Characterization of the in vivo transient responses of the femoral cartilage by means of quantitative ultrasound imaging techniques.

BACKGROUND: Quantitative ultrasound (QUS) techniques are new diagnostic tools able to identify changes in structural and material properties of the investigated tissue. For the first time, we evaluated the capability of QUS techniques in determining the in vivo transient changes in knee joint cartilage after a stressful task. METHODS: An ultrasound scanner collecting B-mode and radiofrequency data simultaneously was used to collect data from the femoral cartilage of the right knee in 15 participants. Cartilage thickness (CTK), ultrasound roughness index (URI), average magnitude ratio (AMR), and Nakagami parameters (NA) were evaluated before, immediately after and every 5 min up to 45 min a stressful task (30 min of running on a treadmill with a negative slope of 5%). RESULTS: CTK was affected by time (main effect: p < 0.001). Post hoc test showed significant differences with CTK at rest, which were observed up to 30 min after the run. AMR and NA were affected by time (p < 0.01 for both variables), while URI was unaffected by it. For AMR, post hoc test showed significant differences with rest values in the first 35 min of recovery, while NA was increased compared to rest values in all time points. CONCLUSION: Data suggest that a single running trial is not able to modify the integrity of the femoral cartilage, as reported by URI data. In vivo evaluation of QUS parameters of the femoral cartilage (NA, AMR, and URI) are able to characterize changes in cartilage properties over time.

Mechanical and metabolic power in accelerated running-Part II: team sports.

PURPOSE: This manuscript is devoted to discuss the interplay between velocity and acceleration in setting metabolic and mechanical power in team sports. METHODS: To this aim, an essential step is to assess the individual Acceleration-Speed Profile (ASP) by appropriately analysing training sessions or matches. This allows one to estimate maximal mechanical and metabolic power, including that for running at constant speed, and hence to determine individual thresholds thereof. RESULTS: Several approaches are described and the results, as obtained from 38 official matches of one team (Italian Serie B, season 2020-2021), are reported and discussed. The number of events in which the external mechanical power exceeded 80% of that estimated from the subject's ASP ([Formula: see text]) was 1.61 times larger than the number of accelerations above 2.5 m s-2 ([Formula: see text]). The difference was largest for midfielders and smallest for attackers (2.30 and 1.36 times, respectively) due to (i) a higher starting velocity for midfielders and (ii) a higher external peak power for attackers in performing [Formula: see text]. From the energetic perspective, the duration and the corresponding metabolic power of high-demanding phases ([Formula: see text]) were essentially constant (6 s and 22 W  kg-1, respectively) from the beginning to the end of the match, even if their number decreased from 28 in the first to 21 in the last 15-min period, as a consequence of the increased recovery time between [Formula: see text] from 26 s in the first to 37 s in the last 15-min period. CONCLUSION: These data underline the flaws of acceleration counting above fixed thresholds.

Estimating Active Drag Based on Full and Semi-Tethered Swimming Tests.

During full tethered swimming no hydrodynamic resistance is generated (since v = 0) and all the swimmer's propulsive force (FP) is utilized to exert force on the tether (FT = FP). During semi-tethered swimming FP can be made useful to one of two ends: exerting force on the tether (FST) or overcoming drag in the water (active drag: Da). At constant stroke rate, the mean propulsive force (FP) is constant and the quantity FP - FST (the "residual thrust") corresponds to Da. In this study we explored the possibility to estimate Da based on this method ("residual thrust method") and we compared these values with passive drag values (Dp) and with values of active drag estimated by means of the "planimetric method". Based on data obtained from resisted swimming (full and semi-tethered tests at 100% and 35, 50, 60, 75, 85% of the individual FT), active drag was calculated as: DaST = kaST.vST2 = FP - FST ("residual thrust method"). Passive drag (Dp) was calculated based on data obtained from passive towing tests and active drag ("planimetric method") was estimated as: DaPL = Dp.1.5. Speed-specific drag (k = D/v2) in passive conditions (kp) was )25 kg.m-1 and in active conditions (ka) )38 kg.m-1 (with either method); thus, DaST > Dp and DaST > DaPL. In human swimming active drag is, thus, about 1.5 times larger than passive drag. These experiments can be conducted in an ecological setting (in the swimming pool) by using basic instrumentation and a simple set of calculations.

