Figure 3. Average passive stress and normalized active twitch force response to stretch.

The Slow Force Response (SFR) was recorded in WT (closed circles; n=5) and HM (closed squares; n=8) at [Ca²⁺]_o=0.4 mM (which results in each muscle group ~50% of maximum twitch force). Twitch force was first recorded at the baseline diastolic sarcomere length (SL=2.1 μm) for 1.5 mins; at time=0 min, the muscles were quickly stretched during diastole to increase diastolic sarcomere length to SL=2.3 μm (ΔSL=0.2 μm), resulting in an immediate increase in both passive (left panel) and active twitch force (right panel). In the right panel, active twitch force is normalized to peak twitch force of the first elicited twitch following the stretch. Presence of the giant isoform of titin in the HM muscles was associated with significant lower passive stress development following stretch and a ~50% lower final magnitude of the SFR in HM muscles. The kinetics of the SFR, however, were not affected (see also Table 2). [Ca²⁺]_o=0.4 mM; stimulus frequency=1 Hz; temperature=25 °C.