Kinematic stride cycle asymmetry is not associated with sprint performance and injury prevalence in athletic sprinters.

The aims of this study were to (a) quantify the magnitude of kinematic stride cycle asymmetry in high-level athletic sprinters, (b) explore the association between kinematic asymmetry and maximal sprint running performance, and (c) investigate possible associations between kinematic asymmetry and injury prevalence. Twenty-two competitive sprinters (age 23 ± 3 year, height 1.81 ± 0.06 m, body mass 75.5 ± 5.6 kg, personal best 100 m 10.86 ± 0.22 seconds) performed 2-3 flying sprints over 20 m. Kinematics were recorded in 3D using a motion tracking system with 21 cameras at a 250 Hz sampling rate, allowing assessment of six consecutive steps for each athlete. Information about injuries sustained 1 year prior to and after the experiment was continuously registered (type, location, severity/duration, and time of year occurrence). The results showed that ≥11 of the 22 participating athletes displayed large or very large asymmetry for at least 11 of 14 variables, and all athletes displayed large or very large asymmetry for at least three variables. No correlations between individual magnitudes of asymmetry and sprint performance were significant (trivial to moderate). No significant changes in asymmetry between best and worst trial were observed for any of the analyzed variables. In addition, injured and non-injured athletes did not differ in asymmetry, neither for the time period 1 year prior to nor after the test. In conclusion, kinematic asymmetries in the stride cycle were not associated with neither maximal sprint running performance nor the prevalence of injury among high-level athletic sprinters.

Bilateral Asymmetry in Upper Extremities Is More Pronounced in Distal Compared to Proximal Joints.

The authors' aim was to compare spatial and temporal accuracy in proximal versus distal joints in upper extremities. Given the morphological differences in corticospinal and corticomotoneuronal projections for proximal and distal muscles, they hypothesized that bilateral asymmetry would be larger for distal than for proximal joints. Twelve participants performed isolated flexion-extension movements with the shoulders and index fingers. Angular range of motion of finger and shoulder movements was kept constant. The results showed significant bilateral asymmetry for both proximal and distal joints for both spatial and temporal accuracy. More importantly, bilateral asymmetry was significantly larger for the index fingers than for the shoulders for both spatial and temporal variables, as hypothesized. These results at the behavioral level pave the way for further studies that combine direct measures of neural activation with behavioral measures to further illuminate the potential link between bilateral communication and laterality effects in motor performance.

Biomechanical analysis of the herringbone technique as employed by elite cross-country skiers.

This investigation was designed to analyse the kinematics and kinetics of cross-country skiing at different velocities with the herringbone technique on a steep incline. Eleven elite male cross-country skiers performed this technique at maximal, high, and moderate velocities on a snow-covered 15° incline. They positioned their skis laterally (25 to 30°) with a slight inside tilt and planted their poles laterally (8 to 12°) with most leg thrust force exerted on the inside forefoot. Although 77% of the total propulsive force was generated by the legs, the ratio between propulsive and total force was approximately fourfold higher for the poles. The cycle rate increased with velocity (1.20 to 1.60 Hz), whereas the cycle length increased from moderate up to high velocity, but then remained the same at maximal velocity (2.0 to 2.3 m). In conclusion, with the herringbone technique, the skis were angled laterally without gliding, with the forces distributed mainly on the inside forefoot to enable grip for propulsion. The skiers utilized high cycle rates with major propulsion by the legs, highlighting the importance of high peak and rapid generation of leg forces.

Gender differences in endurance performance by elite cross-country skiers are influenced by the contribution from poling.