Impairments in muscle shape changes affect metabolic demands during in-vivo contractions.

The uncoupling behaviour between muscle belly and fascicle shortening velocity (i.e. belly gearing), affects mechanical output by allowing the muscle to circumvent the limits imposed by the fascicles' force-velocity relationship. However, little is known about the 'metabolic effect' of a decrease/increase in belly gearing. In this study, we manipulated the plantar flexor muscles' capacity to change in shape (and hence belly gearing) by using compressive multidirectional loads. Metabolic, kinetic, electromyography activity and ultrasound data (in soleus and gastrocnemius medialis) were recorded during cyclic fixed-end contractions of the plantar flexor muscles in three different conditions: no load, +5 kg and +10 kg of compression. No differences were observed in mechanical power and electrophysiological variables as a function of compression intensity, whereas metabolic power increased as a function of it. At each compression intensity, differences in efficiency were observed when calculated based on fascicle or muscle behaviour and significant positive correlations (R2 range: 0.7-0.8 and p > 0.001) were observed between delta efficiency (ΔEff: Effmus-Efffas) and belly gearing (Vmus/Vfas) or ΔV (Vmus-Vfas). Thus, changes in the muscles' capacity to change in shape (e.g. in muscle stiffness or owing to compressive garments) affect the metabolic demands and the efficiency of muscle contraction.

The interplay between gastrocnemius medialis force-length and force-velocity potentials, cumulative EMG activity and energy cost at speeds above and below the walk to run transition speed.

NEW FINDINGS: What is the central question of the study? Are the changes in force potentials (at the muscle level) related with metabolic changes at speeds above and below the walk-to-run transition? What is the main finding and its importance? The force-length and force-velocity potentials of gastrocnemius medialis during human walking decrease as a function of speed; this decrease is associated with an increase in cumulative EMG activity and in the energy cost of locomotion. Switching from fast walking to running is associated to an increase in the force potentials, supporting the idea that the 'metabolic trigger' that determines the transition from walking to running is ultimately driven by a reduction of the muscle's contractile capacity. ABSTRACT: The aim of this study was to investigate the interplay between the force-length (F-L) and force-velocity (F-V) potentials of gastrocnemius medialis (GM) muscle fascicles, the cumulative muscle activity per distance travelled (CMAPD) of the lower limb muscles (GM, vastus lateralis, biceps femori, tibialis anterior) and net energy cost (Cnet ) during walking and running at speeds above and below the walk-to-run transition speed (walking: 2-8 km h-1 ; running: 6-10 km h-1 ). A strong association was observed between Cnet and CMAPD: both changed significantly with walking speed but were unaffected by speed in running. The F-L and F-V potentials decreased with speed in both gaits and, at 6-8 km h-1 , were significantly larger in running. At low to moderate walking speeds (2-6 km h-1 ), the changes in GM force potentials were not associated with substantial changes in CMAPD (and Cnet ), whereas at walking speeds of 7-8 km h-1 , even small changes in force potentials were associated with steep increases in CMAPD (and Cnet ). These data suggest that: (i) the walk to run transition could be explained by an abrupt increase in Cnet driven by an upregulation of the EMG activity (e.g., in CMAPD) at sustained walking speeds (>7 km h-1 ) and (ii) the reduction in the muscle's ability to produce force (e.g., in the F-L and F-V potentials) contributes to the increase in CMAPD (and Cnet ). Switching to running allows regaining of high force potentials, thus limiting the increase in CMAPD (and Cnet ) that would otherwise occur to sustain the increase in locomotion speed.

Mechanical and Metabolic Power in Accelerated Running-PART I: the 100-m dash.

