The Flexibility Trainer:

Feasibility Analysis, Prototype- and Test Station Development for a

Sports Device for Hip-joint Flexibility and Strength Enhancement

Hölbling Dominik

1,3 a

, Grafinger Manfred

, Baca Arnold

and Dabnichki Peter

Centre for Sport Science and University Sports, University of Vienna, Auf der Schmelz 6A, A-1150 Vienna, Austria

Department for Virtual Product Development, TU Wien, Getreidemarkt 9/307-4, 1060 Wien, Austria

School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, GPO Box 2476,

Melbourne VIC 3001, Australia

Keywords: Flexibility, Strength, Sports Engineering, Hip Joint, Martial Arts, Dancing, Gymnastics.

Abstract: Martial Arts, dancing, and gymnastics are among the sports that depend on outstanding hip-joint flexibility

and strength to successfully perform high level techniques. Latest research suggests that flexibility and

antagonistic strength are strongly related and require agonistic strength training. Therefore, the aim of the

study was to develop a model prototype of a device that utilises flexibility-enhancement reflexes and provides

appropriate means for strength training and delivers hip joint range-of-motion (ROM) increase. The device

provisionally called Flexibility Trainer (FT) is equipped with sensors that measures and estimate the athletes’

hip joint moments during training. The FT aims to utilise the athletes body weight while performing controlled

leg spreads. Its main components are a rail system with 2 slides and foot mounts (tiltable and turnable), a

hydraulic braking system for force independent constant velocity slide (nearly isokinetic), a force sensor and

a holding device. It is hypothesised that the leg-spread movement activates the reciprocal-inhibition reflex

and increases antagonistic strength, whereas the leg-closing movement activates the autogenic-inhibition

reflex and leads to serial hypertrophy. A model for hip-joint moment calculation based on force sensor and

motion capturing data is proposed.

1 INTRODUCTION

Hip-joint flexibility (HJF) and strength are essential

pre-requisites for most sports, but particularly Martial

Arts, dancing and gymnastics that are highly

dependent on it (Hölbling, Preuschl, Hassmann, &

Baca, 2017; Shan, 2005; Weber, Bedi, Tibor, Zaltz,

& Larson, 2015).

Training this ability can be challenging, as the

improvement of the range-of-motion (ROM) involves

physiological and neuronal adaptations (Alter, 2004;

Moreira & Gonzaga, 2012) and it decreases with age

and periods of immobility (Roaas & Andersson,

1982). However, recent research suggests different

physiological and neuronal methodical approaches

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result in measurable short- and long-term adaptions

of active and passive hip-joint flexibility.

1.1 Methodical Approaches for

Short-term Adaptions of ROM

Most short-term approaches are based on neuronal

adaptions, such as decreased residual muscle tone and

increased stress tolerance (Sharman, Cresswell, &

Riek, 2006). Current publications have outlined the

benefits of reciprocal and autogenic inhibition

activation (Rowlands, Marginson, & Lee, 2003).

1.1.1 Reciprocal Inhibition

Reciprocal inhibition is “a process that inhibits the

stretch reflex in antagonistic pairs of muscles. When

Dominik, H., Manfred, G., Arnold, B. and Peter, D.

The Flexibility Trainer: Feasibility Analysis, Prototype- and Test Station Development for a Sports Device for Hip-joint Flexibility and Strength Enhancement.

DOI: 10.5220/0010019400220029

In Proceedings of the 8th International Conference on Sport Sciences Research and Technology Support (icSPORTS 2020), pages 22-29

ISBN: 978-989-758-481-7

one muscle contracts it sends inhibitory nerve

impulses to its opposing muscle causing it to relax”

(Kent, 2006, p. 458). Therefore, the reflex can be used

inversely to increase the flexibility in one muscle, by

contracting its antagonist.

1.1.2 Autogenic Inhibition

Autogenic inhibition (or inverse myotatic reflex)

describes a ”reflex inhibition of a motor unit in

response to excessive tension in the muscle fibres it

supplies” (Kent, 2006, p. 62). The reflex results in

increased ROM after contraction of the limiting

muscles.

