Muscle response while holding an unstable load

Objective. Basic biomechanical measurement was applied to explore the physical risk associated with holding an unstable load. Design. We investigated muscle response, before and after the initiation of the balance-recovery responses from perturbation when handling unstable loads. Background. Most studies of muscle response and postural control during load handling focused on stable (fixed) loads. Investigation of muscle response while handling an unstable load deserves similar attention. Methods. Twelve male subjects experienced time-variant loads (18 kg), with additional sudden 12 kg m/s impact momentum, under three different load-shift conditions (stable, posterior-to-anterior rolling, or anterior-to-posterior rolling load), and in three foot placements (straddle, wide, and narrow stance). Normalized electromyography (expressed as percentage of maximum voluntary contraction) for the brachioradialis, biceps, erector spinae, and hamstrings were measured. Results. The maximal exertions for brachioradialis (91.0%) and biceps (42.5%) were observed approximately 0.1 s after posterior-to-anterior impact, with maximal measured contractions for erector spinae (57.2%) and hamstrings (59.9%) noted approximately 0.2 s after impact. Conclusions. The central nervous system detects and responds to the need to stabilize the joints close to the location of load-shift perturbation, with these responses determined according to the anatomical and functional limitation of each biomechanical link.