Reliability of neuromuscular measurements during explosive isometric contractions, with special reference to electromyography normalization techniques

INTRODUCTION: This study determined the between-session reliability of neuromuscular measurements during explosive isometric contractions, with special consideration of electromyography (EMG) normalization. METHODS: Following familiarization, 13 men participated in 3 identical measurement sessions involving maximal and explosive voluntary contractions of the knee extensors, while force and surface EMG were recorded. Root mean square EMG amplitude was normalized to different reference measures: (evoked maximal M-wave peak-to-peak amplitude and area, maximum and sub-maximum voluntary contractions). RESULTS: Explosive voluntary force measurements were reliable on a group level, whereas within-subject reliability was low over the initial 50 ms and good from 100 ms onward. Normalization of EMG during explosive voluntary contractions, irrespective of the reference method, did not reduce the within-subject variability, but it did reduce substantially the variability between-subject. CONCLUSIONS: The high intra-individual variability of EMG and early phase explosive voluntary force production may limit their use to measuring group as opposed to individual responses to an intervention.