1.1.3 CRAC- PNF Training

Proprioceptive neuromuscular facilitation (PNF)

training describes “an effective stretching technique

for increasing the joint’s range of motion. All PNF

procedures require a partner and involve a pattern of

alternating muscles’ contraction and relaxation while

stretched. In this way the Golgi tendon organs are

stimulated (Kent, 2006, p. 441). A particular

promising variant called Contract-relax-agonist-

contract (CRAC) combines a contraction of the

targeted (stretching) muscle (TM), followed by

relaxation and contraction of the opposing muscle

(OM) to reach a wider position culminating in

additional passive stretch (Sharman et al., 2006). This

method is believed to activate both, reciprocal and

autogenic inhibition, which results in a significantly

higher ROM increase, compared to traditional

flexibility training methods. Sharman et al. (2006).

report a gain of 3-9° in joint angle by this method,

which would theoretically result in 6-18° for full leg

spreading.

1.2 Methodical Approaches for

Long-term Adaptions of ROM

Long-term enhancements in flexibility are normally

accompanied by physiological adaptions. Thereby,

active ROM is strongly dependent on OM (agonist of

the spreading movement) strength, whereas passive

flexibility is primarily related to the stretching

muscle‘s length and elasticity. Recent research

reported that full-ROM strength training (if

performed in both opposing movement directions)

leads to both increase of OM strength and serial

hypertrophy (particularly eccentric motions) of the

TM (Csapo, Alegre, & Baron, 2011; Franchi et al.,

2014).

1.2.1 Agonistic Strength

The agonistic muscle (OM) strength constitutes a key

factor to significantly enhance active flexibility,

especially in fast sports movements opposing the

gravitational pull. Increase in maximum strength can

be achieved by intramuscular strength (IC) or

(parallel) hypertrophy training (HT). This requires 1

to 3 repetitions at 90-100% of one repetition

maximum (1rpm) resistance for IC or 8-12 with 60-

80% of 1rpm for HT, varying between authors.

1.2.2 Serial Hypertrophy of TM

Recent research states that full-ROM strength

training, particulalry (but not solely) in eccentric

motion leads to increased muscle length, due to

additional serial muscle fibre sarcomere production

(Csapo et al., 2011; Franchi et al., 2014).

1.3 Isokinetic Training

To enable a sport device to allow for different number

of repetitions and percentages of 1 rpm and to

simultaneously decrease the injury propability, an

isokinetic mode is believed to be advantageous.

Isokinetics in general describes “exercise with an

accommodating resistance and a fixed speed”

(Brown, 2000, p. 6).

1.4 Device Development

The aim of the study is (a) to design a device which

combines static and dynamic CRAC-PNF with full-

ROM strength training in an isokinetic training mode

for an increase of hip joint flexibility in side

(abduction/ adduction) and front split (flexion/

extension) direction, (b) to investigate its’ feasibility

propose an appropriate participant specific test setup.

2 MATERIALS AND METHODS

The design process was structured in four main

phases product planning, conception, product design

and detailing for production according to Pahl and

Beitz (2013) and VDI2221 (2019). The first 2 phases

are considered as pre-analysis phases and can

therefore be found in the methods section, whereas

later include detailed calculations, which can be

found in the results and discussion section.

The Flexibility Trainer: Feasibility Analysis, Prototype- and Test Station Development for a Sports Device for Hip-joint Flexibility and

Strength Enhancement

Planning Phase: A requirement list was

developed to address customers’ needs as outlined in

Hölbling (2016) and illustrated in Table 1.

Table 1: Requirement list. The list covers customer needs

and patent specification from Hölbling (2016). Table

column 2 distinguishes between fixed requirements (F) and

wanted requests (W).

Function F/W Description

Sliding

system

F Linear movement direction

Sliding

system

F Coupled motion for symmetric

exercises

Sliding

system

W Foldable for easy

transportation

Feet

fixations

F Fixation of feet on sliders

rotatable around 360°

Feet

fixations

F Foot fixations must be tiltable

>40° to avoid ankle injuries

Damping

system

F Damper to slow down foot

rotation in the foot ankle joint

Brake

system

F Movement with adjustable

velocity from 0,1 m/s to 0,3

m/s, at design load of

maximum expected load.

Frame W Minimized weight for easy

transportation

Handle F Adjustable in height

Force

Sensor

W Measure the combined leg

spreading and closing force,

withstanding maximum loads.

Conception Phase: To achieve the main functions,

the product was segmented into functional modules:

rail system, sliders, braking system, holding device

and force sensors.

The design phase includes detailed descriptions of the

main device components of a first market-ready

version, including stress analysis, due to the external

loads during usage. Therefore, this section covers aim

a-b.

In the detailing phase some changes of the device

design are proposed, to address aim c.

A setup for device testing and analysis of general

and sport specific flexibility is proposed as stated in

aim d).