PURPOSE: Acceleration phases require additional mechanical and metabolic power, over and above that for running at constant velocity. The present study is devoted to a paradigmatic example: the 100-m dash, in which case the forward acceleration is very high initially and decreases progressively to become negligible during the central and final phases. METHODS: The mechanical ([Formula: see text]) and metabolic ([Formula: see text]) power were analysed for both Bolt's extant world record and for medium level sprinters. RESULTS: In the case of Bolt, [Formula: see text] and [Formula: see text] attain peaks of ≈ 35 and ≈ 140 W kg-1 after ≈ 1 s, when the velocity is ≈ 5.5 m s-1; they decrease substantially thereafter, to attain constant values equal to those required for running at constant speed (≈ 18 and ≈ 65 W kg-1) after ≈ 6 s, when the velocity has reached its maximum (≈ 12 m s-1) and the acceleration is nil. At variance with [Formula: see text], the power required to move the limbs in respect to the centre of mass (internal power, [Formula: see text]) increases gradually to reach, after ≈ 6 s a constant value of ≈ 33 W kg-1. As a consequence, [Formula: see text] ([Formula: see text]) increases throughout the run to a constant value of ≈ 50 W kg-1. In the case of the medium level sprinters, the general patterns of speed, mechanical and metabolic power, neglecting the corresponding absolute values, follow an essentially equal trend. CONCLUSION: Hence, whereas in the last part of the run the velocity is about twice that observed after ≈ 1 s, [Formula: see text] and [Formula: see text] are reduced to 45-50% of the peak values.

Energetics (and Mechanical Determinants) of Sprint and Shuttle Running.

Unsteady locomotion (e. g., sprints and shuttle runs) requires additional metabolic (and mechanical) energy compared to running at constant speed. In addition, sprints or shuttle runs with relevant speed changes (e. g., with large accelerations and/or decelerations) are typically short in duration and, thus, anaerobic energy sources must be taken into account when computing energy expenditure. In sprint running there is an additional problem due to the objective difficulty in separating the acceleration phase from a (necessary and subsequent) deceleration phase.In this review the studies that report data of energy expenditure during sprints and shuttles (estimated or actually calculated) will be summarized and compared. Furthermore, the (mechanical) determinants of metabolic energy expenditure will be discussed, with a focus on the analogies with and differences from the energetics/mechanics of constant-speed linear running.

Influence of muscle-belly and tendon gearing on the energy cost of human walking.

This study combines metabolic and kinematic measurements at the whole-body level, with EMG and ultrasound measurements to investigate the influence of muscle-tendon mechanical behavior on the energy cost (Cnet ) of walking (from 2 to 8 km·h-1 ). Belly gearing (Gb = Δmuscle-belly length/Δfascicles length) and tendon gearing (Gt = ∆muscle-tendon unit length/∆muscle-belly length) of vastus lateralis (VL) and gastrocnemius medialis (GM) were calculated based on ultrasound data. Pendular energy recovery (%R) was calculated based on kinematic data, whereas the cumulative activity per distance travelled (CMAPD) was calculated for the VL, GM, tibialis anterior, and biceps femoris as the ratio between their EMG activity and walking speed. Finally, total CAMPD (CMAPDTOT ) was calculated as the sum of the CMAPD of all the investigate muscles. Cnet and CMAPDTOT showed a U-shaped behavior with a minimum at 4.2 and 4.1 km·h-1 , respectively; while %R, VL, and GM belly gearing showed an opposite trend, reaching a maximum (60% ± 5%, 1.1 ± 0.1 and 1.5 ± 0.1, respectively), between 4.7 and 5 km·h-1 . Gt was unaffected by speed in GM (3.5 ± 0.1) and decreased as a function of it in VL. A multiple stepwise linear regression indicated that %R has the greatest influence on Cnet, followed by CMAPDTOT and GM belly gearing. The role of Gb on Cnet could be attributed to its role in determining muscle work: when Gb increases, fascicles shortening decreases compared with that of the muscle-belly, thereby reducing the energy cost of contraction.

Mechanical work as a (key) determinant of energy cost in human locomotion: recent findings and future directions.