Greater gender differences have been found in exercise modes where the upper body is involved. Therefore, the present study investigated the influence of poling on gender differences in endurance performance by elite cross-country skiers. Initially, the performance of eight male and eight female sprint skiers was compared during four different types of exercise involving different degrees of poling: double poling (DP), G3 skating, and diagonal stride (DIA) techniques during treadmill roller skiing, and treadmill running (RUN). Thereafter, DP was examined for physiological and kinematic parameters. The relative gender differences associated with the DP, G3, DIA and RUN performances were approximately 20%, 17%, 14%, and 12%, respectively. Thus, the type of exercise exerted an overall effect on the relative gender differences (P < 0.05). In connection with DP, the men achieved 63%, 16%, and 8% higher VO2peak than the women in absolute terms and with normalization for total and fat-free body mass (all P < 0.05). The DP VO2peak in percentage of VO2max in RUN was higher in men (P < 0.05). The gender difference in DP peak cycle length was 23% (P < 0.05). In conclusion, the present investigation demonstrates that the gender difference in performance by elite sprint skiers is enhanced when the contribution from poling increases.

Comparison of bilateral force deficit in proximal and distal joints in upper extremities.

Bilateral force deficit refers to the phenomenon that maximal generated force during simultaneous bilateral muscle contractions is lower than the sum of forces generated unilaterally. Based on the notion that neural inhibition is the main source for bilateral force deficit and existing differences in neural inhibiting interhemispheric organization of proximal and distal muscles, we expected differences in bilateral deficit in proximal and distal joints. The aim of the current behavioral experiment was to compare bilateral force deficit in proximal compared to distal upper extremity joints. Ten young adults performed single-joint maximal voluntary contractions in isometric flexions of the shoulder and index finger unilaterally and bilaterally. The results showed a significant absolute bilateral force deficit for both proximal (140.01 ± 86.99 N) and distal muscles (4.64 ± 4.86 N). More importantly, relative bilateral force deficit for shoulder flexion was significantly larger than for index finger flexion, -20.51 ± 7.8% and -5.07 ± 3.84% respectively. The hypothesis of a more pronounced bilateral force deficit for proximal compared to distal muscles was confirmed in our results. Thus, our findings, in combination with the neuroanatomical differences for proximal and distal muscles, make it worthwhile to further explore the hypothesis that the commissural fibers provide differences in interhemispheric inhibitory interactions during bimanual actions for proximal and distal muscles.

Metabolic rate, cardiac response, and aerobic capacity in fibromyalgia: a case-control study.

OBJECTIVES: Several studies report reduced aerobic capacity in patients with fibromyalgia (FM). The purpose of our study was to investigate whether a reduction in aerobic capacity in these patients is accompanied by alterations in metabolic rate and heart rate (HR) response. METHOD: Twelve women with FM and 12 healthy controls (HCs) matched on sex and age, and with similar leisure time physical activity, participated in the study. All subjects performed an incremental submaximal cycle ergometer test to anaerobic threshold [AT; i.e. blood lactate concentration (bLa) ≥ 4 mmol/L], followed by a stepwise test to exhaustion to estimate maximal oxygen consumption (VO(2max)). RESULTS: Oxygen consumption and workload were lower among patients than HCs both at AT and at termination of the VO(2max) test (p < 0.011 for all comparisons). Two patients (18%) and nine HCs (75%) reached VO(2max) criteria. The relationship between metabolic rate and workload did not differ between groups at exercise below AT. At exercise above AT, the metabolic rate increased disproportionally to workload in the patients. Although the patients had a higher anaerobic contribution to the total metabolic rate at the end of the submaximal test, the anaerobic contribution at the end of the maximal test did not differ between groups. HR responses were largely similar between groups. CONCLUSIONS: The current study indicates that patients with FM have similar metabolic and cardiovascular responses to submaximal exercise as HCs. However, these patients have reduced ability to reach VO(2max) and a possible deficit in the metabolic system when exercising above the AT.

Effects of frequency on gross efficiency and performance in roller ski skating.

