Annette M. Gabaldón

Mechanical function of two ankle extensors in wild turkeys: shifts from energy production to energy absorption during incline versus decline running.

SUMMARY We investigated the mechanical function of two ankle extensor muscles, the lateral gastrocnemius (LG) and peroneus longus (PL), in wild turkeys Meleagris gallopavo during steady speed running. We hypothesized that mechanical work output of the LG and PL during running parallels the demand for mechanical work on the body. The turkeys ran on level, inclined (+6°, +12°) and declined (–6°, –12°) treadmills to change the demand for mechanical work. Simultaneous measurements of muscle length (from sonomicrometry) and muscle force (from tendon strain gauges) were used to calculate mechanical work output. During level running at a speed of 2 m s–1, the LG and PL were both active in stance but produced peak force at different times, at approximately 21% of stance duration for the LG and 70% for the PL. The LG and PL also had different length patterns in stance during level running. The LG underwent little shortening during force production, resulting in negligible net positive work (2.0±0.8 J kg–1). By contrast, the PL produced force across a stretch–shorten cycle in stance and did significant net positive work (4.7±1.6 J kg–1). Work outputs for both the LG and PL were directly proportional to running slope. When we increased the demand for net positive work by running the turkeys on an incline, the LG and PL increased stance net positive work output in direct proportion to slope (P<0.05). Stance net positive work output increased to 7.0±1.3 J kg–1 for the LG and 8.1±2.9 J kg–1 for the PL on the steepest incline. Increases in stance net positive work for the LG and PL were associated with increases in net shortening strain and average shortening velocity, but average force in stance remained constant. The LG and PL muscles were also effective energy absorbers during decline running, when there is demand for net negative work on the body. During decline running at 2 m s–1 on the steepest slope, the LG absorbed 4.6±2.2 J kg–1 of net work in stance and the PL absorbed 2.4±0.9 J kg–1 of net work. Shifts in muscle mechanical function from energy production during incline running to energy absorption during decline running were observed over a range of running speeds from 1–3 m s–1 for both the LG and PL. Two fundamentally different mechanisms for changing work output were apparent in the mechanical behavior of the LG and PL. The LG simply altered its length pattern; it actively shortened during incline running to produce mechanical energy and actively lengthened during decline running to absorb mechanical energy. The PL changed mechanical function by altering its length pattern and by shifting the timing of force production across its stretch–shorten cycle. During incline running, the PL produced force during late stance shortening for positive work, but during decline running, the timing of force production shifted into early stance, to align with lengthening for negative work. In addition, during decline running, the PL greatly reduced or eliminated late stance shortening, thus reducing the potential for positive work. Our results show that the changing demands for whole body work during steady speed running are met, at least in part, by an ability of single muscles to shift mechanical function from net energy production to net energy absorption.

Interpreting muscle function from EMG: lessons learned from direct measurements of muscle force

Electromyography is often used to infer the pattern of production of force by skeletal muscles. The interpretation of muscle function from the electromyogram (EMG) is challenged by the fact that factors such as type of muscle fiber, muscle length, and muscle velocity can all influence the relationship between electrical and mechanical activity of a muscle. Simultaneous measurements of EMG, muscle force, and fascicle length in hindlimb muscles of wild turkeys allow us to probe the quantitative link between force and EMG. We examined two features of the force-EMG relationship. First, we measured the relaxation electromechanical delay (r-EMD) as the time from the end of the EMG signal to time of the end of force. This delay varied with locomotor speed in the lateral gastrocnemius (LG); it was longer at slow walking speeds than for running. This variation in r-EMD was not explained by differences in muscle length trajectory, as the magnitude of r-EMD was not correlated with the velocity of shortening of the muscle during relaxation. We speculate that the longer relaxation times at slow walking speeds compared with running may reflect the longer time course of relaxation in slower muscles fibers. We also examined the relationship between magnitude of force and EMG across a range of walking and running speeds. We analyzed the force-EMG relationship during the swing phase separately from the force-EMG relationship during stance phase. During stance, force amplitude (average force) was linearly related to mean EMG amplitude (average EMG). Forces during swing phase were lower than predicted from the stance phase force-EMG relationship. The different force-EMG relationships during the stance and swing phases may reflect the contribution of passive structures to the development of force, or a nonlinear force-EMG relationship at low levels of muscle activity. Together the results suggest that any inference of force from EMG must be done cautiously when a broad range of activities is considered.

Muscle strain is modulated more with running slope than speed in wild turkey knee and hip extensors.

SUMMARY We examined the length changes and electromyographic (EMG) activity of two hindlimb muscles in wild turkeys, to determine how these muscles modulate mechanical function with changes in running speed and slope. The muscles studied were the iliotibialis lateralis pars postacetabularis (ILPO), a biarticular knee and hip extensor, and the femorotibialis lateralis (FT), a knee extensor. Muscle length changes were recorded using sonomicrometry, and EMG activity was recorded from indwelling bipolar electrodes as the animals walked and ran at a range of speeds (1–3.5 m s–1). Treadmill slope was also varied, from a 12° uphill slope to a downhill slope of –12°. To test the hypothesis that the strain pattern in active muscles reflects the demand for mechanical work, we compared strain in the ILPO and FT across the range of slopes. Both muscles underwent active lengthen–shorten cycles during stance. We analyzed the lengthening and shortening part of the strain pattern separately to determine the response of muscle strain to surface slope. In both muscles stance phase shortening strain increased over the range of slopes studied, from 7.8±3.5% (ILPO) and 1.9±2.2% (FT) during downhill running at –12°, to 30.3±3.9% (ILPO) and 8.2±5.6% (FT) during uphill running at 12°. Stance-phase lengthening strain was also modulated with slope, from– 15.6±3.2% (ILPO) and –22.1±9.6% (FT) during downhill running at –12°, to –4.2±2.5% (ILPO) and– 9.0±5.6% (FT) during uphill running at 12°. The results suggest that for the ILPO and FT a change in net mechanical work output with running slope is likely mediated by a change in both the lengthening, energy absorbing portion of the contraction and the shortening, energy producing part of the contraction. We also found changes in the timing of EMG activity, and the relative portion of the stance period spent lengthening, which were consistent with a shift in muscle function from energy absorption during downhill running, to net energy production during uphill running. Generally, muscle strain was less affected by speed than by slope. Shortening strains were not significantly correlated with running speed. Only FT lengthening strain changed significantly with speed, ranging from– 6.8±4.3% at 1 m s–1 to –15.3±4.7% at 3.5 m s–1. The consistent patterns of strain changes with running slope are evidence that strain pattern is modulated to meet the changes in demand for net mechanical work. The relatively poor relationship between strain and running speed may reflect the fact that changes in running speed during level running are not associated with a change in demand for net mechanical work. Taken together, the speed and slope results suggest that the demand for mechanical work is an important determinant of muscle length patterns in running and walking.

Thermoregulation during Aging

Old mammals exhibit alterations in their thermoregulatory system that compromise their ability to maintain homeothermy when encountering hot or cold environments. Despite the difficulties in obtaining consistent data on humans, there are now numerous examples of age-related changes in both behavioral and physiological responses, the latter involving heat production as well as heat conservation/dissipation. Studies on old laboratory rodents provide further evidence for blunted thermoregulatory effectiveness with age, for gender differences in cold tolerance, for individual variation (even in in bred strains), and for the concept that the mechanisms underlying age-related changes in thermal responses do not occur in a linear manner. This chapter discusses studies evaluating the effects of age on specific thermal responses/effectors in humans as well as studies on laboratory rodents where mechanistic questions have been addressed.