Increasing compliance to instructions in the squat jump.

The purposes of this investigation were to evaluate the occurrence of a small amplitude counter-movement (SACM) in SJ (squat jump) trials of elite athletes to determine the efficacy of gross observation and the use of a portable position transducer to determine whether or not a SACM occurred. The subjects (N = 30, 20.1 +/- 3.0 years, 199.0 +/- 8.4 cm, and 87.2 +/- 9.5 kg) were a combination of high-performance (National Team) and elite athletes (Olympian) from the sports of athletics, swimming, and volleyball. All subjects performed SJ trials on a force platform, with a linear position transducer attached to a bar placed across the shoulders. Subjects performed the SJ from a depth that allowed for a 90 degrees knee angle, with the subject's instructed to maintain a 3-second isometric hold preceding the concentric action of the jump. One hundred twenty-five SJ trials were observed for a SACM and analyzed (using the force plate data and position transducer data) for a SACM. Of the 125 SJ trials, 69 trials (55.2%) were observed to have a SACM by the researchers. In the remaining 56 trials, 43 of these trials contained a force unload (>/=10% body mass) before initiation of the concentric action, indicating a SACM. Of the 119 SJ trials where a force unload was observed and detected by the force-time graph, 118 (99.2%) of these trials also showed a change in displacement using the displacement-time graph from the linear position transducer. The results of this study indicate that achieving compliance to protocol in the SJ is difficult, and that gross observation is inadequate in detecting a SACM in the SJ. From a practical perspective, these results suggest that using a force plate or a linear position transducer would allow the strength and conditioning coach to ensure compliance to instructions in the SJ.