The purpose of the present study was to examine the effect of frequency on efficiency and performance during G3 roller ski skating. Eight well-trained male cross-country skiers performed three submaximal 5-min speeds (10, 13, and 16 km/h) and a time-to-exhaustion (TTE) performance (at 20 km/h) using the G3 skating technique using freely chosen, high, and low frequency at all four speeds. All tests were done using roller skis on a large treadmill at 5% incline. Gross efficiency (GE) was calculated as power divided by metabolic rate. Power was calculated as the sum of power against frictional forces and power against gravity. Metabolic rate was calculated from oxygen consumption and blood lactate concentration. Freely chosen frequency increased from 60 to 70 strokes/min as speed increased from 10 to 20 km/h. GE increased with power. At high power (20 km/h performance test), both efficiency and performance were significantly reduced by high frequency. In regard to choice of frequency during G3 roller ski skating, cross-country skiers seems to be self-optimized both in relation to energy saving (efficiency) and performance (TTE).

Aerodynamic drag is not the major determinant of performance during giant slalom skiing at the elite level.

This investigation was designed to (a) develop an individualized mechanical model for measuring aerodynamic drag (F(d) ) while ski racing through multiple gates, (b) estimate energy dissipation (E(d) ) caused by F(d) and compare this to the total energy loss (E(t) ), and (c) investigate the relative contribution of E(d) /E(t) to performance during giant slalom skiing (GS). Nine elite skiers were monitored in different positions and with different wind velocities in a wind tunnel, as well as during GS and straight downhill skiing employing a Global Navigation Satellite System. On the basis of the wind tunnel measurements, a linear regression model of drag coefficient multiplied by cross-sectional area as a function of shoulder height was established for each skier (r > 0.94, all P < 0.001). Skiing velocity, F(d) , E(t) , and E(d) per GS turn were 15-21 m/s, 20-60 N, -11 to -5 kJ, and -2.3 to -0.5 kJ, respectively. E(d) /E(t) ranged from ∼5% to 28% and the relationship between E(t) /v(in) and E(d) was r = -0.12 (all NS). In conclusion, (a) F(d) during alpine skiing was calculated by mechanical modeling, (b) E(d) made a relatively small contribution to E(t) , and (c) higher relative E(d) was correlated to better performance in elite GS skiers, suggesting that reducing ski-snow friction can improve this performance.

The physiology of world-class sprint skiers.

The present study investigated the physiological characteristics of eight world-class (WC) and eight national-class (NC) Norwegian sprint cross country skiers. To measure the physiological response and treadmill performance, the skiers performed a submaximal test, a peak aerobic capacity (VO2peak) test, and a peak treadmill speed (V(peak)) test in the skating G3 technique. Moreover, the skiers were tested for G3 acceleration outdoors on asphalt and maximal strength in the lab. The standard of sprint skating performance level on snow was determined by International Ski Federation points, and the training distribution was quantified. WC skiers showed 8% higher VO2peak and twice as long a VO(2) plateau time at the VO2peak test, and a higher gross efficiency at the submaximal test (all P<0.05). Furthermore, WC skiers showed 8% higher V(peak) (P<0.05), but did not differ from NC skiers in acceleration and maximal strength. WC skiers performed more low- and moderate-intensity endurance training and speed training (both P<0.05). The current results show that aerobic capacity, efficiency, and high speed capacity differentiate WC and NC sprint skiers and it is suggested that these variables determine sprint skiing performance.

Effect of physical fatigue on motor control at different skill levels.

The purpose of this experiment was to explore the effect of fatigue on motor coordination, and of prospective adjustment strategies to compensate for fatigue in a multijoint movement. Two male groups (N = 8) participated in the experiment: Highly skilled table tennis players (M age = 27 yr., SD = 2.3, n = 4) and Recreational table tennis players (M age = 25.9 yr., SD = 0.04, n = 4). The task was an attacking forehand drive towards a scaled target on the opposite side of the net. The Highly skilled players adjusted their movement patterns and preserved the task requirements in terms of spatial accuracy under the condition of fatigue by using opportunistic movement coordination. The Recreational players did not adjust their forehand drive, and spatial accuracy deteriorated. The current results support the notion that expertise enhances potential to adjust motor coordination strategies as a reaction to induced physical fatigue.

Coordination specificity in strength and power training.

