An increased level of lactic acid in the body may result from strenuous exercise or other conditions of vital organs such as heart failure, sepsis, liver and lung disease. A standard way of measuring lactate is through blood sampling. The level of lactate in the blood plays a critical role in the performance of muscles when the body is under acute stress.
A noninvasive measurement of lactate level in the body will be of great benefit in many clinical situations, home care, and in the field of sports. It is known that muscle contraction leads to a physiological tremor when the body is under stress, hence, a measure of these muscle tremors can be an indicator of blood lactate level.
The authors of this Biomedical Optics Express article have come up with a noncontact and noninvasive method to measure the tiny movement of skin surface caused by the muscle tremors while under stress, thereby correlating the muscle movements to lactate levels in the body with an optical method based on speckle interferometry. Secondary speckle patterns have been used earlier for remote sensing of blood pressure, intraocular pressure, heart beating, breathing, and chemical concentrations of alcohol and glucose, to name a few. This work focuses on the measurement of muscle tremors under high level of stress and examines its relationship with lactate levels in the blood.
When illuminated with a laser beam, the human skin reflects the light in the form of speckle. The authors used a camera at some distance from the body to collect the scattered light from the skin. The temporal variations of the speckle patterns recorded in this configuration represent the Fourier transform of the speckle distribution on the skin. Any twitch of a muscle (skin vibration) adds a phase to the reflected wavefront of light that translates to a shift in the spatial frequency domain. The temporal oscillations of the skin are then proportional to this shift in the speckle pattern in spatial frequency domain. The temporal frequency of the skin vibration is obtained from the peak in the Fourier transform domain corresponding to the temporal displacement of the speckle pattern between consecutive frames. The amplitude of this peak varies as a function of the amount of lactic acid that changes in response to some physical activity.
The results of this method, when applied to human subjects, show a strong correlation between the amplitude of muscle tremor signal and an elevated lactate level. A drop in the lactate level, as observed from the data, suggests a state of muscle fatigue that supports a connection between the two physiological responses.
The proposed method is a preliminary study on measurement of lactate levels in the blood remotely from the body. A rigorous quantitative study is needed before this method can be implemented in practice. As the authors of this article indicate, there remains a significant difference in the amplitude of the measured levels of lactate between different subjects with close lactate levels.
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