A SYSTEM TO EVALUATE PARALYZED LOWER-LIMB MUSCLE PERFORMANCE DURING FES-INDUCED CONTRACTIONS

D. Drew Pringle, Thomas W.J. Janssen, Roger M. Glaser, José W. Almeyda, and William P. Couch Institute for Rehabilitation Research and Medicine, Wright State University School of Medicine; Rehabilitation Institute of Ohio, Miami Valley Hospital; and Dayton VA Medical Center, Dayton, OH

ABSTRACT

A muscle performance evaluation system for use on lower-limb paralyzed muscles was developed utilizing a Kin-Com isokinetic dynamometer, microcomputer, data acquisition software, and a custom-designed electrical stimulation unit. To test the validity of this system, individuals with SCI underwent isometric testing of the quadriceps, hamstring, gluteal, anterior tibialis and gastroc-soleus muscle groups. The protocol consisted of 20 FES-induced isometric contractions, each followed by a 5-sec rest interval. For each contraction, FES current was ramped from 0-300-0 mA in 24 sec. Results obtained from this evaluation system generally showed that as fatigue progressed, contraction threshold increased, maximum force decreased, the slope of the force/current curve decreased, and force-current hysteresis increased. Occurrence of spasms and antagonistic muscle co-contractions were also determined. Data suggest that this standardized system can provide detailed information about individual muscle performance characteristics, which may be useful in designing and evaluating FES-assisted technologies.

BACKGROUND AND PURPOSE

Therapeutic FES-induced exercise of paralyzed lower-limb muscles is being used in an effort to improve the health status, physical fitness, and rehabilitation potential of individuals with SCI and various other neuromuscular disorders (1,2). Muscles employed for various clinical FES modalities include the quadriceps, hamstring, gluteal, gastroc-soleus, and tibialis anterior. Of course, to obtain a desirable muscle contraction pattern a proper application of FES current is required. However, performance characteristics among the various muscles employed can be quite different with respect to fiber recruitment pattern, maximal force output and fatigability. Thus, each muscle has its own particular FES current operating range for suitable function. Furthermore, the ratio between force developed and FES current decreases at a certain rate as fatigue progresses (3), depending upon the particular muscle and its state of conditioning. Spasms and antagonistic muscle co-contractions can also affect performance. Therefore, it would be desirable to objectively evaluate the contraction characteristics of various lower-limb muscles to determine their particular FES current operating range, and to ascertain how contraction characteristics change with progressive fatigue, as well as during therapeutic exercise conditioning programs. Thus far, however, the literature contains only few descriptions of techniques for objective FES muscle evaluation, and little data concerning the FES-induced contraction characteristics of various lower-limb muscles (3-8).

The purpose of this study was to develop and test a standardized system to objectively evaluate the FES muscle performance (i.e., operating current range, recruitment pattern, and fatigability) of the major lower-body muscle groups during repetitive contractions of ramped intensities. This system can provide information regarding the specific: 1- current threshold for contraction; 2- current to elicit peak force; 3- force developed per unit of stimulation current; 4- spasms and antagonistic muscle co-contractions elicited; and 5- changes in muscle performance during progressive fatigue.

METHODS

Instrumentation. The basis of the FES muscle performance evaluation system is a Kin-Com II isokinetic dynamometer used in conjunction with a specially designed and constructed selectable parameter stimulator. Fig. 1 provides a block diagram of the evaluation system. The stimulator allows for pre-setting of: max current output from 0-300 mA; pulse frequencies from 10-100 Hz; pulse durations from 50-600 (s; and, balanced monophasic or biphasic rectangular, sine, or triangular waveforms. Adjustments are also available for setting threshold current, current ramp rate, the number of contractions induced, rest interval, and the force limit. The FES current and contraction force data are fed into an analog-to-digital converter in the microcomputer which utilizes Asyst data acquisition/ analysis software. Data are also displayed in real time on an X-Y plotter.

Preliminary Testing. Subjects with lower-limb paralysis due to SCI were used to test the Kin-Com FES muscle performance evaluation system. The protocol and procedures followed were approved by the IRB of our institution. An informed consent form was signed by the subjects prior to participation. For this paper, only illustrative examples of the data obtained are presented to demonstrate operation of the instrumentation.

