Research Report

Depression of HofEmann Reflexes Following Voluntary Contraction and Implications for Proprioceptive Neuromuscular Facilitation Therapy

Postconlrraction depression of Hoffmann-reflex (H-reJlex)amplitudes was examined to study the rationale underlying propnoceptive neummuscular facilitation relaxation techniques. The time course of H-reflex amplitude depression was used to assess postcontraction changes in motoneuron reflex excitability. Sixteen healthy female subjects performed voluntary isometric plantar-flexion contractions (65%-75% of maximal voluntary contraction) in a prone position. H-reflex stimulation began at a postcontraction delay of 0.05, 0.1, 0.5, 1, or 5 seconds and continued every 10 secondsfor 1 minute. Reflexes were depressed (X = 67% decrease) Ey 0.05 second postcontraction, reached maximal depression (X = 83.3% decrease)from 0.1 to 1 second postcontraction, recovered to 70% of control amplitudes (X = 30% decrease) by 5 seconds postcontraction, and reached 90% of control amplitudes (X = 10% decrease) b-y 10.05 seconds postcontraction. The results indicate that propn'oceptive neuromuscular facilitation techniques (eg, hold-relax)purported to produce a phase of relaxationfollowing voluntary contraction do appear to produce a strong, but brig neuromuscular inhibition that muy be clinically useful for applying stretch. [MooreM A Kukulka CG. Depression of HoJknunn reflexesfollowing volunta ry contraction and implicationsfor proprioceptive neuromuscular facilitation therapy. Phys Ther. 1991;71:321-333.1

Marjorie A Moore Carl G Kukulka

Key Words: Flexibility training; Muscle stretching; Neuromuscular facilitation; Proprioception; Reflex, mnosynuptic.

Proprioceptive neuromuscular facilitation (PNF) relaxation techniques often utilize voluntary contraction by patients in an attempt to induce a subse-

quent phase of muscle relaxation.1 Such voluntary contraction commonly is used to precede manual stretching of the same muscle group (eg, as in

M Moore, PT, PhD, is Assistant Professor, Physical Therapy Program, The University of Rhode Island, Kingston, RI 02881 (USA). She was a doctoral candidate in the Graduate Program in Physical Therapy, The University of Iowa, Iowa City, 1.4 52242, at the time this study was conducted. Address all correspondence to Dr Moore.

C Kukulka, PhD, PT, is Associate Professor, Graduate Program in Physical Therapy, The University of Iowa. This article was adapted from a poster presentation at the 1988 Joint Congress of the American Physical Therapy Association and the Canadian Physiotherapy Association, June 12-16, 1988, las Vegas, Nev. This projec~:was supported by NIH grant #NS24991 awarded to Dr Kukulka This study was approved by The University of Iowa Human Subjects Committee

the hold-relax technique) in the hope of reducing stretch reflex responses and thereby minimizing resistance to muscle elongation2 Previous research has revealed both excitatory and inhibitory effects on neurons and muscles in response to prior activity. Excitatory effects include (1) postcontraction sensory discharge (from muscle spindles); (2) post-tetanic twitch (force) potentiation; (3) post-tetanic potentiation of monosynaptic reflexes and of motor end-plate potentials; and (4) postcontraction facilitation of stretch, tendon tap, and vibration reflexes. These effects have been

This article was submitted September 8,1989,and was accepted November 20,1990.

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321 / 69

previously reviewed in the literature.3 Inhibitory effects were described by Sherrington and Forbes4 as a component of successive induction. They observed that, following reflex activation of an agonist, a reversal process sometimes occurred, involving antagonist facilitation and reciprocal inhibition of the agonist. This process is called successive induction.4 Forbes noted that the presence of either subsequent "augmentation" (facilitation) or subsequent depression (inhibition) of agonist activity was dependent on the strength of the reflex stimulus. These reflex findings were generalized to voluntary contractions during the development of PNF, in the hope of utilizing successive induction to increase agonist strength and promote antagonist relaxation. Changes in muscle excitability are difficult to quantify, especially when the subject is at rest. The status of the motoneurons (MNs) controlling the contractile state of a muscle, however, can be monitored indirectly. Hoffmann reflexes (H-reflexes)5.6 can be used to measure changes in the reflex excitability of an MN pool. An electrical stimulus is delivered to a peripheral nerve, action potentials in afferent fibers result in excitatory postsynaptic potentials to the MNs, and a reflex muscle twitch may be evoked. The amplitude of the triphasic electromyographic (EMG) response associated with this twitch is measured to provide a quantitative indication of the number of motor units that were reflexly recruited. The neuronal circuitry involved in this reflex is controversial.7

