Beginning of Strokes Preparation (StPrB)

Strokes preparation phase includes underwater lower limbs actions (except for breaststroke, which includes one lower limb action and one upper limb cycle). Detection method for sacrum, head, and right wrist is threshold-based and the idea is using thresholds on peak magnitude, peak prominence or signal variation depending on sensor location (for sacrum; EXT(|Acc_x|, TH_SPSA = g, TH_SPSAvar = 0.06 g), for head; EXT(|Acc_y|, TH_SPHE = −0.5 g, TH_SPHEprom = 0.1 g), for right wrist; EXT(|Acc|, TH_SPRW = −0.9 g). On right shank, the first positive peak of Acc_y is StPr_B for backstroke, while for other swimming techniques, the peak is detected with EXT(|Acc_x|, TH_SPRS = 1.3 g) and then the next sample on |Acc| passing from g is StPr_B.