Fig. 1. Block diagram of the Kin-Com FES muscle performance evaluation system.

Protocol. Bipolar surface electrodes were placed over the motor points of the muscle group being tested. Body position and Kin-Com adjustments were made to insure proper joint positioning. The stimulator was set to biphasic rectangular wave pulses that were 300 (s in duration at a frequency of 35 Hz and a max current of 300 mA. Twenty FES-induced contractions were performed with a linear ramped increase in current from 0-300 mA in 12 sec, followed by an identical ramped decrease in current from 300-0 mA. Rest intervals between contractions were set to 5 s. Stimulation intensity was permitted to ramp upward so long as force remained less than 150 N. However, if 150 N was reached, stimulation current was automatically reversed to reduce force. This maximal force limit was set as a safety precaution to prevent injury.

RESULTS AND DISCUSSION

A representative example of the FES current-contraction force data with respect to time obtained during the 1st, 5th, 10th, 15th and 20th contraction (shown for clarity) of the quadriceps muscle of a subject with quadriplegia is illustrated in Fig. 2. The 150 N force limit was achieved for the first 10 contractions. Then, force progressively decreased with fatigue since the 300 mA current limit was reached and there was no additional fiber recruitment. The threshold current was about 50-60 mA and approximately 250 mA was required for full fiber recruitment for this particular muscle.

Fig. 3 illustrates the current vs force data for the same muscle in Fig. 2, but without regard to time. The 150 N force limit was achieved on the 1st contraction at about 150 mA. In contrast, this force production required about 175 mA on the 5th and 250 mA on the 10th contraction. On the 15th and 20th contractions, force markedly decreased despite the 300 mA current application. In addition, force-current hysteresis tended to increase with fatigue, and it took less current to maintain a given contraction force, once achieved.

Fig. 4 illustrates the ratio between FES current and contraction force for the same quadriceps muscle in Figs. 2 and 3 for each of the 20 repetitive contractions. The slope of this line provides indices of the muscle's responsiveness to FES, as well as of its fatigability for FES-induced contractions.

Fig. 2. Composite FES current-contraction force data with respect to time for the quadriceps muscle.

Fig. 3. FES current vs force produced relationship for the same quadriceps muscle in Fig. 2.

Fig. 4. The ratio between contraction force vs FES current for 20 repetitive contractions of the same quadriceps muscle in Fig. 2. To illustrate the occurrence of spasms elicited by FES sensory effects, Fig. 5 represents the FES current vs contraction force relationship for the gastroc-soleus muscle group. A spasm causes the loss of contraction smoothness as shown on the 1st contraction.

Fig. 5. The FES current vs contraction force for the gastroc-soleus muscle group showing a spasm that occurred on the 1st contraction.

To illustrate the occurrence of antagonistic muscle co-contractions, Fig. 6 represents the FES current vs contraction force relationship for the anterior tibialis muscle. As contraction threshold is exceeded, the force began to rise in a positive fashion. However, as FES current increased to a certain level, co-contractions of the antagonist (gastroc-soleus) muscles become stronger than the agonist (anterior tibialis) muscle. This can be seen by the negative force readings that follow the positive force readings. The less negative force readings that occurred during the 5th to 20th contractions may indicate progressive fatigue of the gastroc-soleus muscle group.

Fig. 6. The FES current vs contraction force for the anterior tibialis muscle. Co-contractions of the antagonistic gastroc-soleus muscle group are indicated by the negative force readings.

Conclusion. The described FES muscle performance measurement system appears to provide a valid, reliable, and objective technique to standardize evaluation of contraction characteristics for lower-limb paralyzed muscles. Information derived can also be useful for evaluating exercise training adaptations, as well as for developing computer algorithms for more precise FES control of paralyzed lower-limb muscles.

REFERENCES

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ACKNOWLEDGMENTS

This work was supported by the Rehabilitation Research and Development Service of the United States Department of Veterans Affairs.

D. Drew Pringle, Ed. D. Wright State University 3171 Research Blvd. Kettering, OH 45420, USA Tel: (513) 259-1326 Fax: (513) 259-1310 E-mail: dpringle@desire.wright.edu