NEW FINDINGS: What is the topic of this review? This narrative review explores past and recent findings on the mechanical determinants of energy cost during human locomotion, obtained by using a mechanical approach based on König's theorem (Fenn's approach). What advances does it highlight? Developments in analytical methods and their applications allow a better understanding of the mechanical-bioenergetic interaction. Recent advances include the determination of 'frictional' internal work; the association between tendon work and apparent efficiency; a better understanding of the role of energy recovery and internal work in pathological gait (amputees, stroke and obesity); and a comprehensive analysis of human locomotion in (simulated) low gravity conditions. ABSTRACT: During locomotion, muscles use metabolic energy to produce mechanical work (in a more or less efficient way), and energetics and mechanics can be considered as two sides of the same coin, the latter being investigated to understand the former. A mechanical approach based on König's theorem (Fenn's approach) has proved to be a useful tool to elucidate the determinants of the energy cost of locomotion (e.g., the pendulum-like model of walking and the bouncing model of running) and has resulted in many advances in this field. During the past 60 years, this approach has been refined and applied to explore the determinants of energy cost and efficiency in a variety of conditions (e.g., low gravity, unsteady speed). This narrative review aims to summarize current knowledge of the role that mechanical work has played in our understanding of energy cost to date, and to underline how recent developments in analytical methods and their applications in specific locomotion modalities (on a gradient, at low gravity and in unsteady conditions) and in pathological gaits (asymmetric gait pathologies, obese subjects and in the elderly) could continue to push this understanding further. The recent in vivo quantification of new aspects that should be included in the assessment of mechanical work (e.g., frictional internal work and elastic contribution) deserves future research that would improve our knowledge of the mechanical-bioenergetic interaction during human locomotion, as well as in sport science and space exploration.

The influence of in vivo mechanical behaviour of the Achilles tendon on the mechanics, energetics and apparent efficiency of bouncing gaits.

In this study, we used kinematic, kinetic, metabolic and ultrasound analysis to investigate the role of elastic energy utilization on the mechanical and physiological demands of a movement task (hopping) that primarily involves the plantar-flexor muscles to determine the contribution of tendon work to total mechanical work and its relationship with apparent efficiency (AE) in bouncing gaits. Metabolic power (PMET) and (positive) mechanical power at the whole-body level (PMEC) were measured during hopping at different frequencies (2, 2.5, 3 and 3.5 Hz). The (positive) mechanical power produced during the Achilles tendon recoil phase (PTEN) was obtained by integrating ultrasound data with an inverse dynamic approach. As a function of hopping frequency, PMEC decreased steadily and PMET exhibited a U-shape behaviour, with a minimum at about 3 Hz. AE (PMEC/PMET) showed an opposite trend and was maximal (about 0.50) at the same frequency when PTEN was also highest. Positive correlations were observed: (i) between PTEN and AE (AE=0.22+0.15PTEN, R2=0.67, P<0.001) and the intercept of this relationship indicates the value of AE that should be expected when tendon work is nil; (ii) between AE and tendon gearing (Gt=Δmuscle-tendon unit length/Δmuscle belly length; R2=0.50, P<0.001), where a high Gt indicates that the muscle is contracting more isometrically, thus allowing the movement to be more economical (and efficient); (iii) between Gt and PTEN (R2=0.73, P<0.001), which indicates that Gt could play an important role in the tendon's capability to store and release mechanical power.

Cervical Spine Motion During Vehicle Extrication of Healthy Volunteers.

