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Results: Cycle time [CT] (P0.01) and relative (%CT) arm recovery time (P0.05) decreased at higher inclinations. Cycle rate and relative poling time both increased (both P0.01). No differences were found for the relative durations of leg-ground contact and leg swing. All the force values measured were greatest at the steepest inclination, including peak pole and leg forces, forefoot force in ground position and impulses of pole and leg push-off (all P0.01). Elbow [EA], hip [HA] and knee [KA] angle minima, EA at pole plant, KA at ski plant and HA maximum before push-off decreased (all P0.01). The elbow extension range of motion [ROM] and knee (both P0.01), as well as hip
push-off extension ROM (P0.05), increased with inclination. Angular push-off flexion velocity (P0.01) decreased, along with higher angular push-off extension velocities in the ankle (P0.05), knee and hip joints (both P0.01) at steeper inclinations.
Conclusion: Elite skiers adapt the DIA technique to increased inclinations through substantial changes in pole and leg kinetics and joint kinematics, with only minor changes in specific phase durations. Arm and leg actions show larger amplitudes, higher angular velocities, longer poling times, higher forces and impulses generated at higher frequencies. We suggest that XC skiers should use technique, endurance and strength training at different inclinations in order to train these aspects specifically.
Bilodeau B, Boulay M, Roy B. (1992). Propulsive and gliding phases in four crosscountry skiing techniques. Med Sci Spor Exer, 24, 8, 917KNEE JOINT LOADING DURING LEVEL WALKING WITH MBT SHOES IN OVERWEIGHT MEN BUCHECKER, M., WAGNER, H., PFUSTERSCHMIED, J., MÜLLER, E.
UNIVERSITY OF SALZBURGFelson (1996) identified obesity and overweight as main risk factors for the onset of knee osteoarthritis (OA). According to Gushue et al.
(2005) high body mass initiates primarily greater dynamic loads on the medial joint compartment as assessed by a higher peak external knee adduction moment during walking. Furthermore DeVita & Hortobágyi (2003) stated reduced knee flexion excursion in healthy obese adults, which indicates not only ’quadriceps avoidance’ but also higher co-activation between antagonistic muscle pairs (Childs et al.
2004). The aim of the present study was to determine whether an MBT shoe alters walking kinetics and muscle coordination in overweight men with regard to knee OA prevention.
Ten healthy overweight men (32.0±7.9 yr, 179.2±5.8 cm, 91.3±7.0 kg, 28.4±1.8 kg/m2), familiar with MBT, participated. Subjects were instructed to walk 6 trails with their own and MBT shoes at their preferred speed (±0.2 km/h), which was controlled by a wireless Brower timing system. Kinetics and kinematics were obtained simultaneously with an AMTI force platform (2000 Hz) and a Vicon MX13-system consisting of eight cameras (250 Hz). Surface EMG activities of 6 right-leg muscles were recorded with a 16 channels Biovision measurement system (2000 Hz). Data were synchronised by an external trigger signal and processed by Nexus 1.2 (Oxford Metrics) and IkeMaster 2004 (Ike Software Solutions). Paired sample t-test and Cohen’s effect size dz were used to compare peaks of 3D external knee net joint moments, IEMG of vastus lateralis (VL), rectus femoris (RF) and biceps femoris (BF) as well as average VL/BF co-activation during stance phase.
No relevant differences in average VL/BF co-activation (t(9) = -.54, p =.60, dz =.17), muscle intensities (IEMG) of VL (t(9) = -.41, p =.69, dz =.13), RF (t(9) = -.01, p =.99, dz =.00) and BF (t(9) =.39, p =.71, dz =.12) were observed between the control and MBT situation. Similar results could be obtained for peak knee flexion moment (t(9) =.58, p =.58, dz =.18) and peak knee external rotation moment (t(9) = -.78, p =.46, dz =.25). However, concerning peak knee adduction moment strong significant effects were found. In comparison to the control shoes this variable demonstrated an average reduction of 10% (t(9) = 2.64, p =.03, dz =.83) for the MBT condition.
First data exhibit decreased knee joint loads using MBT. In fact, this shoe seems to be advantageous for knee OA prevention in overweight men. Further kinematic and kinetic analysis is warranted to find biomechanical reasons for the smaller adduction moment. Due to its construction and referring to Kälin & Segesser (2004) a lateral shifting of the centre of pressure and thus a reduced frontal knee moment arm are presumed as main factors for this result.
Childs et al. (2004) Clin Biomech, 19 DeVita & Hortobágyi (2003) J Biomech, 36 Felson (1996) Ann Rheum Dis, 55 Gushue et al. (2005) J Pediatr Orthop, 25 Kälin & Segesser (2004) Orthopädie Schuhtechnik, 12
RELATIONSHIP OF REARFOOT MOTION TO VERTICAL AND LEG STIFFNESS IN RUNNINGELTARVÅG, B., ERIKSRUD, O., SMITH, G.
