Continuous relative phases, a new use to determine a modified joint spatiotemporal organization: A preliminary study about rowing movement on ergometer

1, 2 Nicolas Découfour, 1Philippe Pudlo, 1Emilie Simoneau and 1Franck Barbier 1PRES Université Lille Nord de France, F-59000 Lille, UVHC, LAMIH, CNRS, FRE3304,F-59313Valenciennes, Cedex 9, France 2Centre Hospitalier de la région de Saint-Omer, rue Blendecques, 625S70 Helfaut, France ; email: nicolas.decoufour@ch-stomer.fr

SUMMARY In coordination studies, the major difficulty is to determine which joint spatiotemporal organization is modified. In rowing movement, inter-joint coordinations between knee, elbow and trunk are modified especially during recovery phase but authors were not able to identify which joint is at the origin of the modification. Our study aims at giving a new approach of continuous relative phase use in order to determine which joint spatiotemporal organization is modified during rowing on ergometer between two different stroke rates. Method of continuous relative phase is based on Hamill’s method. The two signals used in this method are the considered joint angle computed at 20 strokes per minute and the same joint angle computed at 40 strokes per minute. In this preliminary study, an international level rower participated to experiments. Results show firstly that the spatiotemporal organization of knee is un-modified with the stroke rate increase. Secondly, results show that trunk and elbow organization is modified during recovery movement on ergometer when stroke rate increases from 20 to 40 strokes per minute. In conclusion, the new approach seems to be interesting to determine which joint is disorganized when the condition changes. INTRODUCTION Coordination is difficult to maintain when movement frequency is increased [1]. For a complex movement, coordination is actually defined by: “An ability to maintain a context-dependent and phase-dependent cyclical relationship between different body segments or joints in both spatial and temporal domains.”[2]. This ability is very important for high level practice in competitive sport. Particularly, in rowing, rowers must maintain an optimal movement whatever the stroke rate maintained [3], and, to become one of the best rowers, coaches consider that the rowing movement repeated during training sessions must be reproduced during races [4]. A recent study shows, with continuous relative phases (CRP) analysis, that inter-joint coordination is modified when stroke rate increases, particularly during recovery phase for elbow, trunk and knee coordinations [5]. But these authors were not able to determine which joint provokes the modification of the inter-joint coordination considered. This paper aims at using differently the CRP to determine which joint organization, between elbow trunk and/or knee joints, is modified with stroke rate increase on ergometer during recovery phase. METHODS An international level rower participates to the experiment. 62 skin markers were positioned on the rower. A VICON® 612 motion capture system, composed of 8 cameras (60 Hz, 1880 x 881 pixels) and disposed around the ergometer, was used to

record movements of markers placed on the rowers' skin. The 3D coordinates of the skin markers were filtered bidirectionally, using a second-order low-pass Butterworth filter with a cut-off frequency of 6 Hz. This cut-off frequency allows the filtered signal to be maintained very close to the measured signal with an attenuation of the error induced by instrumental and experimental noises. Instruction given to the rowers was to “row at 20 or 40 strokes per minute (spm), producing the “usual” power at this stroke rate, and remain at this rate as long as possible. I will tell you when to stop”. The motion capture begun when the desired stroke rate was attained, but without informing the rower. The recording session was stopped when at least 10 consecutive strokes had been measured at the desired stroke rate. Among the 10 recorded cycles, only the first one performed at the nearest asked stroke rate has been chosen for analysis and especially recovery phase of this cycle. Only left joint angles ( iθ ) were considered because the rowing movement is symmetric on concept2 ergometer [6]. Considered joints are elbow (e), lumbar sacral (ls) and knee (k). To compute CRP, the method used is based on [7]. The angular velocity of each joint i ( iω ) is computed by derivation, using the centred method at each instant. In order to respect method of phase plane construction [8], joint angles and velocities, respectively, are normalised using equations 1 & 2:

( )

( ) ( )_ _

_ _

2 i i MAX i MINi NORM

i MAX i MIN

tt

θ θ θθ

θ θ× − +

=− (1)

( ) ( )

max{ }i

i NORMi

tt

ωω

ω=

(2) where i is the considered joint; t, the considered instant; and

iω , the absolute value of iω . The phase plane represents the normalised velocity in relation to the normalised angle for each joint considered. The phase plane provides a picture of a joint state over time. The phase angle ( iφ ) is computed using a specific equation for each dial position being studied (Table 1). Table 1: Equations used to compute phase angles

iθ iω iφ

[-1 0[ [-1 0]

