Changes in balance coordination and transfer to an unlearned balance task after slackline training: a self-organizing map analysis.

How humans maintain balance and change postural control due to age, injury, immobility or training is one of the basic questions in motor control. One of the problems in understanding postural control is the large set of degrees of freedom in the human motor system. Therefore, a self-organizing map (SOM), a type of artificial neural network, was used in the present study to extract and visualize information about high-dimensional balance strategies before and after a 6-week slackline training intervention. Thirteen subjects performed a flamingo and slackline balance task before and after the training while full body kinematics were measured. Range of motion, velocity and frequency of the center of mass and joint angles from the pelvis, trunk and lower leg (45 variables) were calculated and subsequently analyzed with an SOM. Subjects increased their standing time significantly on the flamingo (average +2.93 s, Cohen's d = 1.04) and slackline (+9.55 s, d = 3.28) tasks, but the effect size was more than three times larger in the slackline. The SOM analysis, followed by a k-means clustering and marginal homogeneity test, showed that the balance coordination pattern was significantly different between pre- and post-test for the slackline task only (χ 2 = 82.247; p < 0.001). The shift in balance coordination on the slackline could be characterized by an increase in range of motion and a decrease in velocity and frequency in nearly all degrees of freedom simultaneously. The observation of low transfer of coordination strategies to the flamingo task adds further evidence for the task-specificity principle of balance training, meaning that slackline training alone will be insufficient to increase postural control in other challenging situations.

Influence of virtual reality on postural stability during movements of quiet stance.

INTRODUCTION: Balance problems during virtual reality (VR) have been mentioned in the literature but seldom investigated despite the increased use of VR systems as a training or rehabilitation tool. We examined the influence of VR on body sway under different stance conditions. METHODS: Seventeen young subjects performed four tasks (standing with feet close together or tandem stance on firm and foam surfaces for 60s) under three visual conditions: eyes open without VR, eyes closed, or while viewing a virtual reality scene which moved with body movements. Angular velocity transducers mounted on the shoulder provided measures of body sway in the roll and pitch plane. RESULTS: VR caused increased pitch and roll angles and angular velocities compared to EO. The effects of VR were, for the most part, indistinguishable from eyes closed conditions. Use of a foam surface increased sway compared to a firm surface under eyes closed and VR conditions. CONCLUSION: During the movements of quiet stance, VR causes an increase in postural sway in amplitude similar to that caused by closing the eyes. This increased sway was present irrespective of stance surface, but was greatest on foam.

Control of roll and pitch motion during multi-directional balance perturbations.

Does the central nervous system (CNS) independently control roll and pitch movements of the human body during balance corrections? To help provide an answer to this question, we perturbed the balance of 16 young healthy subjects using multi-directional rotations of the support surface. All rotations had pitch and roll components, for which either the roll (DR) or the pitch (DP) component were delayed by 150 ms or not at all (ND). The outcome measures were the biomechanical responses of the body and surface EMG activity of several muscles. Across all perturbation directions, DR caused equally delayed shifts (150 ms) in peak lateral centre of mass (COM) velocity. Across directions, DP did not cause equally delayed shifts in anterior-posterior COM velocity. After 300 ms however, the vector direction of COM velocity was similar to the ND directions. Trunk, arm and knee joint rotations followed this roll compared to pitch pattern, but were different from ND rotation synergies after 300 ms, suggesting an intersegmental compensation for the delay effects. Balance correcting responses of muscles demonstrated both roll and pitch directed components regardless of axial alignment. We categorised muscles into three groups: pitch oriented, roll oriented and mixed based on their responses to DR and DP. Lower leg muscles were pitch oriented, trunk muscles were roll oriented, and knee and arm muscles were mixed. The results of this study suggest that roll, but not pitch components, of balance correcting movement strategies and muscle synergies are separately programmed by the CNS. Reliance on differentially activated arm and knee muscles to correct roll perturbations reveals a dependence of the pitch response on that of roll, possibly due to biomechanical constraints, and accounts for the failure of DP to be transmitted equally in time across all limbs segments. Thus it appears the CNS preferentially programs the roll response of the body and then adjusts the pitch response accordingly.

Anthropometric and performance characteristics of the German rugby union 7s team.

BACKGROUND: Somatotyping is advantageous in sports for the optimal development of performance level and injury prevention. The aim of this study was to describe the anthropometric and physical performance characteristics of the German national rugby union 7s team. Seventeen male rugby players, classified as forwards (N.=9; 24.2±2.1 years) and backs (N.=8; 24.3±5.05 years) were assessed. METHODS: Anthropometric measurements included: body height, weight, height to weight ratio (H/W), five skinfolds, biepicondylar humerus and femur breadth, upper arm- and calf girth, estimated lower body fat percentage and determination of the individual and mean somatotype. The physical performance tests included: sit-and-reach, handgrip strength, one minute of sit-ups, one minute of push-ups, vertical jump performance, peak power performance, bent arm hanging, 40-m sprint, and the Yo-Yo Intermittent Endurance Test. RESULTS: The forward players were significantly taller (P=0.003), heavier (P=0.001) with a smaller H/W (P=0.009) compared to the backs. Humerus and femur bone breadths (P<0.05) and flexed upper arm and calf girths (P<0.05) were significantly different between the groups. Handgrip strength left (P=0.04), one minute of sit-ups (P=0.03), and peak power output (P=0.015) were also significantly different between the groups. CONCLUSIONS: The data indicate that German forward and back players have a similar somatotype and performance level. However, a higher body mass of forward players could be advantageous in that their playing position is much more body contact intensive, and requires a significant amount of tackling. The nominative data of this study may assist coaches to detect weak links in rugby specific athletic performance.