Strategies for delaying muscle failure during

repetitive dynamic performance of human skeletal muscle.

M. B. Kebaetse1        S. A. Binder-Macleod2

1 Interdisciplinary Programs in Biomechanics and Movement Science, McKinly Laboratory, University of Delaware, Newark, Delaware, USA., 41429@udel.edu

2 Department of Physical Therapy, University of Delaware, Newark, Delaware, USA., sbinder@udel.edu

 

Abstract

   The purpose of this study was to identify the best stimulation strategy for delaying muscle fatigue and maximizing dynamic muscle performance from the human quadriceps femoris muscle.  Data were collected from 18 healthy subjects (12 females), ages 21-35 years. The quadriceps muscle was stimulated using 20-Hz constant-frequency trains (CFT20), CFT40, or 36-Hz novel stimulation trains consisting of pairs of closely spaced pulses called doublets (doublet-frequency trains (DFT36)).  In addition, two patterns using combinations of trains were tested.  The combinations were: 15 CFT20s followed by DFT36s (CFT20-DFT36), and 15 DFT36s followed by CFT20s (DFT36-CFT20). Each pattern was tested on a separate day. Stimulation began with the knee in 90º of flexion and the train was automatically cut off when the knee reached a target angle of 40º.  Testing was stopped when the subject failed to reach the target three consecutive times. Targets were reached (mean±SD) 35.7±12.7, 43.8±19.7, 45.1±17.5, 56.3±33.1, and 26.8±6.1 times for the CFT20, CFT40, DFT36, CFT20-DFT36, and DFT36-CFT20, respectively. The strategy using the DFT36 followed by the CFT20 produced significantly more successful contractions than any of the other stimulation patterns.  These results showed that the use of lower-frequency CFTs followed by higher-frequency stimulation trains delayed muscle failure during repetitive, non-isometric contractions.

 

1.       Introduction

FES typically activates muscles with equally spaced pulses arranged in stimulation patterns called constant frequency trains (CFTs). Recently, train patterns that vary the frequency within the stimulation train have been shown to produce greater forces from skeletal muscles during isometric contractions [1,2,4,11] and produce greater peak power and excursions during non-isometric contractions [5,8,10] than CFTs, particularly when the muscle is fatigued. The most commonly studied variable-frequency trains (VFTs) have used a high-frequency burst at the beginning of the train, followed by a low-frequency portion of equally spaced pulses [4,9,12].  Interestingly, our laboratory has recently identified a slightly different pattern of pulses that appears capable of producing greater force augmentation than the commonly tested VFTs [6,7].  These trains use closely spaced (~5 ms) pairs of pulses (doublets) separated by longer intervals (inter-doublet intervals) and we have termed these trains doublet-frequency trains (DFTs) [6,7].

 

Although varying the frequency within the train may allow skeletal muscles to produce greater performance than using CFTs, this augmented performance does not occur without some cost. Specifically, repetitive isometric activation with VFTs or DFTs has been shown to be more fatiguing than repetitive activation with CFTs [3,4,6].    However, recent evidence suggests that repetitive non-isometric activation of muscles with CFTs followed by DFTs may produce better performance than activation with CFTs alone or DFTs alone [7].  The purpose of this study was, therefore, to investigate the effect of combining different train patterns on repetitive dynamic quadriceps muscles contraction to determine the stimulation pattern that maximizes the muscle’s performance.

 

2.       Methods

Data were collected from eighteen healthy subjects (12 females) ranging in age from 21 to 35 years.  Subjects were seated on a computer-controlled dynamometer (KinCom III 500-11, Chattecx, Chatanooga, TN) with their hips flexed to ~85°. Self-adhesive stimulating electrodes were placed over the rectus femoris and vastus medialis motor points.  A Grass S8800 stimulator with a SIU8T stimulus isolation unit was used for stimulation.  

Stimulation train patterns: Five different stimulation train patterns were tested using 3 basic trains  (Figure 1). Each train was ­­≤1200 ms in duration.  Each pattern was tested on a separate day.  Sessions were at least 48 hours apart. Each train was identified by the pattern (CFT or DFT) followed by the train’s mean frequency in Hz (total number of pulses in a 1-s period). The train patterns were a CFT20, a CFT40, a DFT36 (consisted of a 5-ms doublet interpulse interval and a 50-ms interdoublet interval), a CFT20-DFT36 combination pattern (15 CFT20s followed by DFT36s), and a DFT36-CFT20 combination pattern (15 DFT36s followed by CFT20s).


Figure 1. The three basic trains used to make the five stimulation patterns. Each vertical line represents a 600 µs pulse. Trains could continue for up to 1200 ms.

 


Experimental Testing: Testing first involved measuring the subject’s maximum voluntary isometric contraction (MVIC) force for the quadriceps muscle with the knee at 90° of flexion. The stimulation intensity was then set while the knee was held at 90° of flexion so that a 1000-ms CFT20 produced 20% of the subject’s MVIC force. The intensity was not changed for the remainder of the test. The dynamometer was then switched to the isotonic mode, which was set to provide a load equal to 50% of the subject’s electrically elicited force produced by the 1000-ms CFT (i.e., 10% of the subject’s MVIC force). A predetermined train pattern was delivered to the muscle. Stimulation was interrupted when the knee reached the 40°-flexion target angle (i.e., a 50° range of motion).  Stimulation was repeated every 2000 ms.  Testing was terminated when the leg failed to reach the target 3 consecutive times.

A One-way repeated measure analysis of variance (ANOVA) was used to determine the effect of stimulation train pattern (CFT40, CFT20, DFT36, CFT20-DFT36, or DFT36-CFT20) on the number of times the target was reached. Post-hoc paired testing compared the CFT20-DFT36 train combination to each of the other patterns.

 

3.      
Results

Figure 2. Number of times (means and S.E.) the target was reached with each stimulation pattern. Significance is shown for the CFT20-DFT36 vs. the CFT20, DFT36, CFT40, and DFT36-CFT20 . *p≤0.05; **p≤0.01.

 

The ANOVA showed significant differences in the number of times each pattern produced the targeted excursion (p≤0.001; F=8.012). The CFT20-DFT36 train combination produced a significantly greater number of contractions than all other train patterns (see Figure 2).

 

4.       Summary and Conclusions

In the present study, we investigated the effect of using individual or combined train patterns on repetitive dynamic contractions during knee extension. Consistent with our hypothesis, repetitively activating the quadriceps femoris muscles with the CFT20 immediately followed by the DFT36 resulted in the leg completing the targeted excursion the most times.  Future studies should evaluate the effectiveness of additional train combinations and frequencies.  In addition, the effects of more sophisticated switching strategies from one train to the next and the effects of such patterns when testing paralyzed human skeletal muscles need to be explored. 

 

Acknowledgment

This work was supported by NIH Grant HD-41254 to Dr. Binder-Macleod. 

 

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