NORWEGIAN SCHOOL OF SPORT SCIENCESDuring running, the lower extremities function much like a mass-spring system. Near the middle of stance, the leg spring is maximally compressed. This is approximately the time of maximal pronation, commonly described by rearfoot eversion. Few studies have explored the relationship between rearfoot motion and leg stiffness (Viale, 1998). The current study which focuses on foot motion was part of a larger project evaluating the influence of lower extremity joint kinematics on leg and vertical stiffness.
Methods: 15 female subjects were all tested while running over a 20 m lab runway at a target speed of 3.8 m·s-1. Three dimensional kinematic data were obtained from reflective markers on the lower extremity using a Pro-Reflex motion capture system (Qualisys AB, Sweden). Three-dimensional kinetic data were measured using three floor mounted force plates (AMTI, USA) in sequence placed midway in the runway. Three or more successful trials within a 5% range of 3.8 m·s-1 were analyzed using MatLab and Visual3D. Vertical and leg stiffness were calculated based on the method by McMahon and Cheng (1990). Rearfoot angle was calculated from tracking markers in Visual3D as the relative angle between the foot and shank segments with 0° taken as the positioning in a static, standing trial. Maximum rearfoot eversion (EvMax) along with eversion range of motion (EvROM: angular change from heelstrike to EvMax) were determined. The relationships of EvMax and EvROM to vertical and leg stiffness were evaluated using Pearson's product moment correlation (r).
Results: Mean EvROM during early stance of running was 14.4 ± 4.2° while EvMax was 11.5 ± 2.7°. Vertical stiffness for the runners was
31.8 ± 4.7 kN·m-1 while leg stiffness averaged 10.3 ± 1.5 kN·m-1. EvRom and EvMax were not significantly correlated with neither vertical nor leg stiffness.
Discussion: The mass-spring model applies to the whole running leg as a system which collectively exhibits elastic properties. The model does not attempt to identify which anatomical structures produce these characteristics. However, it is likely that structures surrounding ankle, knee and hip all contribute in some amount to this response. Foot kinematics during stance may help to absorb shock during impact with the ground, but in addition, foot motion provides a mechanism for partially explaining the elastic characteristics of the running leg. EvROM was not found to be significantly related to leg stiffness. This is only one plane of a complex foot motion and only one of several joints which could contribute to leg stiffness. Further analysis of the ankle and foot in other planes of motion as well as other joints needs to be analyzed to better understand this important fraction of leg mechanics.
McMahon TA & Cheng GC (1990) The mechanics of running: how does stiffness couple with speed. J Biomech 23:65-78 Viale F et al. (1998). Leg stiffness and foot orientation in running. Foot & Ankle 19:761-765
CHANGES IN MOTOR UNIT CHARACTERISTICS AFTER ECCENTRIC ELBOW FLEXOR EXERCISEPIITULAINEN, H., HOLOBAR, A.,3, AVELA, J.
1. UNIVERSITY OF JYVÄSKYLÄ, 2. POLITECNICO DI TORINO, 3. UNIVERSITY OF MARIBORIntroduction: Morphological evidence suggests that predominantly fast-twitch fibers are prone to sarcolemmal disruption during eccentric exercise (Jones et al., 1986). However, it is unclear if the function of motor units (MUs) is selectively affected.
Methods: High-density surface electromyographs (sEMG) were recorded with 5×13 electrode array (LISiN-OT Bioelettronica, Torino, Italy, inter-electrode distance of 8 mm) and decomposed with Convolution Kernel Compensation approach (Holobar and Zazula, 2004) to examine MU characteristics in m. biceps brachii after eccentric elbow flexor exercise (50 repetitions, 20 s intervals). Nine healthy subjects were measured at wide range of isometric contraction levels: 10%, 20%, 30%, 40% 50% and 75% of maximal voluntary contraction (MVC)
before (BEF) the exercise, and two hours (2H), two days (2D) and four days (4D) post-exercise. Number of extracted MUs per session was:
BEF: 471 MUs, 2H: 228 MUs, 2D: 426 MUs and 4D: 470 MUs. The following MU characteristics were extracted for each identified MU:
mean instantaneous discharge rate (IDR), mean muscle fiber conduction velocity (CV) (McGill and Dorfman, 1984), and mean power frequency (MNF).
Results: As a result of the exercise, MVC force decreased by 21.3 ± 5.6 % 2H (p 0.001) and by 12.6 ± 11.1 % 2D (p 0.01) post-exercise.
