Acute response on general and sport specific hip joint flexibility to training with novel sport device

Figures & data

Figure 1. Design illustration of Flexibility Trainer, with the braking system (isokinetic hydraulic dampers) and the force sensor (connection between dampers and left slide). Rotational damper (front of foot mount), steel cable with guide rollers (from left to right) and its opposing sided mounts on the sliders (Hoelbling et al., 2020)

Figure 2. Visualisation of flexibility tests recorded with a 3D Vicon® motion capturing system, based on (Hoelbling et al., 2020a)

Table 1. Flexibility trainer exercise description for total ~60s

Part	Duration	Exercise	Anticipated Function
Pre	~1 min	Rest	Recovery
1	Individual ~5s	Maximum voluntary leg spreading (hip abduction; hip flexion/extension)	Activation of reciprocal inhibition and full-ROM strength training
2	5s	Relaxation at maximum ROM	Passive flexibility and stress tolerance training
3	Individual ~5s	Maximum voluntary leg closing (hip adduction; hip extension/flexion)	Activation of autogenic inhibition and full-ROM strength training
4	2s	Rest	Rest
5–8	Individual ~5s	Repeating part 1–4	Same as above
9	Individual ~5s	Maximum voluntary leg spreading (hip abduction; hip flexion/extension)	Activation of reciprocal inhibition and full-ROM strength training
10	8s	Static contraction of leg-closing muscles	Static activation of autogenic inhibition
11	8s	Static contraction of leg-spreading muscles	Static activation of reciprocal inhibition
12	Individual ~5s	Maximum voluntary leg closing (hip adduction; hip extension/flexion)	Activation of autogenic inhibition and full-ROM strength training
Post	~1 min	Rest	Recovery

Figure 3. Phases and nodes illustration of the double side kick, with specifications of kicking movement 1 and 2 (KIM1; KIM2), which together forms the total kicking movement (TKIM)

Figure 4. Illustration of forces and moments acting during device training, adapted from. Hoelbling et al. (2020)

Table 2. Results of main (within-subject) effects and group differences of ANOVA (or Friedman test for variables, which significantly differ from normal distribution). Variables: φ = leg spreading angle; INI = Kick initiation; FLX1 = First flexion; MT1 = First measuring time; KE1 = First knee extension; FLX2 = Second flexion; MT2 = Second measuring time; KE2 = Second knee extension; FC = Floor contact; KIM = Kicking motion (Hoelbling et al., 2017)

	Avg. hip flex.	Comb. hip abd.	φINI⁺	φFLX1	φMT1	φKE1	φFLX2	φMT2	φKE2	φFC⁺	φKIM1max	φKIM2max
p_Sphericity/p_Friedman	<0.01*~	<0.01*	0.30	0.07~	<0.01*	<0.01*	<0.01*	0.11~	0.04*~	0.86	<0.01*	<0.01*
Willks λ/Friedman χ^2	0.33*	0.25*	3.64	0.32*	0.24*	0.44*	0.39*	0.57	0.38*	0.76	0.30*	0.29*
Part. Eta Squared	0.75	0.67			0.76	0.56	0.60		0.62		0.70	0.70

⁺Variable significantly differs from normal distribution.

*Significant difference between groups (p < 0.05).

~Greenhouse-Geisser was used due to significant Mauchly-Test and correction value <0.75.

Figure 5. Static flexibility test results training (TR) and control (CO) group before (Pre) and after (Post) intervention or rest. * above bars marks significant differences of Bonferroni post-hoc test between test times, whereas * between the bars on the bottom means significant difference of the ANOVA test between test times (further details in Table 2)

Figure 6. Results for vector spreading angles (VSA) during the kicking movements (KIM) of training (TR) and control (CO) group before (Pre) and after (Post) intervention or rest. * marks significant differences between variables and groups (further details to group differences for VSA at the nodes are shown in Table 3)

Table 3. Bonferroni post-hoc test for ANOVA for comparison of group differences of the nodes with significant ANOVA outcomes during the kick execution. Mean and standard deviation of VSAs are given for each group and brackets below mark significant differences of the Bonferroni test and the respective test times. Nodes: MT1 = first measuring time 1; KE1 = first knee extension; FLX2 = second knee flexion; KE2 = Second knee extension

Table

Equation	Description
$F_s=F_{rm}+F_x$	The spreading force (Fs) equals the horizontal components of the muscle force (Frm) and the gravitational force (Fx). Fs is measured with the sensor for both legs combined.
$F_{leg}=F_g/2$	The force of each leg applied to the foot mount (Fleg) equals half of the gravitational force (Fg).
$F_x=F_{leg}cos\propto$	Fx can be calculated by multiplying the force applied by each leg (Fleg) with the cosine of the leg angle (α).
$F_m=\frac{F_{rm}}{cos\left(90-\propto \right)}$	Now the muscle force (Fm) can be calculated by dividing its horizontal component Frm by the cosine of 90 minus α.
$M_h=F_m{l}_{leg}$	The cumulated (all directions) hip moment (Mh) equals Fm times the leg length (lleg).
$M_h=\frac{\left|F_s\right|-1/2F_{g}cos\propto }{cos\left(90-\propto \right)}{l}_{leg}$	All the above can be summarised to this equation.
$M_i=\frac{M_h}{\left|M_{Vx}\right|+\left|M_{Vy}\right|+\left|M_{Vz}\right|}\left|M_{Vi}\right|$	To filter the respective ‘motion-direction component’ (i) for each spreading movement (abduction or flexion/extension), Mh was split into its 3 components (Mi = Mx; My; Mz) based on the Vicon® Nexus model, which calculated flexion/extension (MVx), abduction/adduction (MVy) and internal/external rotation (MVz) moments solely on the kinematic and anthropometric data.
$\mathrm{\varnothing }{M}_i=\frac{\sum \left|M_{ir}\right|+\sum \left|M_{il}\right|}{n_{M_i}m}$	Finally, the average for both legs is calculated.

Hoelbling, D., Grafinger, M., Baca, A., & Dabnichki, P. (2020). The flexibility trainer: Feasibility analysis, prototype- and test station development for a sports device for hip-joint flexibility and strength enhancement [Paper presented]. The proceedings of the 8th international conference on sport sciences research and technology support (icSPORTS 2020), Budapest, Hungary.

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Hoelbling, D., Preuschl, E., Hassmann, M., & Baca, A. (2017). Kinematic analysis of the double side kick in pointfighting, kickboxing. Journal of Sports Sciences, 35(4), 317–324. https://doi.org/10.1080/02640414.2016.1164333

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