We therefore used H-reflexes in our study to assess the presence of reflex inhibition following voluntary contraction. The purpose of the study was to determine the onset latency, duration, and magnitude of the postcontraction reflex depression. This information should increase the knowledgeability of the therapist in optimally applying therapeutic techniques such as the hold-relax technique by optimally timing the hold phase prior to the desired relaxation interval.

Method Subjects Sixteen female subjects, aged 22 to 25 years @=23.6, SD=0.96), with no known neurological disease volunteered for this study. All subjects signed informed consent forms prior to participation in the study.

Procedure Subject positioning. Each subject was positioned prone on a treatment plinth (Fig. I), with the left foot secured by an ankle cuff and turnbuck-

The response of an MN pool is determined by the summation of all inputs impinging on the MNs. The amplitude of an evoked reflex will therefore -

depend partly on the background excitability level of the MN pool. In the presence of a constant background input, a given stimulus should elicit a consistent reflex response. If inhibitory influences are present at the time the reflex is elicited, however, the response of the MN pool may be depressed, resulting in reflex activation of fewer MNs and a smaller reflex muscle twitch.8 According to this reasoning, if H-reflex amplitudes are reduced following therapeutic procedures (such as PNF relaxation techniques), we can infer that these techniques have produced an inhibition of the reflex excitability of the MN pool and its associated muscle.

-

-

-

*Genisco Technology Corp, 650 Easy St, Simi Valley, CA 93065. +Therapeutics Unlimited Inc, 2835 Friendship St, Iowa City, 1A 52240. *~ewlett-~ackard Co, 19310 Pruneridge Ave, Cupertino, CA 95014. $Tektronix Inc, Howard Vollum Industrial Park, PO Box 500, Beaverton, OR 97077 " ~ r a sInstrument s Co, 101 Old Colony Ave, PO Box 516, Quincy, MA 02169.

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les to a footplate instrumented with a force transducer.* To facilitate access to the tibia1 nerve, the knee was flexed about 30 degrees and the ankle was plantar flexed about 15 degrees. The signal from the force plate was amplified with a high-impedance differential amplifiert and stored on an FM tape recorder.*

Electromyographic recording apparatus. Surface EMG electrodest were positioned over three muscles: (1) the soleus muscle to record the H-reflex (electrode position was standardized as the lower electrode edge at one third the distance from the lateral malleolus to the fibular head on the calf posterior midline), (2) the triceps surae muscle to signal the end of voluntary plantar flexion (adjacent electrode positioned immediately proximal to the soleus muscle electrode), and (3) the tibialis anterior (TA) muscle to assess any reciprocal activity (electrode positioned over the muscle belly visible during voluntary dorsiflexion). The EMG signals were amplified by on-site preamplifiers and a high-impedance differential amplifier. The triceps surae muscle EMG signal was also "integrated" by means of electronic root-mean-square (RMS) processing. High and low frequency cutoffs of 75 Hz and 5 kHz, respectively, were used for the raw EMG signals, and a 110-millisecondtime constant was used for the RMS processing. The raw soleus muscle EMG signal was displayed on a Tektronix model 7313 analog oscilloscopes for H-reflex monitoring during the experiment and recorded on FM tape* for later analysis. This EMG signal was also led to an audio amplifier.llThe raw TA muscle EMG signal was monitored on the same scope and recorded on tape. The triceps surae muscle RMS EMG signal was led to an electronic voltage level detection (VLD) circuit that provided a positive output pulse whenever EMG voltage dropped below a preset level (ie, a mean of 80.33%2 12.22%of voluntary contraction EMG voltage). This VLD pulse served as the signal for the end of the voluntary contraction, initiating the nerve stirnu-

Physical Therapy /Volume 71, Number 4 /April 1991

1

EMGIFORCE PROCESSING

NERVE STIMULATION

I

, ,

STIMULUS ISOLATION UNITS

CI:

-7

STIMULATOR

DIGITIMER

-

+--

TO TAPE TO TAPE FORCE AMP

VOLTAGE LEVEL DETECTOR

Figure 1. Schematic of experimental setup. Subject is positioned prone on plinth, with foot attached to a footplate instrumented by a force transducer. Suface electromyographic (EMG) actir~itywas recorded from soleus (SOL), triceps surae (TS),and tibialis anterior (TA) muscles. Tibia1 nerve stimulation mas applied via a cathode in the popliteal fossa and an anode on the anterior thigh. lation. Both RMS EMG activity and VLD pulse signals were displayed to the subject on a Tektronix model 5110 digital oscilloscopes and recorded on tape.