In this study, we tested the hypothesis that specificity of inter-joint coordination in power training improves training effects. We compared two different training regimes, one with and one without the possibility to exploit the coordination between knee and ankle, on performance and coordination in maximal vertical jumping and explosive squat movements. 22 subjects were divided into two groups for a 5-week training study. One group (Tsingle) trained squats (SQ) and plantar flexions (PL) in separate activities and the other group (Tmulti) trained squats ending with plantar flexion in one movement (SQPL), three times a week. Both groups increased their peak power during training movements between 2 - 15 % (depending on the training movement) but there were no group effects. There were no effects on vertical jumping performance. However, our data indicate different coordinative changes between groups in the vertical jump after the training period. The group specific training movements resulted in an increased power potential that is shown in the training movements themselves but did not transfer to an increased vertical jump performance in either group. However, some training movement specific coordination effects were seen during vertical jumping. In this study, these coordinative changes are specific to the training groups and may be a forerunner to improvements in vertical jumping.

Effects of contraction history on control and stability in explosive actions.

In this model study, the effect of contraction history in skeletal muscle on joint movement and stability was investigated. A joint system was constructed with two identical (antagonistic) muscles. The muscles were modelled either according to the Hill tradition or as a modified Hill system including history-dependent properties. The joint system underwent movements with full activity of both muscles, mimicking explosive actions with high stability demands. Movements starting away from a balanced mid-position, as well as perturbation experiments, were simulated. The comparison between the Hill and modified Hill systems showed that contraction history improved stability (stiffness under perturbation) and, under certain conditions, caused a shift in the final joint position, which depended on the task characteristics (starting position and perturbations characteristics). This result indicates that modulations of muscle activity, required to move a joint to a particular end-position, do not only depend on the end-position but also on the preceding movements. This finding does not agree with the equilibrium-point hypothesis and is discussed accordingly.

Muscle efficiency: the controversial role of elasticity and mechanical energy conversion in stretch-shortening cycles.

The analysis of muscle efficiency was performed on a variety of simulated muscle stretch-shortening cycles of in situ rat gastrocnemius muscle. The processes of biochemical energy conversions (phosphorylation and contraction-coupling) and mechanical conversions (internal work to external work) were incorporated in the efficiency calculations. Metabolic cost was determined using a simple linear model. Special attention was drawn to the interacting roles of series elastic compliance and contraction dynamics. The results showed that series elastic compliance affected the efficiency of muscle contraction to a great extent. Stiff muscle was well designed to perform efficient contractions in which muscle merely shortened while active. Compliant muscle performed best in true stretch-shortening contractions utilising the storage and release of series elastic energy effectively. However, both stiff and compliant series elastic elements showed similar optimal muscle efficiency values in shortening contractions and stretch-shortening contractions, respectively. The findings indicate that the storage and re-utilisation of series elastic energy does not enhance overall muscle efficiency, but that optimal efficiency is obtained by a proper design of the muscle with regard to the dynamics of the movement task. Furthermore, it was found that although biochemical efficiency determined the feasible range of muscle efficiency, mechanical work conversions had the strongest influence on the exact value of overall muscle efficiency in stretch-shortening contractions.

Muscle contraction history: modified Hill versus an exponential decay model.

In recent years, it has been recognised that improvements to classic models of muscle mechanical behaviour are often necessary for properly modelling coordinated multi-joint actions. In this respect, the purpose of the present study was to improve on modelling stretch-induced force enhancement and shortening-induced force depression of muscle contraction. For this purpose, two models were used: a modified Hill model and a model based loosely on mechano-chemistry of the cross-bridge cycle (exponential decay model). The models were compared with a classic Hill modeland experimental data. Parameter values were based, as much as possible, on experimental findings in the literature, and tested with new experiments on the gastrocnemius of the rat. Both models describe many features of slow-ramp movements well during short contractions (300-500 ms), but long-duration behaviour is described only partly. The exponential decay model does not incorporate a force-velocity curve. Therefore, its good performance indicates that the status ofthe classic force-velocity characteristic may have to be reconsidered. Like movement-induced force depression and enhancement, it seems a particular manifestation of time-dependent force behaviour of muscle, rather than a fundamental property of muscle (like the length-tension curve). It is argued that a combination of the exponential decay model (or other models based on the mechano-chemistry of contraction) and structurally based models may be fruitful in explaining this time-dependent contraction behaviour. Furthermore, not in the least because of its relative simplicity, the exponential decay model may prove more suitable for modelling multi-joint movements than the Hill model.