Objective: Prehospital spinal motion restriction as a prevention technique for secondary neurological injury is a key principle in emergency medicine. Our aim was to evaluate the effectiveness of different cervical spinal cord motion restriction techniques of awake and cooperative healthy volunteers during extrication.Methods: Twenty-three healthy volunteers were asked to exit a car (unassisted) with a rigid cervical collar (CC condition) or without it (autonomous exit: AE; instructed exit: IE); they were also extricated by two rescuers after setting a rigid cervical collar and by using an extrication device (CC + XT condition). Eight 3 D infrared cameras were calibrated around the vehicle to measure cervical spine angle, angular speed and acceleration in the sagittal plane. Surface wireless EMG electrodes were used to record superior trapezius, erector spinae and rectus abdominis muscle activity. All measures were recorded during two phases: device positioning (maneuver) and vehicle exiting.Results: The lowest range of motion was observed in CC during maneuver and exit (about 17°), the greatest in AE and IE (about 45°); when the extrication device was utilized along with the cervical collar (CC + XT) an increase, rather than a further decrease, in the range of motion was observed (about 25° during maneuver and exit). Larger values of angular speed and acceleration were observed in CC + XT when compared to CC, both during maneuver and exit (p < 0.001). The lowest EMG activity was observed during maneuver in CC and CC + XT; during exit a lower EMG activity was observed in CC + XT compared to CC (p < 0.001). Thus, when an extrication device is utilized (CC + XT), a lower active control of the cervical spine region is associated with faster and more brisk movements of the cervical spine compared to CC alone.Conclusions: Our findings support the idea that spinal motion restriction via rigid cervical collar of awake and cooperative trauma patients is effective in reducing cervical spine motion in the sagittal plane during vehicle extrication.

Gastrocnemius Medialis and Vastus Lateralis in vivo muscle-tendon behavior during running at increasing speeds.

This study combines in vivo ultrasound measurements of the Vastus Lateralis (VL) and Gastrocnemius Medialis (GM) muscles with electromyographic, kinematic, and kinetic measurements during treadmill running at different speeds (10, 13, and 16 km/h) to better understand the role of muscle and tendon behavior in two functionally different muscle-tendon units. In addition, the force-length and force-velocity relationships of VL and GM were experimentally assessed by combining dynamometry and EMG data with ultrasound measurements. With increasing running speed, the operating length of the fascicles in the stance phase shifted toward smaller lengths in the GM (P < .05; moving down the ascending limb of the F-L relationship) and longer lengths in the VL (P < .05; moving down the descending limb) at all speeds; however, both muscles contracted close to their optimal length L0 , where isometric force is maximal. Whereas the length of VL SEE did not change as a function of speed, GM SEE lengthened and shortened more at higher speeds. With increasing running speed, the contribution of elastic strain energy to the positive power generated by the MTU increased more for GM (from 0.75 to 1.56 W/kg) than for VL (from 0.62 to 1.02 W/kg). Notwithstanding these differences, these results indicate that, at increasing running speeds, both the VL and GM muscles produce high forces at low contraction velocities, and that the primary function of both muscle-tendon units is to enhance the storage and recovery of elastic strain energy.

The energy cost of swimming and its determinants.

The energy expended to transport the body over a given distance (C, the energy cost) increases with speed both on land and in water. At any given speed, C is lower on land (e.g., running or cycling) than in water (e.g., swimming or kayaking) and this difference can be easily understood when one considers that energy should be expended (among the others) to overcome resistive forces since these, at any given speed, are far larger in water (hydrodynamic resistance, drag) than on land (aerodynamic resistance). Another reason for the differences in C between water and land locomotion is the lower capability to exert useful forces in water than on land (e.g., a lower propelling efficiency in the former case). These two parameters (drag and efficiency) not only can explain the differences in C between land and water locomotion but can also explain the differences in C within a given form of locomotion (swimming at the surface, which is the topic of this review): e.g., differences between strokes or between swimmers of different age, sex, and technical level. In this review, the determinants of C (drag and efficiency, as well as energy expenditure in its aerobic and anaerobic components) will, thus, be described and discussed. In aquatic locomotion it is difficult to obtain quantitative measures of drag and efficiency and only a comprehensive (biophysical) approach could allow to understand which estimates are "reasonable" and which are not. Examples of these calculations are also reported and discussed.

The influence of Achilles tendon mechanical behaviour on "apparent" efficiency during running at different speeds.