Nerve stimulation apparatus. An acrylic plastic stimulating electrode housing," containing the cathode (a 5-mm stainless-steel ball covered by moistened gauze and felt), was strapped to the subject's left popliteal fossa, over the tibia1 nerve. This device ensured stability of electrode location and pressure on the nerve. The anode was a 5- X 5-cm moistened pad and metal plate placed under the anterior !eft thigh. The H-reflex stimulus consisted of 1-millisecond square-wave pulses delivered by a Grass S88 stimulatoJ1 through a Grass SIU5 stimulus isolation unit.' Initiation of stimulation was controlled by the VLD pulse, used as ' input to a Dl00 ~ i ~ i t i r n e r ,which controlletl timing of subsequent

H-reflex stimuli. These Digitimer pulses were also recorded on tape.

Preliminary data collection. After the subject was positioned in the apparatus, each experiment began by determining the subject's maximal voluntary contraction (MVC) peak EMG voltage level for 3 seconds of isometric plantar flexion (best of 3-5 trials) to choose the target intensity for the experimental voluntary isometric plantar-flexion contractions. Enoka et all0 have shown that there is no significant difference in H-reflex depression following contractions at 50% o r 100% of MVC. Therefore, 65% to 75% of each subject's MVC peak EMG voltage level was chosen to be the target contraction level. This target range was drawn on the oscilloscope screen, and subjects were then responsible for monitoring their own triceps surae muscle EMG activity, keeping voluntary contractions within the desired range. Previous research has shown no significant difference in reflex depression for contractions

'~edical Sjatems Corp, 1 Plaza Rd, Greenville, NY 11548.

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lasting 2.5 o r 5.0 seconds1° o r for 10 to 20 seconds" A contraction duration of 3 seconds (with a 1-minute rest between trials) was therefore chosen to minimize fatigue. For each subject, an H-reflex recruitment curve was generated with the subject at rest. A stimulus intensity was selected to produce the largest H-reflex possible without an M-wave. Prior to each trial, resting H-reflexes were elicited to ensure that the reflex had returned to control levels from any postcontraction depression produced by previous trials. If necessary, small adjustments were made in stimulus intensity to maintain an appropriate resting H-reflex amplitude.

Experimental protocol. Each trial consisted of resting (control) H-reflexes, followed by a single voluntary plantar-flexion contraction, then postcontraction H-reflexes. It was desirable to test H-reflex depression at several times shortly after contraction. The MN pool, however, needs time to recover from previous stimuli. To avoid depression of the H-reflex amplitude by

323 / 71

the preceding reflex, H-reflexes should not be elicited at intervals of less than 5 o r 10 seconds.12 To overcome this problem and allow reflex testing at various times shortly after voluntary contraction, subjects performed multiple trials, with reflex stimulation beginning at one of five different postcontraction delays (0.05, 0.1, 0.5, 1,o r 5 seconds) on each trial. Following the initial stimulus, stimuli continued every 10 seconds (postdelay interval) for 1 minute, resulting in a total of seven postcontraction H-reflexes per trial. The postcontraction times at which these seven reflexes were elicited for any given delay are shown in Table 1. The combination of the five postcontraction delays with the seven postdelay intervals allowed assessment of H-reflexes at 35 different times postcontraction (Tab. 1). The mean precontraction reflex measurement (time 0.0) was also utilized in the data analysis, making a total of 36 reflex times. Each subject performed five trials at each of the five postcontraction delays (25 trials total) in a randomized order, providing 175 (35 x 5) postcontraction H-reflex measurements per subject.