Legend:

= Gravitational force

= Normal component of gravitational force

= Horizontal component of gravitational force

= Horizontal component of muscular force

= Spreading force

3 RESULTS AND DISCUSSION

3.1 General Components

The outlined design consists of two sliding

mechanisms (1), running on U-Profiles (2) with

rotating foot straps (3), shown in Figure 1. The foot

straps can be set by locking pin at every 90°. The

sliders are connected with a circulating steel cable (4)

guide rollers are located at the outer end (5) of the U-

Profiles to ensure symmetric motion. The damper unit

of the braking system (6) is placed inside the U-

Profile. The trainer can be folded at the connection

plate (7) for easy transportation. The handles (8) are

height adjustable by lock pins (9) and are mounted in

injection moulded foldable plates (10).

Figure 1: Design of Flexibility trainer.

3.2 Calculations of External Loads

The expected user generated maximum loads are

calculated using maximum hip moment and the user’s

weight (see Figure 2).

Figure 2: Illustration of the force calculation method in

abduction

= Cumulated moment of hip joint

g = Gravitational acceleration

α = angle leg axis to floor

leg

= leg length

eff

= effective component of leg length

icSPORTS 2020 - 8th International Conference on Sport Sciences Research and Technology Support

For the calculation of F

, the possible M

from an

Isokinetic test is approximately linearized (based on

the isokinetic hip abduction and adduction test of a

highly skilled Martial Artist, shown in Appendix A

and divided by effective height.

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

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





.

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∗°

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(1)

The component F

of bodyweight F

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(2)

The resulting maximal force on the slider is the sum

of F

and F

equal to approx. 2200 N at the minimal

possible α of 15°. Leg length is assumed 1 m and the

body weight up to 110 kg (see Figure 3).

Figure 3: Horizontal forces on slider depending on α.

This is also the assumed load for the braking system

and the other slider components. The selected damper

should work for speed up to 0.3 m/s at this load.

The force sensors should have a range from 100 N to

5000 N in pulling and pushing directions.

3.3 Components and Specific Loads

3.3.1 Rail System

The main part of the rail systems are the U-Profiles

100x80x10 mm. For weight reduction purpose the

chosen material is Polyamid. Maximal stress and

deformation is obtained via FEA (Finite Elements

Analysis; (Klein, 2012)). Maximum stress level of 25

MPa is located around the mounting holes as

expected and is well below the critical stress level

(Figure 4). The detailed model description is

available in Stummer (2016)

Figure 4: Von Mises stress on polyamid U-Profile

(Stummer, 2016).

Inside the rail track two adjustable hydraulic brake

cylinders HB-40-500 and HB-40-600 (6) from

supplier ACE (Figure 5) are placed in series. The

maximum allowed force is 4000 N for the longer

600 mm cylinder.

Figure 5: Rail and slider system of one side.

The sliders are running on the U-Profile with SKF 635

ball bearings (11) to minimize the friction. Both

sliders are clamped to the steel cable (4) by sheet

metal (12) and M5 screws. For cable turn at the end

of the U-Profile guide pulleys 10/3 (13) from Ingo

Quirnbach Industrieservice are applied (Figure 6).

The two upstanding rollers 20K JGV 050-2 (14) from

Dematech are for transportation issues.

Figure 6: Rollers at the outer end of the U-Profile.

3.3.2 Sliders

The main frame of the slider is profiled from a metal

sheet of 3 mm thickness and connected to the braking

system by a detachable lock pin (15) as shown in

Figure 7. This enables dismantling for folding and

transportation.

The Flexibility Trainer: Feasibility Analysis, Prototype- and Test Station Development for a Sports Device for Hip-joint Flexibility and

Strength Enhancement

Figure 7: Slider with ankle tilt system and foot mount.

The ankle tilt mechanism, consisting of roller

bearings with housing SY 12 TF from SKF (16) and

rotational damper WRD-H-3015-C from Weforma

(17), is mounted on the slider frame. The torque of the

selected damper is adjustable from 2 to 14 Nm and

should reduce unintended ankle wobble.

A turntable (18) is placed between the foot mount

and the ankle tilt mechanism to allow the foot mount

rotation of 360° for spread training lateral and

longitudinal. It can be fixed every 90° by the lock pin

(19).

The foot mount (3) is manufactured by FDM

(Fused Deposition Modelling) for the prototype. For

serial production it will be injection moulded.

3.3.3 Holding Device

The choice of material for the tubes (8), of this

essential safety feature as shown in Figure 8, is CFRP

(carbon fibre reinforced plastic). Pre-ordered tubes

can be glued together with aluminium T-piece (20) to

form the handlebar. For ergonomic reasons, the

handlebar is height adjustable and fixed by a lock pin

(9).