PURPOSE: We investigated the role of elastic strain energy on the "apparent" efficiency of locomotion (AE), a parameter that is known to increase as a function of running speed (up to 0.5-0.7) well above the values of "pure" muscle efficiency (about 0.25-0.30). METHODS: In vivo ultrasound measurements of the gastrocnemius medialis (GM) muscle-tendon unit (MTU) were combined with kinematic, kinetic and metabolic measurements to investigate the possible influence of the Achilles tendon mechanical behaviour on the mechanics (total mechanical work, WTOT) and energetics (net energy cost, Cnet) of running at different speeds (10, 13 and 16 km h-1); AE was calculated as WTOT/Cnet. RESULTS: GM fascicles shortened during the entire stance phase, the more so the higher the speed, but the majority of the MTU displacement was accommodated by the Achilles tendon. Tendon strain and recoil increased as a function of running speed (P < 0.01 and P < 0.001, respectively). The contribution of elastic energy to the positive work generated by the MTU also increased with speed (from 0.09 to 0.16 J kg-1 m-1). Significant negative correlations (P < 0.01) were observed between tendon work and metabolic energy at each running speed (the higher the tendon work the lower the metabolic demand) and significant positive correlations were observed between tendon work and AE (P < 0.001) at each running speed (the higher the tendon work the higher the efficiency). CONCLUSION: These results support the notion that the dynamic function of tendons is integral in reducing energy expenditure and increasing the "apparent" efficiency of running.

Comprehensive mechanical power analysis in sprint running acceleration.

Sprint running is a common feature of many sport activities. The ability of an athlete to cover a distance in the shortest time relies on his/her power production. The aim of this study was to provide an exhaustive description of the mechanical determinants of power output in sprint running acceleration and to check whether a predictive equation for internal power designed for steady locomotion is applicable to sprint running acceleration. Eighteen subjects performed two 20 m sprints in a gym. A 35-camera motion capture system recorded the 3D motion of the body segments and the body center of mass (BCoM) trajectory was computed. The mechanical power to accelerate and rise BCoM (external power, Pext ) and to accelerate the segments with respect to BCoM (internal power, Pint ) was calculated. In a 20 m sprint, the power to accelerate the body forward accounts for 50% of total power; Pint accounts for 41% and the power to rise BCoM accounts for 9% of total power. All the components of total mechanical power increase linearly with mean sprint velocity. A published equation for Pint prediction in steady locomotion has been adapted (the compound factor q accounting for the limbs' inertia decreases as a function of the distance within the sprint, differently from steady locomotion) and is still able to predict experimental Pint in a 20 m sprint with a bias of 0.70 ± 0.93 W kg-1 . This equation can be used to include Pint also in other methods that estimate external horizontal power only.

Metabolic and kinematic responses while walking and running on a motorised and a curved non-motorised treadmill.

The purpose of this study was to assess metabolic and kinematic parameters (contact and flight time, step length and frequency) while walking at the preferred speed (1.44 ± 0.22 m · s-1) and while performing an incremental running test (up to exhaustion) on a motorised treadmill (MT) and on a curved non-motorised treadmill (CNMT). Twenty-five volunteers (24.1 ± 3.4 years; 64.7 ± 11.2 kg) participated in the study. Maximal running speed on MT was significantly larger (P < 0.001) than on CNMT (4.31 ± 0.50 vs. 3.75 ± 0.39 m · s-1) but no differences in heart rate or oxygen uptake (V˙O2) were observed at this speed. The energy cost of walking (Cw) and running (Cr) were significantly greater (P < 0.001) on CNMT than on MT (37 and 17%, respectively). No major differences in kinematic parameters were observed at paired, submaximal, running speeds (2.22-3.89 m · s-1) but V˙O2 was systematically larger in CNMT (of about 340 ml · min-1 · kg-1). This systematic difference can be expressed in terms of a larger "equivalent speed" on CNMT (of about 0.42 m · s-1) and should be attributed to factors other than the kinematic ones, such as the belt characteristics (e.g. friction, type of surface and curvature).

Mechanical power, thrust power and propelling efficiency: relationships with elite sprint swimming performance.

