Muscle strength testing: evaluation of tests of explosive force production.

The purpose of the study was to evaluate four tests of explosive force production (EFP). Specifically, the main aims of the study were to assess the reliability of different EFP tests, to examine their relationship with maximum muscle strength, and to explore the relationship between EFP tests and functional movement performance. After an extensive preliminary familiarization with the tasks, subjects ( n=26) were tested on maximum explosive strength of the elbow extensor and flexor muscle, as well as on rapid elbow extension and flexion movements performed in both an oscillatory and a discrete fashion. In addition to maximum force ( F(max)), four different EFP tests were assessed from the recorded force-time curves: the time interval elapsed between achieving 30% and 70% of F(max) ( F(30-70%)), the maximum rate of force development (RFD), the same value normalized with respect to F(max) (RFD/ F(max)), and the force exerted 100 ms after the contraction initiation ( F(100 ms)). Excluding F(30--70%), all remaining EFP tests revealed either good or fair reliability (intraclass correlation coefficients being within 0.8-1 and 0.6-0.8 intervals, respectively) which was also comparable with the reliability of F(max). RFD and F(100 ms) demonstrated a positive relationship with F(max), but not T(30-70%) and RFD/ F(max). Stronger elbow flexor muscles also demonstrated higher values of RFD and F(100 ms) than weaker elbow extensor muscles, while no difference was observed between either T(30-70%) or RFD/ F(max) recorded from two muscles. Despite the simplicity of the tested movement tasks, the relationship observed between the EFP tests and the peak movement velocity remained moderate and partly insignificant. It was concluded that most of the EFP tests could be reliable for assessing neuromuscular function in their muscle-force- (or, indirectly, muscle size) dependent (such as RFD and F(100 ms)), or muscle-force-independent ( T(30-70%) and RFD/ F(max)) forms. However, their "external validity" when applied to assess the ability to perform rapid movements could be questioned.