Keywords: sports biomechanics, step exercise, kinetic analysis, inverse dynamics, pressure insoles.

Recreational Exercise aiming to improve or maintain health and fitness constitutes a group of physical activities performed by a large number of participants worldwide, independently of age and physical or health status. The main objectives of these physical activities are to provide healthy mechanical and metabolic stimuli as well as self-enjoyment. The purpose of the present study was to determine mean and maximal magnitudes of ground reaction forces (GRF), joint reaction forces (JRF) and moments (M) in ankle, knee and hip joints, using inverse dynamics, and also plantar pressures using pressure insoles, concerning four Step Exercise techniques. Also, we intend to identify potential differences between different techniques (single and alternate leading leg, dominant and non dominant sides, propulsion and non propulsion steps), leading to further development in the future.

Using a 130 bpm music speed, and regular fitness shoes, these Step exercises were performed on an AMTI force platform (17 cm height) for stepping up and on a KISTLER force platform on the ground for stepping down. Two trials of each technique were selected for analysis. Digital images (JVC digital camera) and ground reaction forces (GRF) were collected. Data was processed using BIOPAC (Acqknowlede) and APAS (Analysis Performance Ariel System) software's. An analytical model of the lower limb was developed to estimate net joint reaction forces (JRF) and moments (M) on ankle, knee and hip joints, by inverse dynamics in right and left legs. Using the same music speed, these Step exercises were also performed on a Step Reebok bench platform (15 cm height) for stepping up and the ground for stepping down, using NOVEL-PEDAR pressure insoles. Seven trials of each technique were selected for analysis. These data was processed using NOVEL-PEDAR EXPERT software.

The GRF were obtained and the JRF in ankle, knee and hip joints were calculated for the right and the left legs, in four Step Exercise techniques (Basic Step, Run Step, Knee Lift and Knee Hop). Graphic analysis shows the different moments and force magnitudes obtained. Also, it allows the analysis of the pattern of force-absorbing adjustments, concerning the resultant GRF and JRF, in right and left legs. Concerning the four techniques, GRF are similar to JRF in the ankle. Ankle JRF decreases about 1%* (*mean value in both legs for the 4 techniques). JRF in the knee and hip, are smaller than GRF. Knee JRF decreases about 5%*. Hip JRF decreases about 15%*. GRF values found for Basic Step is similar to those obtained in previous studies. In general, GRF and JRF are greater during the action of leading leg; during propulsion phases; in step down phase of the leading leg; and during propulsion steps of the non dominant side. Maximal values for the M were found in the knee for Basic Step and Knee Hop Step. Moments are greater in techniques with propulsion. These relations need further assessment. Other parameter to assess mechanical load is the maximal values of plantar pressure (PP). Greater PP maximal values were found in the right leg (dominant side) and in the propulsion steps. However mean values for plantar pressure are greater in the non dominant side. Mean values are also greater in propulsion legs. As a general conclusion, concerning health bone, the values for GRF we found are all close to 2 BW, which seem to demonstrate that this physical activity induces to osteogenic process. Concerning the other loading parameters, potential differences between different techniques were identified (single and alternate leading leg, dominant and non dominant sides, propulsion and non propulsion steps), leading to further research. However, results suggest that this physical activity is not symmetric concerning mechanical load leading us to take special care in choosing exercises to be performed by people of different fitness status. Different force magnitudes and moments were obtained during different Step training techniques. In practical terms, Step exercise can provide a healthy mechanical stimulus, if safely performed, once its mechanical load is located between the amount of load provided by walk and run. However, if technique adaptations occur, especially on knee joint, together with greater GRF, JRF and M and decreased time for contact and force transfer, step frequency, bench height and step technique (the most important determinants of exercise intensity) should carefully be chosen in Step classes, considering participants experience in this activity and age.

The main goal for future studies is to develop a biomechanical model for the lower limb, including joint forces and moments, and muscle forces. The advantage of modelling is that this kind of studies provides more information about mechanical load over a large range of performance conditions, which will help to correct technique and prevent musculoskeletal disorders.

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