Kinetics and energetics during exercise: A model evaluation

During exercise the skeletal muscular tissues are exposed to both mechanical and metabolic loading. In 2000, a simple biomechanical model was proposed in order to assess both mechanical and metabolic loading of musculoskeletal system during load carrying horizontally, uphill and downhill [1]. The aim of the present study was to test this model for prediction of whole-body energy turnover from kinematic and anthropometric data during running on an elliptic ergometer. Experimental protocol Three healthy, non smoking, male volunteers [age 25±1y, height 1.79±0.01m and weight 75±3Kg] participated in the study. The participants were instructed to arrive at the laboratory rested and well-hydrated, at least 3 hours after their last meal and at least after 48 hours after training. Before the tests, participants were asked to lie in quite position for 10 minutes and energy expenditure rate at rest was measured by indirect calorimetry. All participants performed running tests on an elliptic cardiovascular training equipment (Vario, Technogym, Italy) at two frequencies (90, 100 spm) and two power levels (1, 5). Load levels were fixed under their estimated anaerobic threshold. Each combination of frequency and load was maintained for 6 minutes, and 6 minutes of rest were imposed between trials. During the tests, kinematic (SmartE, BTS, Milan, Italy) and gas exchange data (Quark b2, Cosmed, Italy) were acquired. Caloric consumption was calculated using gas exchange data, through indirect calorimetry [2]. Energy expenditure rate at rest was subtracted in order to obtain net energy consumption required for the exercise. Body segment trajectories were obtained from kinematic data using CAST [3]. A 12 segment model of the subject was reconstructed. Changes in the total body energy, , result when work is done by the subjects; an increase in the curve of the energy level with time corresponds to a phase of positive work; a decrease in the curve to a phase of negative work. It is assumed that energy transfers both between body segments and within body segments and that the muscles have to perform both the positive (slope of >0) and the negative work (slope of <0). Total muscular metabolic task cost was calculated according to [1]. For this calculation the efficiency for positive work was set to 25% and for negative work to -120%. A resting Oxygen uptake of 0.21 l O2 min-1 was added giving the total muscle metabolic rate. Model results of energy expenditure were smaller but close to the values obtained experimentally. However, at the higher frequency, the model underestimated more measured Oxygen consumption. Preliminary results obtained in comparing model predictions with experimental data are promising. The underestimation of energy expenditure predicted by the model at the higher frequency can be explained with an increase in energy consumption of the non muscular mass with movement velocity [4]. In the work by Laursen [1] results were totally in agreement with experimental data: this may be due to the fact that the model was applied to walking uphill, downhill and horizontally, at the same relatively low speed.