Data Analysis H-reflex data. The raw H-reflexes were digitized off-line at a rate of 20,480 samples per second by playing the FM tape onto a digital oscilloscope, using cursors to measure the peak-to-peak (maximal positive peak to maximal negative peak) reflex amplitude. To allow comparisons across subjects, the measured amplitudes were normalized by expressing them as a percentage of each subject's resting H-reflex amplitude.10 The normalized reflex amplitudes for each postcontraction time were averaged across the five trials for all subsequent analyses.

-

Table 1. H-Reflex Stimulus Postcontraction Times (in Seconds)" Postcontraction Delay (s)

Post-Delay Interval (s) 0

10

20

30

40

50

60

"There were a total of 35 postcontraction times for delivery of H-reflex stimuli and measurement of H-reflex amplitude (5 delaysx7 intervals=35). The mean precontraction H-reflex (time 0) was also included in the data analysis, as a control.

To test the main effect of postcontraction time elapsed (the independent variable) on H-reflex amplitude (the dependent variable), a two-way (16 subjects X36 times) analysis of variance (ANOVA) was performed, using a repeated-measures design. In the presence of significant I; values, planned post hoc paired t tests (with Bonferroni's correction for multiple comparisons) were made of leastsquares means. To determine the total duration of H-reflex depression, pairwise comparisons were made of the mean precontraction H-reflex amplitude (control) with each postcontraction H-reflex amplitude (dependent variable), using only postcontraction times from 0.05 to 25 seconds (independent variable). Second, to determine the time of maximal H-reflex depression, all possible pair-wise comparisons were made of H-reflex amplitudes (dependent variable) for postcontraction times of 0.0, 0.05, 0.1, 0.5, 1, and 5 seconds (independent variable).

Force data. Force data from five trials (all 5-second postcontraction delay trials) were analyzed for each subject to assess fatiglie. Force data were digitized off-line at a rate of 20,480 Hz by playing the FM tape onto a digital 0scilloscope, using cursors to measure (1) the reflex twitch peak force,

**SASComputer Program, SAS Institute Inc, Cary, NC 27511.

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(2) the reflex twitch half-relaxation time (time required for force to decline from peak to 50% of peak), (3) the voluntary contraction peak force, and (4) the MVC peak force. For statistical analysis, data were averaged across subjects. A separate twoway (5 trials x 7 postcontraction reflex times) ANOVA was performed for each dependent variable (peak force and force half-relaxation time), using a repeated-measures design. All statistical analyses were performed using the Statistical Analysis System (SAS) package.** Tibialis anterior muscle EMG activity was monitored in an attempt to control for a possible confounding variable. If subjects had unintentionally contracted their dorsiflexors during or after their voluntary plantar-flexion contractions, reciprocal inhibitory effects from the dorsiflexors could conceivably have affected the soleus muscle H-reflex. The TA muscle EMG activity was therefore visually examined to rule out such undesirable contractions. The data revealed minimal activity, consistent with rest. These data, therefore, were not analyzed further.

Results H-Reflex Data H-reflex amplitudes were profoundly depressed following voluntary contraction (P.05) (Tab. 7). We did not examine the reliability of these measurements within the configuration of this study, but similar measurements are routinely obtained in our laboratory with a high level of reproducibility.

Table 3. Means and Standard Deviations for H-ReJex Amplitude at Various Postcontraction Times

Postcontraction Time (s)'

a

Percentage of Resting H-Reflex Amplitude

x

SD

Time 0.0 represents the mean preconrraction control reflex. Values for times 0.0 to 5 seconds postcontraction are averaged across subjects and trials. Values for times 10 to 60 seconds postcontraction are averaged across subjects, trials, and five delay times (eg, time 50 seconds postcontraction is an average of actual stimulation times of 50.05, 50.1, 50.5, 51, and 55 seconds postcontraction).

Table 4. Post Hoc Analysis of Duration of Postcontraction Depresion of H-Reflexes Postcontraction Time (s)

0.0

H-Reflex Amplitude Leastsquares Mean (% Precontraction) Pa

100.00 (control)

-+-----+----+----+---+-----+-----+-----+----+-----+-----+-----+-

0.0

0.05

0.1

0.5

I

5

10

20

30

40

50

60

PostcontractionTime (s)

Figure 3. Time course of depression and recovery of H-reflex amplitude. Means (horizontal dashes) and standard deviations (vertical lines) are shown for pooled data across trials and subjects. Data for times 10 to 60 seconds postcontraction are also each averaged acrossfive times (eg, time 5 0 seconds postcontraction is an average of times 50.05, 50.1, 50.5, 51, and 55 seconds postcontraction). Note nonlinear time scale, wed to more clearly show changes immediately after contraction. (Time 0.0 second represents precontraction control amplitude.)

