To investigate in vivo mechanics of the guinea fowl lateral gastrocnemius (LG) muscle during obstacle negotiation while running on a treadmill, we used mouse extensor digitorum longus (EDL) muscles in ex vivo experiments with in vivo strain inputs from perturbed and steady strides obtained in a previous study. In vivo strain trajectories from a stride down from obstacle to treadmill, two strides up from treadmill to obstacle, and a level stride with no obstacle, as well as a sinusoidal strain trajectory at the same amplitude and frequency, were used as inputs in work loop experiments. With five strain trajectories and three activation patterns, each muscle was used in a total of 15 work loop experiments. EDL forces produced using in vivo strain trajectories were more similar to in vivo LG forces (R² = 0.58 – 0.94) than to forces produced using the sinusoidal trajectory (average R² = 0.045). Given the same activation, in vivo strain trajectories produced consistent work loops that showed a shift in muscle function from more positive work during strides up from treadmill to obstacle to less positive work in strides down from obstacle to treadmill. Activation, strain trajectory, and activation*strain trajectory interaction had significant effects on all work loop variables, with the interaction having the largest effect on peak force and work per cycle. These results support the hypothesis that muscle is an active material whose viscoelastic properties are tuned by activation, and which produces forces in response to deformations of length associated with time-varying loads.

Extensor digitorum longus (EDL) muscles were surgically exposed, and 4-0 silk sutures were tied in square knots at the distal and proximal muscle-tendon junctions. Tendons were cut outside the suture knots, and muscles were removed for ex vivo experiments. Extracted EDL muscles (n = 6) were attached to a dual-mode muscle lever system (Aurora Scientific, Inc., Series 300B, Aurora, ON, Canada) via tightened slip knots in the suture at the ends of the muscle. During experiments, the muscles were bathed in a 21°C, 7.4 pH Krebs–Henseleit solution containing (in mmol l^–1): NaCl (118); KCl (4.75); MgSO₄ (1.18); NaHCO₃ (24.8); KH₂PO₄ (1.18); CaCl₂ (2.54); and glucose (10.0). The bath was aerated with a 95% O₂/ 5% CO₂ gas mixture. The muscle was suspended between two platinum electrodes which delivered 1 ms square-wave pulses at tetanic supramaximal stimulation (80 mV, 200 Hz) while finding optimal length (L0). Submaximal stimulation (45 mV, 90 Hz) was used during the experimental protocol to more closely emulate in vivo activation, and to decrease fatigue during experimental trials (James et al., 1995).

To investigate the function of the guinea fowl LG during these different types of strides, we used LG strain trajectories from four strides of a guinea fowl’s instrumented leg in ex vivo EDL work loop experiments: one stride in which the leg stepped down from the obstacle onto the treadmill (Fig. 1A; ‘Down’, yellow line); two in which the instrumented leg stepped up from the treadmill onto the obstacle (Fig. 1A; ‘Up1’ purple line and ‘Up2’ red line); and one in which no obstacle was present (Fig. 1A, ‘No obstacle’, blue line. We included two different Up trajectories because ‘Up1’ had a slight stretch in the strain trajectory (Fig. 1A; ‘Up1’, purple line), whereas ‘Up2’ did not (Fig. 1A; ‘Up2’, red line). Each of the strain trajectories was used in ex vivo EDL work loops at 2 Hz. For each muscle, we also included passive and active sinusoidal work loops at the same strain amplitude and frequency. Two work loop cycles were performed for each active strain trajectory, but only one cycle was performed for the passive sinusoidal strain trajectory. 

Activation patterns for the ex vivo EDL work loops were based on measured in vivo guinea fowl EMG, which typically starts at the longest muscle length at the onset of leg retraction (Daley and Biewener, 2011). All five strain trajectories were tested using three different activation patterns (Normal, Late and Long) at the same submaximal activation level (45 mV, 90 Hz). On average, submaximal activation produced 91% of the maximum isometric force observed during supramaximal activation. At activation levels lower than 45 mV and 90 Hz, the force in sinusoidal work loops was unfused. The onset and duration of the ‘Normal’ activation pattern (Fig. 2A) used in the ex vivo EDL work loops was based on observed electromyographic (EMG) data from the guinea fowl LG for the ‘Up1’ step. We accounted for the differing delay between activation and force onset, measured at ~25 ms in vivo (Daley and Biewener, 2011) versus only 4-5 ms in ex vivo experiments, by activating EDL muscles 20 ms later than observed in vivo. Thus, for ‘Normal’activation, stimulation began 20 ms after the longest muscle length and continued for a duration of 115 ms as observed in vivo during the ‘Up1’ step (Daley and Biewener, 2011). 

For ‘Late’ activation, (Fig. 2B), stimulation started at the onset of increased muscle shortening velocity, on average 32.5 ms later than for ‘Normal’ activation, for the same duration of 115 ms. ‘Long’ activation (Fig. 2C) spanned the other two, as the muscle was activated at the same time as ‘Normal’ activation, but was deactivated at the same time as ‘Late’ activation, for an average duration of 148.4 ms. The order in which strain trajectories and activation patterns were performed was randomized for each muscle. In total, 15 experiments were performed on each muscle, including all combinations of 5 strain trajectories and 3 activation patterns.

The 7 data files are in.csv format.