The purpose of this study was to explore the relationships between mechanical power, thrust power, propelling efficiency and sprint performance in elite swimmers. Mechanical power was measured in 12 elite sprint male swimmers: (1) in the laboratory, by using a whole-body swimming ergometer (W'TOT) and (2) in the pool, by measuring full tethered swimming force (FT) and maximal swimming velocity (Vmax): W'T = FT · Vmax. Propelling efficiency (ηP) was estimated based on the "paddle wheel model" at Vmax. Vmax was 2.17 ± 0.06 m · s-1, ηP was 0.39 ± 0.02, W'T was 374 ± 62 W and W'TOT was 941 ± 92 W. Vmax was better related to W'T (useful power output: R = 0.943, P < 0.001) than to W'TOT (total power output: R = 0.744, P < 0.01) and this confirms the use of the full tethered test as a valid test to assess power propulsion in sprinters and to estimate swimming performance. The ratio W'T/W'TOT (0.40 ± 0.04) represents the fraction of total mechanical power that can be utilised in water (e.g., ηP) and was indeed the same as that estimated based on the "paddle wheel model"; this supports the use of this model to estimate ηP in swimming.

Sprint running: how changes in step frequency affect running mechanics and leg spring behaviour at maximal speed.

The purpose of this study was to investigate the changes in selected biomechanical variables in 80-m maximal sprint runs while imposing changes in step frequency (SF) and to investigate if these adaptations differ based on gender and training level. A total of 40 athletes (10 elite men and 10 women, 10 intermediate men and 10 women) participated in this study; they were requested to perform 5 trials at maximal running speed (RS): at the self-selected frequency (SFs) and at SF ±15% and ±30%SFs. Contact time (CT) and flight time (FT) as well as step length (SL) decreased with increasing SF, while kvert increased with it. At SFs, kleg was the lowest (a 20% decrease at ±30%SFs), while RS was the largest (a 12% decrease at ±30%SFs). Only small changes (1.5%) in maximal vertical force (Fmax) were observed as a function of SF, but maximum leg spring compression (ΔL) was largest at SFs and decreased by about 25% at ±30%SFs. Significant differences in Fmax, Δy, kleg and kvert were observed as a function of skill and gender (P < 0.001). Our results indicate that RS is optimised at SFs and that, while kvert follows the changes in SF, kleg is lowest at SFs.

Mechanical work and efficiency of 5 + 5 m shuttle running.

PURPOSE: Acceleration and deceleration phases characterise shuttle running (SR) compared to constant speed running (CR); mechanical work is thus expected to be larger in the former compared to the latter, at the same average speed (v mean). The aim of this study was to measure total mechanical work (W tot (+) , J kg(-1) m(-1)) during SR as the sum of internal (W int (+) ) and external (W ext (+) ) work and to calculate the efficiency of SR. METHODS: Twenty males were requested to perform shuttle runs over a distance of 5 + 5 m at different speeds (slow, moderate and fast) to record kinematic data. Metabolic data were also recorded (at fast speed only) to calculate energy cost (C, J kg(-1) m(-1)) and mechanical efficiency (eff(+) = W tot (+) C (-1)) of SR. RESULTS: Work parameters significantly increased with speed (P < 0.001): W ext (+)  = 1.388 + 0.337 v mean; W int (+)  = -1.002 + 0.853 v mean; W tot (+)  = 1.329 v mean. At the fastest speed C was 27.4 ± 2.6 J kg(-1) m(-1) (i.e. about 7 times larger than in CR) and eff(+) was 16.2 ± 2.0 %. CONCLUSIONS: W ext (+) is larger in SR than in CR (2.5 vs. 1.4 J kg(-1) m(-1) in the range of investigated speeds: 2-3.5 m s(-1)) and W int (+) , at fast speed, is about half of W tot (+) . eff(+) is lower in SR (16 %) than in CR (50-60 % at comparable speeds) and this can be attributed to a lower elastic energy reutilization due to the acceleration/deceleration phases over this short shuttle distance.