2H after the exercise, IDR increased from 19.5 ± 4.4 pps (BEF) to 22.6 ± 5.0 pps and from 24.5 ± 5.6 pps (BEF) to 28.3 ± 3.0 pps in 50% (p 0.01) and 75% (p0.05) of MVC contraction, respectively. When IDR was plotted against the absolute force level, the regression slope was significantly steeper at 2H than BEF, suggesting increase in the rate coding of MUs. In contracts to IDR, a reduction in CV was observed in 40% (from 4.4 ± 0.3 m/s to 4.2 ± 0.3 m/s, p 0.05), 50% (from 4.5 ± 0.3 m/s to 4.2 ± 0.3 m/s, p 0.01) and 75% (from 4.4 ±
0.3 m/s to 4.2 ± 0.3 m/s, p 0.01) of MVC contraction levels at 2H (BEF vs. 2H). MNF showed reduction only in 75% of MVC contraction (from 102.0 ± 18.6 Hz (BEF) to 91.1 ± 20.4 Hz (2H), p 0.05). When CV and MNF were plotted against absolute force level, the steepness of both regression slopes was significantly reduced at 2H with respect to BEF, implying possible impairment in sarcolemmal conduction. No changes were observed in these variables when the experiment was repeated without the exercise in seven control subjects.
Discussion: Based on Henneman’s size principle, the present results suggest that intensive exercise can selectively disturb the function of high-threshold MUs. These impairments seem to be related to slowing of sarcolemmal action potential conduction and presumably force production of the affected MUs. Thus, the increased rate coding of active MUs seems to be an attempt to compensate this force reduction.
References Jones D et al. (1986). J.Physiol. 375: 435-448.
Holobar A, Zazula D (2004) Med.Biol.Eng.Comput. 42: 487-495.
McGill K, Dorfman L (1984). IEEE Trans.Biomed.Eng. 31: 462-468.
THE RELEVANCE OF SUBTALAR-JOINT-ANATOMY ON THE INCIDENCE OF CHRONIC OVERUSE INJURIES IN THE LOWERLIMBS.
REULE, C.A., ALT, W.W.
UNIVERSITY OF STUTTGARTIntroduction: Chronic overuse injuries (COI), frequent in the Achilles tendon, are of major importance to athletes, especially high performance athletes. Little is known about the predisposing factors of these injuries. Therefore, the aim of this study is to identify the relationship between individual anatomical factors of the subtalar-joint and COI through the use of a measurement system for determination the spatial orientation of the subtalar-joint-axis (SJA) and a treadmill-analysis.
Methods: An ultrasonic pulse-echo based measurement system (Zebris®) determined the spatial orientation of SJA in-vivo and in realtime. The arch-index and the angle of gait were also determined by a treadmill that enables plantar pressure measurement (Zebris®).
Video-analysis allowed for investigation of the rolling motion of the foot during walking. Information about previous injuries and running performance was collected with a questionnaire. Measurements were taken from 495 subjects, mainly consisting of long distance runners with a running distance of at least 25 km per week and a running history of 3 years. Data from 307 subjects was included.
Results: Of 307 subjects, 69% had previously been injured. A total of 664 injuries were counted. 22% of these injuries were located at the ankle, 21% at the knee and 14 % at the Achilles tendon. By means of the Arch-index, it was found that 42% of subjects with Achilles tendon problems also had high arches. In subjects without Achilles tendon problems, only 34% had high arches. The average of the inclination angles of 614 SJA was 42° ± 16° and the average deviation angle (DA) was 11° ± 23°. There was a significant difference (p= 0.002) between the mean DA of people with Achilles tendon problems (18° ± 23°) and people without (10° ± 23).
Conclusion: The injury-rate in this study was 69% higher than that stated by Mayer (1999). The Results of Arch-index analysis confirmed the conclusion by Lohrer (2006), that a high arch is a common cause for Achilles tendon injuries. The mean inclination angle of the SJA found in this study approximates the results of Isman (1969). In contrast, the mean DA measured in vivo was smaller and the SJA was closer to the foot bisection than measured by Isman (1969). However, the DA (18°) of subjects with Achilles tendon problems was higher than recorded by Isman (1969). Based on these results, long distance runners with higher DA are at a higher risk to suffer from Achilles tendon problems.
Alt, W. (2001) Biomechanische Aspekte der Gelenkstabilisierung, Maurer Druck & Verlag, Geislingen Isman, R. E., & Inman, V. T. (1969) Anthropometric studies of the human foot and ankle. Bulletin of prosthetics research, 10/11, 97-129 Lohrer, H. (2006) Die Achillodynie – Ein Überblick. Orthopädieschuhtechnik 7/8, 34-41 Mayer, F. et al. (1999) current changes in running injuries, int j sports med 20, 103 Acknowledgment: supported by Bundesinstitut für Sportwissenschaft Germany.