Force production of the genioglossus as a function of muscle length in normal humans

Resting muscle length affects both maximum force production and force maint enance. The strength and force maintenance characteristics of the genioglos sus as a function of resting muscle length have not been described. We hypo thesized that genioglossus optimum length (L-o) could be defined in vivo an d that the ability of the genioglossus to sustain a given workload would de crease as resting length deviated from L-o. To test this, 11 normal men rep eated maximal isometric genioglossus protrusions at different muscle length s to determine L-o. L-o was also obtained by using submaximal efforts while simultaneously recording electromyographic activity of the genioglossus, w ith L-o defined as the length at which the force-to-genioglossus electromyo graphic activity ratio was maximum. Both methods provided similar results. Force maintenance was measured at four muscle lengths on separate days. Tar get efforts representing 60% of each subject's maximum at L-o and lasting 5 s were performed at 12-s intervals. Time limit of endurance of the geniogl ossus was defined as the time from trial onset at which 90% of the target c ould not be maintained for three consecutive efforts. Time limit of enduran ce was greatest at L-o and fell to 47.5% at L-o + 1 cm, 53.8% at L-o - 1 cm , and 47.4% at L-o - 1.5 cm. We conclude that L-o of the genioglossus can b e determined in vivo and that force maintenance of the genioglossus is decr eased when operating length deviates from L-o.