1tan 180ii

i

NormNorm

ωφθ

− ⎛ ⎞= −⎜ ⎟

⎝ ⎠

]0 +1] [-1 +1]

1tan ii

i

NormNorm

ωφθ

− ⎛ ⎞= ⎜ ⎟

⎝ ⎠

[-1 0[ [0 +1]

1tan 180ii

i

NormNorm

ωφθ

− ⎛ ⎞= +⎜ ⎟

⎝ ⎠

The CRP between a joint “i” at 2 different stroke rates is noted CRPi20,i40. It is computed with Equation 3:

20, 40 20 40i i i iCRP φ φ= − (3)

To determine a real modification in joint organisation, a study of variance was done. A maximal standard deviation of 25° was noted. This value is considered as a threshold of modification. RESULTS AND DISCUSSION

0 10 20 30 40 50 60 70 80 90 100

-75

-50

-25

0

25

50

75

% of phase

Continuo

usRe

lative Ph

ase (Deg.)

Knee

Lumbar-sacral

Elbow

Figure 1: CRP20,40 of knee (–), lumbar-sacral joint (--) and

elbow (..) evolution during recovery phase. For knee, Figure 1 shows that CRPk20,k40 does not cross the threshold of +/- 25° during recovery. Knee joint organization is not modified with stroke rate increase. For lumbar-sacral joint, Figure 1 shows that CRPls20,ls40 cross the threshold of 25° from 25 to 60% of the recovery phase. In this part of recovery, at 40 spm the lumbar-sacral joint is activated in advance compared to 20 spm. At 20 spm, the lumbar-sacral organization is different from 40 spm. Between 25 and 60% of the recovery phase, the trunk is replaced in advance certainly because the duration of the recovery is divided by 3 [8] and trunk replacement permit the rower to push the handle forward his knee. So, the trunk is replaced faster between these two stroke rates. So, the normalization permits us to show difference in both spatial and temporal organization of trunk. For the elbow joint, the CRPe20,e40 cross the threshold of -25° from 10° to 70° of the recovery phase. Three intervals were detected: a phase of decrease (10-27%), a second phase of stabilization (27-45%) and a third phase of increase (45-70%). In the first phase, the continuous relative phase was negative and decreased: the two joints were thus activated later at 40 than at 20 spm, in proportion to their recovery phase durations. In the second phase, the continuous relative phase was stabilised between the 2 stroke rates: it indicates that the rower changed his elbow organization in the same proportion

than at 20 spm but with a delay. In the third phase, the continuous relative phase increased: the representative rower moved his elbow faster at 40 spm than at 20 spm in order to complete the elbow extension to place the rower in the catch position. CONCLUSIONS Results show that knee joint organization is un-modified although the stroke rate increases. In another way, this method of CRP calculation permit us to show that organizations of lumbar-sacral and elbow joints are modified, due to stroke rate increase. So, when CRP is computed with one joint angle produced at two different stroke rates, CRP permit the determination of the joint which provoke a disorganization of a more global inter-joint coordination. It could be hypothesized that this new use of CRP could be used to compare a joint organization modification in two different conditions. This preliminary study must be prolonged with more rowers in order to confirm the useful of this new method. ACKNOWLEDGEMENTS The authors would like to thank the rower for participating in the experiments. This research was supported financially by the European Community, the délégation Régionale à la Recherche et à la Technologie, the Ministère de l’Education Nationale, de la Recherche et de la Technologie, the Région Nord-Pas-de-Calais, the Centre National de la Recherche Scientifique and the competitive pole of CISIT. REFERENCES [1] Kelso JA, Phase transitions and critical behavior in human bimanual coordination, Am J Physiol Regul Integr Comp Physiol, 246(6), R1000-R1004, 1984. [2] Krasovsky T, et al., Toward a Better Understanding of Coordination in Healthy and Post stroke Gait, Neurorehabil Neural Repair, 24(3), 213-224, 2010. [3] Soper C, et al., Towards an ideal rowing technique for performance: the contributions from biomechanics, Sports Med, 34(12), 825-848, 2004. [4] Nolte V, Rowing Faster (Nolte Volker ed.). Champaign IL: Human Kinetics Publisher, Inc, 2005. [5] Découfour N, et al., Variations in expert rower coordination when rate increases on ergometer concept2®, Computer Methods in Biomechanics and Biomedical Engineering, 11(1), 79 – 80, 2008. [6] Colloud F, et al., Symmetry in rowing air-braked ergometers, Journal of Biomechanics, 39(Supplement 1), S458, 2006. [7] Hamill J, et al., A dynamical systems approach to lower extremity running injuries. Clinical Biomechanics, 14(5), 297-308, 1999.