Objectives: Unlike physiological loads, the biomechanical loads of training in running-based sports are still largely unexplored. This study, therefore, aimed to assess the validity of estimating ground reaction forces (GRF), as a measure of external whole-body biomechanical loading, from segmental accelerations. Methods: Fifteen team-sport athletes performed accelerations, decelerations, 90◦ cuts and straight running at different speeds including sprinting. Full-body kinematics and GRF were recorded with a three-dimensional motion capture system and a single force platform respectively. GRF proﬁles were estimated as the sum of the product of all ﬁfteen segmental masses and accelerations, or a reduced number of segments. Results: Errors for GRF proﬁles estimated from ﬁfteen segmental accelerations were low (1–2 N kg−1) for low-speed running, moderate (2–3 N kg−1) for accelerations, 90◦ cuts and moderate-speed running, but very high (>4 N kg−1) for decelerations and high-speed running. Similarly, impulse (2.3–11.1%), impact peak (9.2–28.5%) and loading rate (20.1–42.8%) errors varied across tasks. Moreover, mean errors increased from 3.26 ± 1.72 N kg−1 to 6.76 ± 3.62 N kg−1 across tasks when the number of segments was reduced. Conclusions: Accuracy of estimated GRF proﬁles and loading characteristics was dependent on task, and errors substantially increased when the number of segments was reduced. Using a direct mechanical approach to estimate GRF from segmental accelerations is thus unlikely to be a valid method to assess whole-body biomechanical loading across different dynamic and high-intensity activities. Researchers and practitioners should, therefore, be very cautious when interpreting accelerations from one or several segments, as these are unlikely to accurately represent external whole-body biomechanical loads.