The connection plate (10) is an injection moulded

part. The maximum stress is calculated at a horizontal

force of 300 N on the handlebar that leads to a

bending moment of 270 Nm at the tube mounting

dome. The FEM results show a maximum stress of 42

MPa at a single node (see Figure 9 and Stummer

(2016) for more details

Figure 8: Handlebar of CFRP tubes.

Figure 9: Von Mises stress on connection plate (Stummer,

2016).

The maximum stress in the ribs is less than 20 MPa

i.e. safely below critical stress.

3.3.4 Force Sensor

As stated in section 0, the expected maximum force

at the sensor is about 2200N per leg. Therefore, the

sensor should have a measuring range from 100N-

5000N. A U9c force sensor for forces up to 20kN has

been identified.

3.4 Changes for the Prototype

Due to manufacturing reasons some parts of the

design needed to be changed from the planned serial

manufacturing to prototyping (see Figure 10):

• Due to high costs, the injection moulded

mounting plates were changed to aluminium

plates.

• The minimum length of Polyamid U-Profile was

changed to aluminium profile.

icSPORTS 2020 - 8th International Conference on Sport Sciences Research and Technology Support

• As a consequence of the rail profile change the

sliders are now running on linear guide inside

the profile instead of the roller bearings outside.

• The foot mount changed from a 3D printed part

to mini skates available on the market.

• The holding devices are made of steel.

Figure 10: Manufactured prototype.

3.5 Proposed Test Setup

In order to comprehensively test the device and its

short-term effect on athletes, a testing setup utilizing

a Vicon® motion capturing system and a force sensor

within the device for measuring the force alongside

the rail, is proposed. The setup is specifically

designed to provide a detailed estimation of the hip

joint moments, to evaluate the athletes’ physical

strain during exercising in order to make propose

future training adaptations. In addition, the setup

includes a stage for practical measurements, such as

static flexibility tests and sport specific movements,

as well as a stage for device training.

3.5.1 System Setup

As stated above, the setup comprises both, one sport-

specific (e.g. Martial Arts) and one device-specific

movement analysis volume. The combined volume is

surrounded by 12 high-resolution infrared cameras,

specifically aimed to capture all markers, which are

attached to the athlete during the exercises, see Figure

11.

For Legend, see

Figure 2

Legend:

= Hip flexion moment

= Hip abduction moment

Figure 11: Proposed test setup for sport-specific and general

flexibility tests.

3.5.2 Hip-joint Moment Calculation

To calculate the hip joint moment with inclusion of

the external forces of the flexibility trainer, a

kinematic model of the hip joint centre, based on a

marker model and anthropometric data is required.

The Vicon® plug-in-gait model is applied (Vicon®,

2016). Hip joint angles can be obtained using the

static and dynamic plug-in-gait modelling function.

Centres of rotation are calculated by the functional

skeleton calibration.

Based on

Figure 2

, the cumulative hip moment (for leg

spreading) can be estimated by following inverse

calculation retrieved from the linear sensor force and

reduced by the gravity component

(3)

The cumulated hip-joint moment M

can then be

divided into the 3 movement direction components

, M

)

y splitting it in relation to the

calculated components from Vicon® (M

, M

), as the kinematic parameters, such as movemen

directions are the same with and without resistance.

(4)

(5)

(6)

= Hip rotation moment

= Vicon® calculated hip flexion moment

= Vicon® calculated hip abduction moment

= Vicon® calculated hip rotation moment

The Flexibility Trainer: Feasibility Analysis, Prototype- and Test Station Development for a Sports Device for Hip-joint Flexibility and

Strength Enhancement

3.6 Summary of Outcomes

The proposed final construction fulfils additional

framework conditions, such as weight and design

specifications, which will be necessary for a

commercially viable product, but are too expensive

for a prototype. Therefore, the design was optimized

in order to provide more safety and stability for

significantly lower costs. The testing setup is opted to

provide analysis possibilities for sport-specific and

general flexibility tests, as well as for the device

training analysis. Furthermore, to comprehensively

analyse the physical strain during the training, a

mathematical model for joint moment analysis was

used, based on the Vicon® model and the force sensor

data.

3.7 Limitations

The calculation model does currently solely comprise

the leg spreading motion because at the leg closing

motion the participant is estimated to reduce the

gravitational force by supporting his body weight on

the holding device. Therefore, to analyse the closing

moment, an additional sensor measuring Fg would be

needed, or the result would be given as a function

from 0-100% of body weight reduction. Furthermore,

marker position changes due to skin shifts might add

large error to the results of the separated moments.

