Presented at NACOB 98:
North American Congress on Biomechanics
Canadian Society for Biomechanics - American Society of Biomechanics

University of Waterloo
Waterloo, Ontario, Canada
August 14-18, 1998

PERFORMANCE OF RAT PLANTAR-FLEXOR MUSCLES DURING ISOMETRIC
AND SLOW CONCENTRIC CONTRACTIONS IN VIVO

M.E.T. Willems, W.T. Stauber
Department of Physiology, West Virginia University
Morgantown, WV 26506

INTRODUCTION

Dynamometry has been widely applied in humans to determine skeletal muscle performance in vivo but scarcely in animals. We determined the performance of rat plantar-flexor muscles in vivo during multipositional isometric and pre-loaded slow concentric contractions using a rat dynamometer and recording of force under the sole of the foot.

REVIEW AND THEORY

In human plantar-flexors, maximum torques during isometric and slow concentric contractions (angular velocity 0.52 rad·s-1) were nearly equivalent but compared at different ankle positions (e.g. Fugl-Meyer et al., 1980). Thus, it is not clear whether the force-position relationships during isometric and slow concentric contractions were similar. The hypothesis, that the force-ankle position relationships of rat plantar-flexors in vivo during isometric and slow concentric contractions were similar, was tested.

PROCEDURES

Female Sprague Dawley rats (n = 6, body mass; 268 ± 6 g) were positioned with flexed knee (1.57 rad) and the left foot pressing on an aluminum plate connected to a custom-built dynamometer (Cutlip et al., 1997). The force applied on the plate was translated into a vertical movement relative to a Z-11 / 5 kg load cell (HBM Inc, Marlboro, MA, USA). Ankle angle (i.e. angle between a line from the lateral epicondyle of the tibia and the foot sole) was measured at different plate positions. Plantar-flexors were stimulated via a cuff electrode around the tibial nerve (width pulse 200 µs; 80 Hz; 6.9 ± 1.9 V). At selected plate positions (range of motion 0.61-2.18 rad; increments of 0.09 rad), two isometric contractions (600 ms duration) were performed and compared to three pre-loaded slow concentric contractions (velocity 0.52 rad·s-1; range of motion 0.70-2.09 rad). Fatigue was tested by recording force decline with time in two sustained isometric contractions (3400 ms duration) at a position of 1.40 rad. Data were fitted with polynomials and averaged at selected ankle angles. The length of the gastrocnemius medialis muscle (GM) was measured at the end of the experiment with the knee at 1.57 rad at ankle angles of 0.79 and 2.01 rad.

RESULTS AND DISCUSSION

The isometric force of rat plantar-flexors in vivo was not significantly different (one-way ANOVA) between the ankle angles tested (Fig. 1). Peak isometric force (19.2 N) was 8% lower than the optimum force of plantar-flexors in situ measured in female rats of equivalent weight (Dodd et al., 1995). Similar findings were reported for mice using a dynamometer (Ashton-Miller et al., 1992). Thus, the isometric force of rat plantar-flexors can be measured in vivo by recording of force under the sole of the foot. If GM is functionally similar to gastrocnemius lateralis in situ and in vivo (gastrocnemius represents 87% of the mass of the plantar-flexors), the force-ankle position relationship of rat plantar-flexors agrees with predictions for force capabilities of GM in vivo (Ettema, 1997).

Figure 1: Active force of rat plantar-flexors as a function of ankle angle during isometric (() and slow concentric (() contractions.

Figure 2: Force-time traces of rat plantar-flexors during a sustained isometric (a) (1.40 rad) and pre-loaded slow concentric (b) (range of motion 0.7-2.09 rad) contraction.

Indeed, lengths of GM in vivo (34.9 ± 0.4 mm at 0.79 rad; 30.5 ± 0.4 mm at 2.01 rad) were close to its optimum length in situ (Zuurbier and Huijing, 1993). Our hypothesis was not confirmed as the force-ankle position relationships during multipositional isometric and slow concentric contractions were notably different (Fig. 1) (two-way ANOVA, p < 0.5). Fatigue does not explain this difference as sustained isometric contractions showed force decreases by 6.3 ± 0.9% compared to 23.1 ± 2.1% during slow concentric contraction (Fig. 2). It is concluded that the isometric force-ankle position characteristics of rat plantar-flexors in vivo cannot be obtained with pre-loaded slow concentric contractions. However, rat dynamometry provides a useful tool to study skeletal muscle performance in vivo with longitudinal changes with disease or age.

REFERENCES

Ashton-Miller J.A. et al. J. Appl. Physiol., 72, 1205-1211, 1992.

Cutlip R.G. et al. Med. Biol. Eng. Comput., 35, 540-543, 1997.

Dodd S.L. et al. Muscle Nerve, 18, 190-195, 1995.

Ettema G.J.C. Anat. Rec., 247, 1-8, 1997.

Fugl-Meyer A.R. et al. Eur. J. Appl. Physiol., 45, 221-234, 1980.

Zuurbier C.J. and Huijing P.A. J. Morphol., 218, 167-180, 1993.

ACKNOWLEDGMENTS

This research was supported by the National Institute for Occupational Safety and Health of the Centers for Disease Control (R01-OHAR-02918).