LAPPIN, AK; MONROY, JA; PILARSKI, JQ; PIEROTTI, DJ; ZEPNEWSKI, ED; NISHIKAWA, KC; Northern Arizona University; Northern Arizona University; Northern Arizona University; Northern Arizona University; Northern Arizona University; Northern Arizona University; ; : Modeling ballistic jaw movements using equations of motion for a damped mass-spring system. II. Results.
During ballistic tongue projection, the mm. depressor mandibulae of toads produce high mass specific power (>7 W/g). To account for this high power output, we performed a series of experiments on individual toads, including high-speed (1000 Hz) image analysis with simultaneous EMG recordings from the mm. depressor mandibulae, force-lever experiments, and anatomical measurements. These data were used to model the feeding apparatus as a damped mass-spring system. Image analysis demonstrates that the mm. depressor mandibulae shorten at >20 ML/s during fast mouth opening, and shorten up to 20% of resting length in 18 ms. The duration of EMG activity in the mm. depressor mandibulae that precedes mouth opening varies from 60-250 ms. Jaw velocity and displacement increase with the duration of muscle pre-activation. Force-lever experiments demonstrate that displacement from equilibrium is 26.5 % after 200 ms of isometric pre-activation and 8.8% after 50 ms at in vivo loads of 2.5 � 4.5 grams. Stiffness was 100 �120 N/m at in vivo loads. We used the characteristic equation for an under-damped mass-spring system to model fast mouth opening. Three parameters (mass, stiffness, and displacement) were estimated from experimental results. The damping coefficient was the only “free” parameter and was assumed to be close to the critical value. Analyses demonstrate that the model is very sensitive to displacement, fairly sensitive to stiffness and damping coefficient, and least sensitive to mass. The fit between data and model was very good (r2 >0.999). The major implication is that isometrically pre-activated muscle behaves as though it has experienced a large strain, and the strain increases with the duration of pre-activation.