Usability and effect sizes are not available yet.

4 CONCLUSIONS

The Flexibility Trainer prototype is expected to have

the potential to enhance the training of active and

passive flexibility, even more than with CRAC-PNF

methods, due to cumulated effects of combined

methodical approaches (isokinetic, full-ROM

strength training with CRAC), which might also last

longer. Furthermore, based on the tests and

component data sheets, the device should resist all

expected loads without damages and meet the

requirements. However, future studies are needed to

proof short- and long-term functionality.

REFERENCES

Alter, M. J. (2004). Science of flexibility: Human Kinetics.

Brown, L. E. (2000). Isokinetics in human performance:

Human Kinetics.

Csapo, R., Alegre, L. M., & Baron, R. (2011). Time kinetics

of acute changes in muscle architecture in response to

resistance exercise. Journal of Science and Medicine in

Sport, 14(3), 270-274. doi:10.1016/j.jsams.2011.

02.003

Franchi, M. V., Atherton, P. J., Reeves, N. D., Fluck, M.,

Williams, J., Mitchell, W. K., . . . Narici, M. V. (2014).

Architectural, functional and molecular responses to

concentric and eccentric loading in human skeletal

muscle. Acta Physiologica, 210(3), 642-654.

doi:10.1111/apha.12225\r10.1111/apha.12225.

Hölbling, D. (2016). EP000003269429A1. U. WIEN.

Hölbling, 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. doi:10.1080/02640414.2016.1164333

Kent, M. (2006). The Oxford dictionary of sports science

and medicine: Oxford University Press, USA.

Klein, B. (2012). FEM: Grundlagen und Anwendungen der

Finite-Element-Methode im Maschinen-und

Fahrzeugbau: Springer-Verlag.

Moreira, P. V. S., & Gonzaga, A. C. d. O. (2012). The effect

of stretching on the health and performance: new

perspectives. Terapia Manual., 10(50), 148-157.

Pahl, G., & Beitz, W. (2013). Engineering design: a

systematic approach: Springer Science & Business

Media.

Roaas, A., & Andersson, G. B. (1982). Normal range of

motion of the hip, knee and ankle joints in male

subjects, 30–40 years of age. Acta Orthopaedica

Scandinavica, 53(2), 205-208.

Rowlands, A. V., Marginson, V. F., & Lee, J. (2003).

Chronic flexibility gains: Effect of isometric

contraction duration during proprioceptive

neuromuscular facilitation stretching techniques.

Research Quarterly for Exercise and Sport, 74(1), 47-

51. doi:10.1080/02701367.2003.10609063

Shan, G. (2005). Comparison of repetitive movements

between ballet dancers and martial artists: Risk

assessment of muscle overuse injuries and prevention

strategies. Research in Sports Medicine, 13(1), 63-76.

doi:10.1080/15438620590922103

Sharman, M. J., Cresswell, A. G., & Riek, S. (2006).

Proprioceptive neuromuscular facilitation stretching :

Mechanisms and clinical implications. Sports

Medicine, 36(11), 929-939.

Stummer, M. (2016). Beweglichkeitstrainer:

Weiterentwicklung des bestehenden Systems und

Konstruktion eines Prototyps. (Bachelor Thesis), TU

Wien

VDI2221. (2019). Design of technical products and systems

(Vol. Part 1): VDI-Gesellschaft Produkt- und

Prozessgestaltung

Vicon®. (2016). Plug-in Gait Reference Guide. In (pp. 95).

Retrieved from https://docs.vicon.com/display/Nexus

25/PDF+downloads+for+Vicon+Nexus?preview=/508

88706/50889377/Plug-in%20Gait%20Reference%20

Guide.pdf

Weber, A. E., Bedi, A., Tibor, L. M., Zaltz, I., & Larson, C.

M. (2015). The hyperflexible hip: managing hip pain in

the dancer and gymnast. Sports Health, 7(4), 346-358.

icSPORTS 2020 - 8th International Conference on Sport Sciences Research and Technology Support

APPENDIX

Appendix A: Isokinetic hip joint adduction and abduction

tests of a highly skilled Martial Artist with a height of

1.82m and a weight of 78kg on a Humac® CSMI device.

The Flexibility Trainer: Feasibility Analysis, Prototype- and Test Station Development for a Sports Device for Hip-joint Flexibility and

Strength Enhancement