Isolated Unweighted Fast Knee-Flexions Relate to Maximal Step Frequency and Sprint Speed in Female Track Athletes

We recently introduced an on-field test to quantify knee flexor fatigue in male soccer players using Peak Angular Acceleration of the shank (PAA) during isolated unweighted fast knee flexions. Unexpectedly, we found a positive relation between PAA and 45 m sprint performance in the unfatigued players. 

This study investigates (1) whether PAA is also positively related to sprint performance in a completely different population—16 female athletes— (2) whether it differs between the first and the second 30 m of a 60 m sprint, and (3) its relationship with step frequency and step length in both phases. Participants wore IMUs on both feet and the shank of the dominant leg. Time gates were placed at 30 and 60 m. Participants performed a series of six fast knee flexions (5s in between) with the dominant leg in a prone position on the floor. The three attempts with the highest PAA were used for analysis. Subsequently, they executed a maximal 60m sprint. 

The within-session reliability for PAA of the three fastest attempts was high, with a CV of 1.8% and ICC of 0.97. PAA had very strong significant (p<0.05) relations with step frequency (r≥0.72, p≤0.001), moderate (0-30m: r=-0.49, p=0.05) to small (30-60m: r=-0.22 p=0.42) negative relations with step length, and moderate (0-30m: r=0.49, p=0.06) to strong positive relations with speed (30-60m: r=0.57, p=0.02) 

In conclusion, PAA was moderately related to sprint speed during acceleration and at maximal speed. This was probably due to the very strong relations between PAA and step frequency.