A new method to measure elastic properties of plastic-viscoelastic connective tissue.

An experimental protocol was tested to measure elastic properties of connective tissue displaying viscoelastic as well as plastic properties. The protocol consisted of a slow rate, linear elongation (0.88 mms(-1), 8 mm) in combination with a superimposed sinusoidal vibration of small amplitude (50 Hz, 0.1 mm). Using digital filters and mathematical algorithms, the force responses to linear elongation and to vibration were obtained. The method was tested on excised fibromuscular tissue of the vaginal wall obtained from women who suffered a vaginal prolapse. The force-stiffness and force-elongation relationships based on the vibration response were unaffected by any long-term deformation of the specimens. The directly measured force-elongation curves were strongly affected by these deformations. It was therefore concluded that with the new method, it is possible to determine the elastic properties accurately. Furthermore, this method seems more sensitive to small changes in elastic properties than the classic tensile test.

Stretch-shorten cycle compared with isometric preload: contributions to enhanced muscular performance.

To isolate any difference muscular contraction history may have on concentric work output, 40 trained male subjects performed three separate isokinetic concentric squats that involved differing contraction histories, 1) a concentric-only (CO) squat, 2) a concentric squat preceded by an isometric preload (IS), and 3) a stretch-shorten cycle (SSC) squat. Over the first 300 ms of the concentric movement, work output for both the SSC and IS conditions was significantly greater (154.8 +/- 39.8 and 147.9 +/- 34.7 J, respectively; P < 0.001) compared with the CO squat (129.7 +/- 34.4 J). In addition, work output after the SSC test over the first 300 ms was also significantly larger than that for the corresponding period after the IS protocol (P < 0.05). There was no difference in normalized, integrated electromyogram among any of the conditions. It was concluded that concentric performance enhancement derived from a preceding stretch of the muscle-tendon complex was largely due to the attainment of a higher active muscle state before the start of the concentric movement. However, it was also hypothesized that contractile element potentiation was a significant contributor to stretch-induced muscular performance under these conditions.

Mechanical behaviour of rat skeletal muscle during fatiguing stretch-shortening cycles.

The effects of muscle fatigue on the mechanical efficiency of muscle work performance were studied in situ, in stretch-shortening cycles that resembled physiological conditions. The experiments were performed on the medial gastrocnemius muscles of seven rats. To calculate efficiency, a simple Hill-type model was used, consisting of the contractile component (CC) and the series elastic component (SEC). Mechanical efficiency was defined as the external work done by the muscle-tendon complex divided by the external work done upon the muscle-tendon complex plus work done by the CC. The fatiguing protocol consisted of a l Hz stretch-shortening cycle with a 0.3 duty factor (i.e. the muscle was stimulated for 300 ms in each cycle). In total, 240 cycles were imposed. The mechanical performance (work, force and mechanical efficiency) was determined throughout the fatiguing process. Fatigue reduced muscle power and force production to approximately 17 and 30%, respectively. SEC stiffness increased, but did not have any effect on muscle performance and probably reflects altered cross-bridge characteristics. Mechanical efficiency was enhanced during the imposed stretch-shortening cycles from 0.74 to 0.83. As a result of the lower peak forces, proportionally less series elastic energy was wasted during the relaxation period of each strength-shortening cycle in the fully fatigued muscles. These results indicate that, in vivo, the timing of muscle contractions in the fresh and the fatigued state may be different in order to perform muscle work efficiently. After a period of 30 min recovery, the condition of the muscles had (partially) returned to the fresh state in all aspects.

