Mechanical work in shuttle running as a function of speed and distance: Implications for power and efficiency

Biomechanics (and energetics) of human locomotion are generally studied at constant, linear, speed whereas less is known about running mechanics when velocity changes (because of accelerations, decelerations or changes of direction). The aim of this study was to calculate mechanical work and power and to estimate mechanical efficiency in shuttle runs (as an example of non-steady locomotion) executed at different speeds and over different distances. A motion capture system was utilised to record the movements of the body segments while 20 athletes performed shuttle runs (with a 180° change of direction) at three paces (slow, moderate and maximal) and over four distances (5, 10, 15 and 20 m). Based on these data the internal, external and total work of shuttle running were calculated as well as mechanical power; mechanical efficiency was then estimated based on values of energy cost reported in the literature. Total mechanical work was larger the faster the velocity and the shorter the distance covered (range: 2.3–3.7 J m⁻¹ kg⁻¹) whereas mechanical efficiency showed an opposite trend (range: 0.20–0.50). At maximal speed, over all distances, braking/negative power (about 21 W kg⁻¹) was twice the positive power. Present results highlight that running humans can exert a larger negative than positive power, in agreement with the fundamental proprieties of skeletal muscles in vivo. A greater relative importance of the constant speed phase, associated to a better exploitation of the elastic energy saving mechanism, is likely responsible of the higher efficiency at the longer shuttle distances.     

The effect of exercise intensity on joint power and dynamics in ergometer double-poling performed by cross-country skiers

The purpose of this study was to examine the effect of increasing exercise intensity on the role of joint powers in ergometer double poling (DP), while taking specific dynamic constraints into account. One main question was whether lower-body power contribution increased or decreased with increasing intensity. Nine male Norwegian national-level cross-country skiers performed ergometer DP at low, moderate, high and maximal intensity. Kinematics, and ground (GRF) and poling (F_poling) reaction forces were recorded and used in link segment modeling to obtain joint and whole-body dynamics. Joint powers were averaged over the cycle, the poling (PP) and recovery (RP) phases. The contribution of these average powers was their ratios to cycle average poling power. At all intensities, the shoulder (in PP) and hip (mostly in RP) generated most power. Averaged over the cycle, lower-body contribution (sum of ankle, knee and hip power) increased from ∼37% at low to ∼54% at maximal intensity (p < .001), originating mostly from increased hip contribution within PP, not RP. The generation of larger F_poling at higher intensities demanded a reversal of hip and knee moment. This was necessary to appropriately direct the GRF vector as required to balance the moment about center of mass generated by F_poling (control of angular momentum). This was reflected in that the hip changed from mostly absorbing to generating power in PP at lower and higher intensities, respectively. Our data indicate that power-transfer rather than stretch-shortening mechanisms may occur in/between the shoulder and elbow during PP. For the lower extremities, stretch-shortening mechanisms may occur in hip, knee and trunk extensors, ensuring energy conservation or force potentiation during the countermovement-like transition from body lowering to heightening. In DP locomotion, increasing intensity and power output is achieved by increased lower-body contribution. This is, at least in ergometer DP, partly due to changes in joint dynamics in how to handle dynamic constraints at different intensities.     

Effect of metastable Cr(Mo)s/Cr(Mo)a powders on mechanical alloying the immiscible Cu–Mo–Cr system

Metastable-state alloy powders are usually synthesised by milling and occur as products of mechanical alloying (MA), but they are rarely used as starting materials for other MA processes. In this work, metastable-state alloy powders, including supersaturated solid-solution Cr(Mo)s and amorphous Cr(Mo)a were prepared in advance. Then, the Cr(Mo)s/Cr(Mo)a powder mixture was mechanically alloyed with elemental Cu to form Cu–Mo–Cr alloy. The effects of the metastable-state powder Cr(Mo)s/Cr(Mo)a on MA of the immiscible Cu–Mo–Cr system were evaluated. Phases and microstructures of the milled powders were analysed by XRD and TEM, respectively. The results show that amorphous Cu-60wt.%Cr(Mo) and supersaturated solid-solution Cu-20wt.%Cr(Mo) alloy powders can be synthesised by MA. It is concluded that MA of the Cu–Mo–Cr ternary alloy system is significantly promoted when elemental Cu powder is milled with metastable-state alloy powder Cr(Mo)s/Cr(Mo)a. Furthermore, the promoting effect of amorphous Cr(Mo)a on MA the Cu–Mo–Cr alloy system is much greater than that of supersaturated solid-solution Cr(Mo)s, during the milling process.     

Renewal and resurgence phenomena generalize to Amazon's Mechanical Turk

Amazon's Mechanical Turk (MTurk) is a crowdsourcing platform that provides researchers with the potential for obtaining behavioral data for very little cost. However, the extent to which the results of common behavioral phenomena found in basic, translational, and applied laboratories may be reproduced (as a first step towards prospective research) via MTurk remains relatively unexplored. We evaluated renewal and resurgence arrangements using MTurk as the subject recruitment platform as a first step to determining the generality of the obtained data. Results suggested that MTurk participants produced renewal and resurgence data similar to those reported in basic, translational, and applied studies.     

Tool use as a way to assess cognition: how do captive chimpanzees handle the weight of the hammer when cracking a nut?

Tool use in apes has been considered a landmark in cognition. However, while most studies concentrate on mental operations, there are very few studies of apes’ cognition as expressed in manual skills. This paper proposes theoretical and methodological considerations on movement analysis as a way of assessing primate cognition. We argue that a privileged way of appraising the characteristics of the cognitive abilities involved in tool use lies at the functional level. This implies that we focus on how the action proceeds, and more precisely, on how the functional characteristics of the task are generated. To support our view, we present the results of an experiment with five captive chimpanzees investigating the way how chimpanzees adapt to hammers of various weights while cracking nuts. The movement performed in the hammering task is analyzed in terms of energy production. Results show that chimpanzees mobilise passive as well as active forces to perform the compliant movement, that is, they modulate the dynamics of the arm/tool system. A comparison between chimpanzees suggests that experience contributes to this skill. The results suggest that in tool use, movements are not key per se, but only in as much as they express underlying cognitive processes. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.

Examining the reliability and predictive validity of performance assessments by soccer coaches and scouts: The influence of structured collection and mechanical combination of information

Soccer coaches and scouts typically assess in-game soccer performance to predict players’ future performance. However, there is hardly any research on the reliability and predictive validity of coaches’ and scouts’ performance assessments, or on strategies they can use to optimize their predictions. In the current study, we examined whether robust principles from psychological research on selection – namely structured information collection and mechanical combination of predictor information through a decision-rule – improve soccer coaches’ and scouts’ performance assessments. A total of n = 96 soccer coaches and scouts participated in an elaborate within-subjects experiment. Participants watched soccer players’ performance on video, rated their performance in both a structured and unstructured manner, and combined their ratings in a holistic and mechanical way. We examined the inter-rater reliability of the ratings and assessed the predictive validity by relating the ratings to players’ future market values. Contrary to our expectations, we did not find that ratings based on structured assessment paired with mechanical combination of the ratings showed higher inter-rater reliability and predictive validity. In contrast, unstructured-holistic ratings yielded the highest reliability and predictive validity, although differences were marginal. Overall, reliability was poor and predictive validities small-to-moderate, regardless of the approach used to rate players’ performance. The findings provide insights into the difficulty of predicting future performance in soccer.