Contraction speed and type influences rapid utilisation of available muscle force:neural and contractile mechanisms

This study investigated the influence of contraction speed and type on the human ability to rapidly increase torque and utilise the available maximum voluntary torque (MVT) as well as the neuromuscular mechanisms underpinning any effects. Fifteen young, healthy males completed explosive-voluntary knee-extensions in five conditions: isometric (ISO), and both concentric and eccentric at two constant accelerations of 500°.s-2 (CONSLOW and ECCSLOW) and 2000°.s-2 (CONFAST and ECCFAST). Explosive torque and quadriceps EMG were recorded every 25 ms up to 150 ms from their respective onsets and normalised to the available MVT and EMG at MVT, respectively, specific to that joint angle and velocity. Neural efficacy (explosive Voluntary:Evoked octet torque) was also measured, and torque data were entered into a Hill-type muscle model to estimate muscle performance. Explosive torques normalised to MVT (and normalised muscle forces) were greatest in the concentric, followed by isometric, and eccentric conditions; and in the fast compared with slow speeds within the same contraction type (CONFAST>CONSLOW>ISO, and ECCFAST>ECCSLOW). Normalised explosive-phase EMG and neural efficacy were greatest in concentric, followed by isometric and eccentric conditions, but were similar for fast and slow contractions of the same type. Thus, distinct neuromuscular activation appeared to explain the effect of contraction type but not speed on normalised explosive torque, suggesting the speed effect is an intrinsic contractile property. These results provide novel evidence that the ability to rapidly increase torque/force and utilise the available MVT is influenced by both contraction type and speed, due to neural and contractile mechanisms, respectively

EFFECTS OF TAI CHI PRACTICE ON POSTURAL SWAY DURING STANDING BALANCE

The purpose of this study was to investigate the effects of Tai Chi practice on postural sway during standing. Thirty-eight older people participated during COVID-19 extended restrictions, a Tai Chi group (n=18, more than five years’ experience) and a Control group (n=20, no Tai Chi experience). Postural sway was quantified under four different conditions:1) eyes open (EO); 2) eyes closed (EC); 3) eyes open and cross step with right leg forward (ER) and 4) cross step with left leg forward (EL). Significantly less postural sway was observed in Tai Chi group, particularly during EO and EL conditions. The findings of this study support the positive effects of Tai Chi practice on balance control. During the COVID-19, although older people in the nursing home limited their outdoor mobility, Tai Chi practice maintained their physical function during standing balance

Limitations of functionally determined joint centres for analysis of athletic human movement: a case study of the upper limb

Much research is ongoing into improving the accuracy of functional algorithms to determine joint centres (JC), but there has been limited testing using human movement data. This paper is in three parts: Part 1, errors in determining JCs from real human movement data using the SCoRE method; Part 2, variability of marker combinations during a punch; Part 3, variability in the JC due to reconstruction. Results indicate determining the JC of the shoulder or elbow with a triad of markers per segment with an accuracy greater than 20 mm is unlikely. Part 2 suggests conducting a pilot study with abundant markers to obtain triads which are most stable due to differences of 300 to 400% in variability between triads. Variability due to the choice of reference frame for reconstruction during the punch ranged from 2.5 to 13.8 mm for the shoulder and 1.5 to 21.1 mm for the elbow. It would appear pertinent to enhance the practical methods in situ than to further improve theoretical accuracy of functional methods

The role of the heel pad and shank soft tissue during impacts: a further resolution of a paradox

This article was accepted for publication in the Journal of Biomechanics [© Elsevier]. It is also available at: www.elsevier.com/locate/jbiomechThe aim of this study was to test the hypothesis that by accounting for soft tissue motion of the lower leg during the impacts associated with in vivo testing, that the differences between in vivo and in vitro estimates of heel pad properties can be explained. To examine this a two-dimensional model of the shank and heel pad was developed using DADS. The model contained a heel pad element and a rigid skeleton to which was connected soft tissue which could move relative to the bone. Simulations permitted estimation of heel pad properties directly from heel pad deformations, and from the kinematics of an impacting pendulum. These two approaches paralleled those used in vitro and in vivo respectively. Measurements from the pendulum indicated that heel pad properties changed from those found in vitro to those found in vivo as relative motion of the bone and soft tissue was allowed. This would indicate that pendulum measures of the in vivo heel pad properties are also measuring the properties of the whole lower leg. The ability of the wobbling mass of the shank to dissipate energy during an impact was found to be significant. These results demonstrate the important role of both the heel pad and soft tissue of the shank to the dissipation of mechanical energy during impacts. These results provide a further clarification of the paradox between the measurements of heel pad properties made in vivo and in vitro

Soft tissue motion during impacts : their potential contributions to energy dissipation

Cette étude montrent que les déformations des tissus mous ont un rôle important dans la cinétique d'un impact d'un segment corporel (l'avant-bras dans cette expérimentation

Contraction type influences the human ability to use the available torque capacity of skeletal muscle during explosive efforts

The influence of contraction type on the human ability to utilise the torque capacity of skeletal muscle during explosive efforts has not been documented. Fourteen male participants completed explosive voluntary contractions of the knee extensors in four separate conditions: concentric (CON) and eccentric (ECC); and isometric at two knee angles (101°, ISO101; and 155°, ISO155). In each condition torque was measured at 25-ms intervals up to 150-ms from torque onset, and then normalised to the maximum voluntary torque (MVT) specific to that joint angle and angular velocity. Explosive voluntary torque after 50-ms in each condition was also expressed as a percentage of torque generated after 50-ms during a supramaximal 300-Hz electrically evoked octet in the same condition. Explosive voluntary torque normalised to MVT was >60% larger in CON than any other condition after the initial 25-ms. The percentage of evoked torque expressed after 50-ms of the explosive voluntary contractions was also greatest in CON (ANOVA; P<0.001), suggesting higher concentric volitional activation. This was confirmed by greater agonist EMG normalised to Mmax (recorded during the explosive voluntary contractions) in CON. These results provide novel evidence that the ability to utilise the muscle’s torque capacity explosively is influenced by contraction type, with concentric contractions being more conducive to explosive performance due to a more effective neural strategy