Author(s): LECOCQ, T., MOCHIZUKI, L., TORDI, N., Institution: UNIVERSITÉ DE FRANCHE COMTÉ, Country: FRANCE, Abstract-ID: 1648

INTRODUCTION:
Still rings routine has evolved during the last century toward more and more strength. The gymnast’s relative strength is paramount to overcome the effect of gravity with his own upper limbs’ features. Authors measured upper limb relative strength with force plates placed under the gymnast (Bango, 2013, 2017) or under his hands’ palm (Dunlavy, 2007) in various positions. Despite being the gold standard, force plates are big, heavy, and expensive pieces of material for a gymnastic association interested in the relative strength measurement. The aim is to quantify how accurately a pair of strain gauges placed in the cables can measure the vertical force produced by gymnasts.
METHODS:
14 gymnasts (32 ± 11 years, 74,5 ± 7,1 kg, 172,6 ± 6,4 cm) performed 19 maximal isometric contractions in the iron cross position. After measuring the gymnast’s arm span with shoulder at 90° and hands in the rings, the following instruction were given “Try to lift your body for 5 seconds by pushing your hands down while maintaining straight arm”. Gymnasts stood on two force plates (K-Deltas, Kinvent, Montpellier) to measure the vertical ground reaction force (vGRF) at rest and during the maximal contraction task. The cable load (Tension) was measured by a pair of one-dimension strain gauges (K-Pull, Kinvent, Montpellier) attached to the rings cable, rigged on a high bar, 50 cm apart. vGRF and Tension were synchronously recorded on a tablet with the manufacturer software at a frequency of 1 kHz. Signals were smoothed with a 100ms window moving average. The angle between the vertical and the cable was calculated using : Angle=sin-1(((Armspan-50)/2)/(Cable length)) and the vertical component of the cable force was corrected with : vForce = (cos(Angle))×Tension. Right and left vForce were summed (total vForce) and a mean value over 2-seconds in the middle of the contraction was compared with vGRF. The linear correlation between both methods’ mean value (total vForce and vGRF) were calculated using Pearson´s r. Bland-Altman plot was utilized to check the agreement and paired-sample Student T-Test to quantify the differences between total vForce and vGRF.
RESULTS:
Total vForce and vGRF are correlated (R=0.99). The strain gauge method presents a systematic error of -0.42 kg (P < 0.001) and a 95% confidence interval of 1.15 kg compared to the force plate.
CONCLUSION:
The systematic under-estimation might be explained by the discrepancy between the arm span measurement position and the maximal contraction position. By removing 5 cm to all athletes’ arm span, the systematic error decreases to -0.12 kg (P = 0.068), with a 1.02 kg confidence interval. The 2.5 cm reduction on each side could be explained by scapula retraction, slight arms bending and hands position modification. The strain gauges are a valid tool to measure the upper limb relative strength provided that the arm span doesn’t change during the contraction since it affects the angle of the cable and therefore the vertical component of the force.