Table 4.

Twitch characteristics of muscle utilized for k_tr experiments

	Fdev,mN/mm2	Frest, mN/mm2	TTP, ms	+dF/dt mN·mm−2·s−1	+dF/dt/F, s−1	RT50, ms	−dF/dt, mN·mm−2·s−1	−dF/dt/F, s−1
Nonfailing
Lopt	23.2 ± 1.7	7.5 ± 0.7	212.6 ± 12.1	179.8 ± 16.6	7.8 ± 0.5	137.0 ± 4.0	−127.6 ± 9.1	−5.5 ± 0.2
L95	9.3 ± 1.1*	4.2 ± 0.5*‡	174.7 ± 8.7*	84.8 ± 11.8*	9.0 ± 0.5*	123.9 ± 5.6*	−51.8 ± 5.4*	−5.7 ± 0.2
L90	4.9 ± 0.6*†‡	2.4 ± 0.3*†‡	155.4 ± 7.1*†‡	48.6 ± 8.1*†‡	9.5 ± 0.7*‡	114.7 ± 5.3*‡	−30.8 ± 3.7*†‡	−6.3 ± 0.3*†‡
Lopt,repeat	8.9 ± 1.1*	6.8 ± 0.8	181.0 ± 8.7*	79.1 ± 12.1*	8.6 ± 0.5*	126.7 ± 3.7	−49.0 ± 5.7*	−5.6 ± 0.2
Failing
Lopt	18.5 ± 3.0	10.6 ± 1.6	181.0 ± 10.8	154.8 ± 21.0	8.8 ± 0.5	136.4 ± 5.9	−96.6 ± 15.3	−5.3 ± 0.2
L95	10.8 ± 2.3*‡	5.7 ± 1.4*‡	159.3 ± 8.8*‡	96.7 ± 17.2*	9.6 ± 0.5*‡	120.4 ± 4.4*	−59.8 ± 12.9*	−5.5 ± 0.2
L90	7.3 ± 1.7*‡	3.7 ± 1.8*‡	147.7 ± 8.3*‡	71.7 ± 14.1*‡	10.4 ± 0.5*†‡	109.8 ± 2.7*‡	−43.8 ± 9.3*‡	−6.3 ± 0.3*†‡
Lopt,repeat	14.9 ± 3.4	10.6 ± 2.6	177.3 ± 10.5	127.6 ± 27.3	8.9 ± 0.5	126.0 ± 6.1	−78.9 ± 17.7	−5.3 ± 0.3

All parameters were means ± SE (measured at 1 Hz); n = 9 nonfailing, n = 10 failing (1 trabecula/heart). F_dev, active developed force (i.e., total minus resting); F_rest, resting force (lowest force during a twitch); +dF/dt/F, maximal velocity of contraction normalized to active developed force; −dF/dt/F, maximal velocity of relaxation normalized to active developed force. P < 0.05 as determined with ANOVA for repeated measures with Bonferroni post hoc analysis:

^*vs. L_opt,

^†vs. L₉₅,

^‡vs. L_opt,repeat of the same group.