A METHOD TO EVALUATE INTRACYCLE PROPULSIVE FORCE AND BODY VELOCITY CHANGES

  • Federico ALVES
  • P. Santos
  • A. Veloso
  • I. Pinto Correia
  • J. Gomes-Pereira

Abstract

Propulsive force in swimming show a periodical variation during each strike, resulting in either acceleration or deceleration of the body. Development of a computer-based system to measure force and velocity during- semi-tethered swinimill-g enables us to quantify intracycle variations and obtain an individual diagnosis of swimmiilg proficiency. The testing apparatus consists of a 314" shaft supported oh radial bearings in which a 4" barrel is welded. The measurements were effectuated' using 15 meters of stainless steel light cable coiled around the barrel, with its free end connected to a harness belt attached to the swimmer's waist. The force generated against the cable by the swimmer when moving away from the apparatus was sensed by a force transducer located between the harness belt and the cable, and converted to a proportional voltage output. The data is relayed to a computer through an a/d converter and processed to calculate force for each swimming stroke or for any given time period. Swimming velocity was measured based on the rotational velocity of the wheel. An adapted potentiometer was attached to an end of the shaft. The angle signal is also interfaced and converted to a digital signal. At the other end an electromagnetic brake was mounted, to prevent turning velocity of the wheel to fluctuate due to its own inertia, keeping the cable always stretched. A constant force of 30 N was necessary to initiate the rolling of the barrel. This procedure does not primarily aim at the quantitative acquisition of the overall velocity and force values, but at a qualitative evaluation of the intracycle fluctuations.