Reproducibility of Blood Lactate Concentration Rate under Isokinetic Force Loads

(1) Background: Maximum isokinetic force loads show strongly increased post-load lactate concentrations and an increase in the maximum blood lactate concentration rate ([Formula: see text] La(max)), depending on load duration. The reproducibility of [Formula: see text] La(max) must be known to be able to better assess training-related adjustments of anaerobic performance using isokinetic force tests. (2) Methods: 32 subjects were assigned to two groups and completed two unilateral isokinetic force tests (210° s(−1), Range of Motion 90°) within seven days. Group 1 (n = 16; age 24.0 ± 2.8 years, BMI 23.5 ± 2.6 kg m(−2), training duration: 4.5 ± 2.4 h week(−1)) completed eight repetitions and group 2 (n = 16; age 23.7 ± 1.9 years, BMI 24.6 ± 2.4 kg m(−2), training duration: 5.5 ± 2.1 h week(−1)) completed 16 repetitions. To determine [Formula: see text] La(max), capillary blood (20 µL) was taken before and immediately after loading, and up to the 9th minute post-load. Reproducibility and variability was determined using Pearson and Spearman correlation analyses, and variability were determined using within-subject standard deviation (S(w)) and Limits of Agreement (LoA) using Bland Altman plots. (3) Results: The correlation of [Formula: see text] La(max) in group 1 was r = 0.721, and in group 2 r = 0.677. The S(w) of [Formula: see text] La(max) was 0.04 mmol L(−1) s(−1) in both groups. In group 1, [Formula: see text] La(max) showed a systematic bias due to measurement repetition of 0.02 mmol L(−1) s(−1) in an interval (LoA) of ±0.11 mmol L(−1) s(−1). In group 2, a systematic bias of −0.008 mmol L(−1) s(−1) at an interval (LoA) of ±0.11 mmol L(−1) s(−1) was observed for repeated measurements of [Formula: see text] La(max). (4) Conclusions: Based on the existing variability, a reliable calculation of [Formula: see text] La(max) seems to be possible with both short and longer isokinetic force loads. Changes in [Formula: see text] La(max) above 0.11 mmol L(−1) s(−1) due to training can be described as a non-random increase or decrease in [Formula: see text] La(max).