0.05. Although peak passive torque was greater in control subjects, P〈0.05, there was no difference in time-dependent passive torque response between control (33%) and SCI (38%) subjects. Initial and final 5-s IEMG ranged from 1.8 to 3.4 μ V.s and did not change during a stretch or differ between control and SCI subjects. Frequency domain analysis yielded similar results in both groups, with an equal energy distribution in all harmonics, indicative of ‘white noise’. The present data demonstrate that no measurable EMG activity was detected in either group during the static stretch maneuver. Therefore, the decline in resistance to static stretch was a viscoelastic stress relaxation response.

Notes: The effect of an early rehabilitation program, including postural training, on ankle joint function after an ankle ligament sprain was investigated prospectively. Ninety-two subjects, matched for age, sex, and level of sports activity, were randomized to a control or training group. All subject received the same standard information regarding early ankle mobilization. In addition, the training group participated in supervised physical therapy rehabilitation (1 h. twice weekly) with emphasis on balance training. Postural sway, position sense and isometric ankle strength were measured 6 weeks and 4 months after the injury, and at 12 months re-injury data were obtained. In the training group, there was a significant difference between the injured and uninjured side for plantar flexion (P〈0.01), eversion (P〈0.01) and inversion (P〈0.05), but not for dorsiflexion at 6 weeks. In the control group, there was a significant difference between the injured and uninjured side for plantar flexion (P〈0.01), eversion (P〈0.01), inversion (P〈0.01), and dorsiflexion (P〈0.05) at 6 weeks. Postural sway, but not position sense, differed between the injured and uninjured side in both groups (P〈0.01) at 6 weeks. The side-to-side percent differences were similar in both groups for all variables (P〉0.05) at 6 weeks, and there were no side-to-side differences at 4 months in either group. In the control group, 11/38 (29%) suffered a re-injury. while this number was only 2/29 (7%)) in the training group (P〈0.05). These data showed that an ankle injury resulted in reduced ankle strength and postural control at 6 weeks, but that these variables had normalized at 4 months, independent of the supervised rehabilitation. However, the findings also demonstrated that supervised rehabilitation may reduce the number of re-injuries, and therefore may play a role in injury prevention.

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: Cross-sectional area, stiffness, viscoelastic stress relaxation, stretch tolerance and EMG activity of the human hamstring muscle group were examined in endurance-trained athletes with varying flexibility. Subjects were defined as tight (n=10) or normal (n=8) based on a clinical toe-touch test. Cross-sectional area was computed from magnetic resonance imagining (MRI) images. Torque (Nm) offered by the hamstring muscle group, electromyographic (EMG) activity, knee joint angle and velocity were continuously monitored during two standardized stretch protocols. Protocol 1 consisted of a slow stretch at 0.087 rad/s (dynamic phase) to a pre-determined final angle followed by a 90-s static phase. In the dynamic phase final angle and stiffness was lower in tight (28.0±2.9 Nm/rad) than normal subjects (54.9±6.5 Nm/rad), P〈0.01. In the static phase tight subjects had lower peak (15.4±1.8 Nm) and final torque (10.8±1.6 Nm) than normal subjects (31.6±4.1 Nm, 24.1±3.7 Nm, respectively)(P〈0.01), but torque decline was similar. Protocol 2 consisted of a slow stretch to the point of pain and here tight subjects reached a lower maximal angle, torque, stiffness and energy than normal subjects (P〈0.01). On the other hand, stiffness was greater in tight subjects in the common range (P〈0.01). Cross-sectional area of the hamstring muscles and EMG activity during the stretch did not differ between the groups. However, lateral hamstring cross-sectional area was positively related to mid-range stiffness (P〈0.05), but inversely related to final stiffness, peak torque and the toe-touch test (P〈0.01). Final angle and peak torque in protocol 1 combined to improve the predictability of the toe-touch test (R2=0.77, P〈0.001). These data show that the toe-touch test (R2=0.77, P〈0.001). These data show that the toe-touch test is largely a measure of hamstring flexibility. Further, subjects with a restricted joint range of movement on a clinical toe-touch test have stiffer hamstring muscles and a lower stretch tolerance.

Notes: The purpose of this study was (1) to evaluate the reproducibility of a new method of measuring passive resistance to stretch in the human hamstring muscle group, in vivo, using a test re-test protocol and 2) to examine the effect of repeated stretches. Passive resistance offered by the hamstring muscle group during knee extension was measured in 10 subjects as knee flexion moment (Nm) using a KinCom dynamometer. The knee was passively extended at 5 deg/s to the final position where it remained stationary for 90 s (static phase). EMG of the hamstring muscle was also measured. The test re-test protocol included 2 tests (tests 1 and 2) administered 1 h apart. On a separate occasion 5 consecutive static stretches were administered (stretches 1–5) separted by 30 s. Stretch 6 was administered one hour after stretch 5. In the static phase passive resistance did not differ between test 1 and test 2. Resistance declined in both tests 1 and 2, whereas EMG activity remained unchanged. The decline in resistance was significant up to 45 s. For the repeated stretches there was an effect of time (90 s) and stretch (1–5) with a significant interaction i.e., resistance diminished with stretches, and the 90-s decline was less as more stretches were performed. Passive resistance in stretch 6 was lower than in stretch 1. The present study has demonstrated a reliable method for studying resistance to stretch of the human hamstring muscle group. A viscoelastic response of the human hamstring muscle was shown. With 5 repeated stretches, resistance to stretch diminished and each stretch exibited a viscoelastic response, albeit less with each subsequent stretch. The effect of 5 repeated stretches was significant 1 h later.

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.