Balancing training load and recovery is crucial for optimal soccer performance. While monitoring devices help optimize performance, most devices focus on trunk-worn sensors for whole-body load indicators, therefore specific insights for the lower extremities are lacking. This potentially leads to underestimation of physical load. Therefore, this thesis aimed to evaluate the construct validity and reliability of new training load indicators derived from the Smart Sensor Short in field-based settings, and subsequently, to use this technology within a soccer-specific practical context. The Smart Sensor Short includes sensors (IMUs) on the lower back, both thighs, and both shanks, which allows to collect data on the soccer pitch. New training load indicators for the lower extremities were developed that can be used simultaneously with whole-body training load indicators. Several field tests were conducted with soccer players from different playing levels. The studies showed that the lower extremity training load indicators are unrelated to the whole-body load indicators. This indicates that the lower extremity training load indicators offer complementary insights into training load, which demonstrates the potential to use the Smart Sensor Short together with traditional methods for training load quantification. Furthermore, the biomechanical load indicators were sensitive for changes in movement intensities and to discriminate activities, thereby supporting their validity and use in practice. Discriminative validity was only partially confirmed in the standardised training drill. Lastly, the potential to use the Smart Sensor Short in practice is demonstrated during on-field rehabilitation sessions, which may, in future, provide more detailed insights in soccer performance.