Discussion Magnitude of Reflex Depression The inrense postcontraction depression of H-reflex amplitudes observed in this study provides support for the theory underlying PNF relaxation techniques. These results also support previous findings of postcontraction H-reflex depression.l0J1 It was difficult to compare the magnitude of maximal H-reflex depression found in this study with that of other investigations. Although these data were not provided in the text of the reports of the previous studies, the magnitude of depression can be inferred from their illustrations. Figure 2 in the article by Enoka et all0 indicates a maximum postcontraction depression to about 35% of control amplitudes (65% decrease). Figure 2 in Schieppati and Crenna'sl1 article indicates depressed reflex amplitudes 74 / 326

approaching 0% of control amplitudes (complete suppression) for the fastest contraction relaxation time (0.3 second), but depression to only 45% of control amplitudes (55% depression) for slow releases (2.5 seconds). Rate of relaxation following contraction was not controlled in the study of Enoka et a1 or in our study. In our study, average maximal postcontraction H-reflex depression reached 16.7% of control amplitudes (83.3% decrease), despite a conservative interpretation of the data. Raw H-reflexes were often observed to be completely absent (100% depression) immediately following contraction (0.05-1 second). The presence of H-reflexes with amplitudes smaller than the resolution of the equipment, however, could not be ruled out. Therefore, rather than assigning a zero to such amplitudes, a value corresponding to the equipment noise level (0.1 mV) was used. This practice

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" Probability value for post hoc paired t tests with Bonferroni's correction for multiple comparisons. H-reflex amplitutle at each listed time was compared with the precontraction control amplitude (time 0.0). Because 15 pair-wise comparisons were planned, the critical one-tailed probability value, with an experiment-wise error rate of P s 0 5 , is (.05/15) XZ=.OO67.

was deemed necessary to avoid overestimating the magnitude of H-reflex depression, but this conservative treatment also probably underestimated the actual magnitude of the depression effect. The magnitude of postcontraction reflex depression may thus be even greater than indicated by the statistical results (between 83.3% and 100% depression).

Time Course of Reflex Depression Our study extends earlier investigations by examining reflex changes at shorter latencies following contraction, demonstraring that reflex depression begins immediately (ie, 0.05 second) after contractile EMG activity declines. The duration of the phase of

Physical Therapy /Volume 71, Number 4 /April 1991

Table 5. Post Hoc Anabsis of Time of Maximum Postcontraction Depression of H-Reflexes PostH-Reflex Amplitude contraction Least-Squares Mean (% Precontraction) Tlme (s)

Pa

Probability value for post hoc paired t tests with Bonft:rroni's correction for multiple compariso.ns.

a

H-reflex amplitude at each listed time was compared with the amplitude at each time before it. Only five of these comparisons are shown. Because all 15 possible pair-wise comparisons w~ereplanned, the critical two-tailed probability value, with an experiment-wise error rate of P1.05, is (.05!15)=.0033.

maximal reflex inhibition, however, is very brief. In our study, the period of maximum H-reflex depression lasted from 0.1 to 1 second postcontraction, with recovery to 70% of control reflex amplitudes already occurring within 5 seconds postcontraction. Schieppati and Crenna'sll illustrations indicate that the reflex had recovered to 50% within 1 second (in the case of rapid relaxations, lasting 0.3 second). Recovery to this level was delayed for about 3.5 seconds with slower relaxations (lasting 1 second). It was impossible to determine the duration of significant reflex depression in the study by Enoka et a1,I0 because they did not sample the reflex at any times between their first stimulus (ie, 1-2 seconds postcontraction) and 5 seconds later, by which time the reflex had transiently recovered beyond control levels. From these results, it is important for therapists to recognize that significant postcontraction reflex inhibition is generally brief, probably present for only a few seconds.

