NIR Spectroscopy of Muscle and Skin

There is a number of established methods for the assessment of human performance during sports and exercise, ranging from gas (O2 and CO2) analysis to blood samples. Near-infrared spectroscopy allows the non-invasive measurement of muscle oxygenation and therefore gives a measure for body metabolism.

In the figures the placement of the optical fibres is shown and the lab setting for performance tests during cycling and running. The challenge is to establish new methods in this sometimes rough environment.

Any new method raises the question of data quality. We addressed this by repeating the same cycling exercise trial three times on the same data. For each volunteer (example in the figure) the reproducibility was high with variations in muscle oxygen saturation SO2 better than 1.5 %.

The performance of the human body depends on the oxygen supply by the inhaled air. We tested the influence of altitude conditions on the oxygenation of the muscles of the leg during cycling. An example is shown for a volunteer where the oxygen saturation of the tissue is plotted versus the exercise power. There is a clear decrease both in maximal power and in oxygen saturation with increased altitude.

Here the calf and thigh muscle oxygen saturation is shown as a function of running velocity for healthy volunteers. There is a clear decrease with higher exercise power. The correlation with the whole body energy expenditure as measured by spirometry (volume of inhaled oxygen VO2) is good.

During running the energy efficiency of the body depends on the movement of the limbs, and the experienced athlete optimizes his or her style accordingly. It was tested whether the muscle oxygenation as measured by NIR spectroscopy as well as the body energy consumption is modulated by the step frequency when keeping the velocity constant. The data show that the method is able to discern small changes in tissue oxygenation.

During exercise both muscle and skin blood supply is modulated with the skin e.g. serving to remove heat. The figure shows an example of muscle and skin oxygen saturation and total haemoglobin concentration during an incremental cycling exercise. It is apparent that both organs are coupled, however, that at higher exercise levels there is a drainage of blood from skin to muscle.