Gastrocnemius muscle length in relation to knee and ankle joint angles: verification of a geometric model and some applications.

BACKGROUND: For understanding the relationship between skeletal muscle architecture and muscle function in vivo, the development of accurate geometric models relating muscle length to joint angles is crucial. Therefore, a geometric model of the calf of mammals was developed to predict the length of the gastrocnemius muscle-tendon complex from knee and ankle angles. METHODS: The model requires three skeletal length measurements (radius of femoral condyle, ankle lever, and tibia length) to predict muscle-tendon length. The model was tested on the hopping mouse (Notomys alexis) by comparing polynomial fittings with geometrical fits of muscle length-joint angle measurements (i.e., the equation of the geometric model was used for least square fitting of the data). The model was applied to the hopping mouse and the rat to study (in vivo) joint-angle-muscle length-force relationships. RESULTS: It appeared that small and, on average, statistically nonsignificant length adjustments of the skeletal lengths were needed for the geometrical fit. Combinations of joint angles that normally occur during locomotion yielded muscle lengths close to optimum (i.e., with the highest isometric force potential). CONCLUSIONS: By relying on the geometry of the animal's leg, the calculated moment arms of the model appeared more reliable than those calculated from the polynomial fit. It was concluded that the architecture regarding length-force properties of the gastrocnemius muscle in both hopping mouse and rat is well adapted for the locomotion patterns.

Mechanical efficiency and efficiency of storage and release of series elastic energy in skeletal muscle during stretch-shorten cycles.

The mechanical energy exchanges between components of a muscle-tendon complex, i.e. the contractile element (CE) and the series elastic element (SEE), and the environment during stretch-shorten cycles were examined. The efficiency of the storage and release of series elastic energy (SEE efficiency) and the overall mechanical efficiency of the rat gastrocnemius muscle (N = 5) were determined for a range of stretch-shorten contractions. SEE efficiency was defined as elastic energy released to the environment divided by external work done upon the muscle-tendon complex plus internal work exchange from the CE to the SEE. Mechanical efficiency is external work done by the muscle-tendon complex divided by the external work done upon the muscle-tendon complex plus work done by the CE. All stretch-shorten cycles were performed with a movement amplitude of 3mm (6.7% strain). Cycle frequency, duty factor and the onset of stimulation were altered for the different cycles. SEE efficiency varied from 0.02 to 0.85, mechanical efficiency from 0.43 t 0.92. SEE efficiency depended on the timing of stimulation and net muscle power in different ways. Mechanical efficiency was much more closely correlated with net power. The timing of muscle relaxation was crucial for the effective release of elastic energy. Simulated in vivo contractions indicated that during rat locomotion the gastrocnemius may have a role other than that of effectively storing elastic energy and generating work. Computer simulations showed that the amount of series elastic compliance can affect the internal energetics of a muscle contraction strongly without changing the muscle force generation dramatically.

Contractile behaviour in skeletal muscle-tendon unit during small amplitude sine wave perturbations.

The effects of sinusoidal vibrations (0.2 mm) on contractile behaviour of the medial gastrocnemius muscle of the rat (N = 6) were determined over a wide range of frequencies (10-210 Hz). The performance of the contractile element (CE) during maximal tetanic contractions and vibrations was calculated by correcting muscle performance for series elasticity. For one experimental muscle the contractions were simulated using a computer model based on mechanical characteristics of that particular muscle. It was found that 10 Hz movements increased the slope of the CE force-velocity curve, compared to its isokinetically determined characteristics. At higher frequencies (30 Hz and above) this slope decreased. It is hypothesised that these two changes in CE behaviour are based on the same phenomena in CE behaviour: force enhancement by active stretch and depression by shortening. The time constants of the decay of these processes may cause the different impact on CE force-velocity behaviour. It is concluded that CE performance is affected at all frequencies, but its impact on muscle-tendon performance shows at low frequencies only. At high-frequencies series elastic characteristics play a dominant role.