Reflex depression recedes as recovery occurs. The time course of the recovery from postcontraction reflex depression, however, differed among our study and the studies of Enoka et all0 and Schieppati and Crenna.ll In our study, H-reflex amplitudes gradually recovered over the course of 10 to 15 seconds. From a minimum amplitude at 0.1 to 1 second postcontraction, they reached 70% of control amplitudes by 5 seconds postcontraction and 90% of control amplitudes by 10.05 seconds postcontraction. Reflex depression of 82% to 86% of control amplitudes was present from 10.1 to 11 seconds postcontraction, but amplitudes remained above 90% thereafter for 65 seconds.

ported a transient recovery (to 107% of control values) at approximately 6 to 7 seconds postcontraction, followed by a plateau period of mild reflex depression (reflex amplitude about 80% of control values) from 10 to 30 seconds postcontraction, then gradual recovery from 30 to 45 seconds postcontraction. These different results may perhaps be explained by Schieppati and Crenna's observation that the time course of reflex depression is closely related to rate of relaxation from the preceding voluntary contraction. Slower relaxation was associated with less reflex depression and with more prolonged recovery times.

Neurophysiological Mechanisms These duration results are in closest agreement with those of Schieppati and Crenna,ll who also observed a gradual recovery, with no long-lasting (beyond 10 seconds) reflex depression. In contrast, Enoka et all0 re-

-

Several neurological processes potentially might be considered to be involved in postcontraction reflex depression, including (1) MN afterhyperpolarization (AHP), (2) recur-

Table 6. Two-Way Randomized Block Analysis-of-Variance Summary for Reflex M m l e Twitch Peak Force Data -

Source

-

-

SS

df

Trial

-

4

Time

46.10

-

MS

F

0.12

0.08

Pa

,9882

6

450.43

0.78

0.53

,8760

24

323.65

0.14

0.10

1.OOOO

Error

464

65755.71

1.48

Total

498

66575.89

1.39

Trial xtime

All values were nonsignificant (P> .05).

Table 7. Two-Wa-yRandomized Block Analysis-of-Variance Summary for Reflex Muscle Twitch Half-Relaxation Time Data Source

SS

df

MS

F

Pa

Trial

4

251 1.35

627.84

1.80

,1289

Time

6

2638.25

439.71

1.26

,2759

0.69

,8654

Trial xtime

24

5749.24

239.55

Error

328

11455.08

349.24

Total

362

12540.92

346.55

"All values were nonsignificant (P> .05)

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rent inhibition, (3) muscle spindle pause, (4) Golgi tendon organ autogenic inhibition, and (5) presynaptic inhibition. Although our study did not specifically address these issues, the potential role of each will briefly be considered. Motoneuron AHP occurs following an action potential, making the MN less responsive to stimuli applied during this interval. The AHP in soleus muscle MNs produced by single action potentials lasts only about 90 milliseconds.l7 However, summation of AHPs from repetitive action potentials, such as would occur during voluntary contraction, would increase the magnitude and duration of MN depression over that produced by single potentials.18J9

Force Data The force data provided the ability to rule out neuromuscular fatigue as a possible cause of postcontraction reflex depression. Fatigue is normally associated with a change in peak muscle twitch force and with a slowing of muscle twitch half-relaxation time.16 Because neither of these processes was observed in the force data of our study, fatigue was not considered to be present and therefore could not be responsible for producing postcontraction reflex depression.

Therapeutic Implications The findings of our study indicate that PNF relaxation techniques must be performed rapidly if the desired inhibitory effect is to be achieved. These results are compatible with current practice of techniques such as the hold-relax technique. The patient is usually taken to the end of his or her passive range of motion and resistance applied to a voluntary "hold" contraction, followed by an increase in the manual stretch. The results of our study indicate that these increments in stretch should be applied immediately after the voluntary contraction, preferably within the first second and certainly by 5 seconds postcontraction. The voluntary contraction can then be repeated to re-

76 / 328

new the inhibition, followed by successive increments in stretch. Therapists may also wish to consider that faster relaxation of the contraction appears likely to produce greater inhibition.]' Although our study was performed with the subjects in the prone position, postcontraction reflex inhibition has also been observed with subjects in the sitting position.1° The results of our study, therefore, may be generalized beyond our experimental protocol.

