Temporal resolution and event visibility in smartphone assessment of drop-jump reactive strength

Abstract

To determine how frame timing and shoe–floor event visibility constrain flight-time-based reactive strength index (RSI) in smartphone drop-jump assessment. The index R = gF²/(8C) was evaluated for contact durations C = 120, 150, 180 and 240 ms; flight durations F = 400, 470 and 600 ms; and capture rates of 30, 60, 120 and 240 frames/s. First-frame assignment was examined by exact enumeration of camera phase. Separate time brackets for initial landing, rebound take-off and subsequent landing yielded score extrema through monotonicity. All results are analytical calculations; no participant measurements were collected. For C = 150 ms and F = 470 ms at 120 frames/s, the conservative sampling envelope was −8.59% to +9.67%, while exact phase enumeration gave −1.41% to +2.14%. Conservative 5% sufficient rates at the same contact duration were 241, 226 and 207 frames/s for 400-, 470- and 600-ms flights, respectively. Two calculated scores, 1.844 and 1.659 m/s, had overlapping one-frame event intervals at 120 frames/s. At 240 frames/s, their intervals separated by 0.031 m/s. Increasing only the second attempt’s take-off bracket from 4.167 to 8.333 ms restored overlap. The shared take-off event couples contact and flight errors. A capture-rate threshold bounds quantization, whereas event-time brackets determine whether the timing evidence establishes score ordering. Both calculations require a consistent event definition and an accurate capture time base.