Limitations The results of our study must be applied with at least two limitations in mind. The first limitation is that the relationship between H-reflex amplitude and clinical assessment of muscle excitability has not been established. Although it seems reasonable that changes in the electrically elicited H-reflex would reflect the same physiological phenomena underlying clinically observed changes in muscle excitability, the interpretation of the H-reflex is controversial.7 The second limitation is that our study was performed on neurologically normal subjects. The results are therefore generalizable only to neurologically normal patients, such as patients with only orthopedic disorders. Neurologically impaired patients may exhibit very different time patterns of postcontraction reflex depression. Nevertheless, therapists should consider these findings in planning their use of PNF treatment. Motoneuron discharge also evokes recurrent inhibition (RI). Recurrent collaterals of the MN axon excite Renshaw interneurons that produce inhibition of the previously active MN pool.20 This process is apparently more strongly influenced by activation of large MNs, preferentially suppressing small MNs.z1 Because small MNs are most responsive to stretch and monosynaptic reflex inputz2(and therefore presumably H-reflexes), RI from large MNs activated by voluntary contraction could easily depress H-reflex excitability. Such depression would be most likely to occur follow-

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ing strong contractions that activate large MNs. Hultborn and PierrotDeseilligny23 have determined that both AHP and RI play a role in the depression of H-reflexes produced by a submaximal conditioning stimulus at rest. These mechanisms, however, d o not appear to be involved in postcontraction reflex depression.13 A postcontraction cessation of the muscle spindle afferent firing that normally occurs during contraction could also theoretically depress H-reflex amplitude. In the presence of normal gamma MN innervation, however, muscle spindle activation has been shown to persist following contraction (provided that muscle length is not increased), enhancing sensitivity to static and dynamic stretch.24 This hypothesis is thus an unlikely explanation, because H-reflexes would more likely be augmented than depressed. Primary afferent depolarization produced by Ia afferent terminals synapsing on other Ia afferents reduces muscle spindle Ia responsiveness to reflex stimuli, thereby producing presynaptic inhibition of MNs.25 Ia afferent firing (achieved through alpha-gamma coactivation) during voluntary contraction could thereby cause a reduction in Ia and MN responsiveness to subsequent H-reflex stimuli.ll This mechanism could further contribute to postcontraction H-reflex depression. Golgi tendon organs are a known source of inhibition to MNs and are strongly activated by even minimal muscle c ~ n t r a c t i o nPersistence .~~ of their summated inhibitory postsynaptic potentials produced during contraction, therefore, could theoretically depress MN excitability.27 However, Golgi Ib afferent firing is depressed following contraction24~28and should therefore produce MN disinhibition, making H-reflex augmentation more likely than depression. In summary, it is possible that multiple neurological mechanisms may be involved in postcontraction reflex depression. The most reasonable explanation at this time appears to be presynaptic inhibition. Muscle fatigue,

Physical Therapy /Volume 71, Number 4 /April 1991

reciprocal inhibition, muscle spindles, Golgi tendon organs, AHP, and RI do not appear to be involved, although they cannot be ruled out in our study. Further basic science investigations of possible mechanisms underlying postcontraction reflex depression are warranted. Additional studies are also needed to determine the magnitude and time course of postcontraction reflex inhibition in patients with neurological disorders.

Conciusions Following voluntary contraction, H-reflex :amplitudes were strongly depressed (mean maximum decrease=83.3%>. indicating - " a reduction in MN reflex excitabiliv' This was shOrt-lasting, with maximal depression lasting less than 1 selcond and 70% recovery occurring w i t h 5 seconds. These results provide support for PNF relaxation technique claims of postcontraction inhibition, but indicate the need for speed when applying these techniques. -

,a

Acknowledgments

We particularly thank David Gerleman for his help with the instrumentation. Dr Trudy Burns and Dr Russell Lenth provided statistical consultation, and the staff of The University of Iowa computer center assisted with statistical programming. Dr Erich Luschei, Dr Gary Soderberg, Dr Warren Darling, and Dr Kelly Cole provided helpful comments on the project as members of Dr Moore's dissertation committee. We also thank each of the subjects who donated their time to this project.

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Commentary Dr Moore and Dr Kukulka should be congratulated for this well-designed and well-controlled study. Investigations assessing the mechanisms underlying therapy approaches are obvi-

ously important. In this case, the authors designed a series of experiments to determine whether targeted motoneuron pools are inhibited during the proprioceptive neuromuscular

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facilitation (PNF) hold-relax technique. The results appear to support the notion that the hold-relax technique has a neurophysiological basis when applied to the soleus muscle.