Amplitude of the human soleus H reflex during walking and running

Abstract

1. The objective of the study was to investigate the amplitude and modulation of the human soleus Hoffmann (H) reflex during walking and during running at different speeds. 2. EMGs were recorded with surface electrodes from the soleus, the medial and lateral head of the gastrocnemius, the vastus lateralis and the anterior tibial muscles. The EMGs and the soleus H reflex were recorded while walking on a treadmill at 4.5 km h-1 and during running at 8, 12 and 15 km h-1. 3. The amplitudes of the M wave and the H reflex were normalized to the amplitude of a maximal M wave elicited by a supramaximal stimulus just after the H reflex to compensate for movements of the recording and stimulus electrodes relative to the nerve and muscle fibres. The stimulus intensity was set to produce M waves that had an amplitude near to 25 % of the maximal M wave measured during the movements. As an alternative, the method of averaging of sweeps in sixteen intervals of the gait cycle was applied to the data. In this case the amplitude of the H reflex was expressed relative to the maximal M wave measured whilst in the standing position. 4. The amplitude of the H reflex was modulated during the gait cycle at all speeds. During the stance phase the reflex was facilitated and during the swing and flight phases it was suppressed. The size of the maximal M wave varied during the gait cycle and this variation was consistent for each subject although different among subjects. 5. The peak amplitude of the H reflex increased significantly (P = 0.04) from walking at 4.5 km h-1 to running at 12 and 15 km h-1 when using the method of correcting for variations of the maximal M wave during the gait cycle. The sweep averaging method showed a small but non-significant decrease (P = 0. 3) from walking to running at 8 km h-1 and a small decrease with running speed (P = 0.3). The amplitude of the EMG increased from walking to running and with running speed. 6. The relatively large H reflex recorded during the stance phase in running indicates that the stretch reflex may influence the muscle mechanics during the stance phase by contributing to the motor output and enhancing muscle stiffness.

Figures

Figure 1. H reflex modulation of one subject during walking at 4.5 km h−1

The gait cycle was divided into 16 time slices starting at the bottom of the figure corresponding to heel strike. In each slice 5 events are visible, from left to right: s1, the first stimulus artefact; M, the M wave; H, the H reflex; s2, the second stimulus artefact; and Mmax, the maximal M wave. The duration of each sweep was 120 ms. The H reflex appeared after approximately 35 ms and the second stimulus (s2) was elicited 60 ms after the first stimulus (s1). The signals in each time slice represent an average of multiple sweeps obtained by the method described earlier by Capaday & Stein (1987). It can be seen that the H reflex was facilitated during the stance phase, but suppressed during the swing phase.

Figure 2. Recordings from the same subject illustrated for 4 gait situations

The gait situations were: walking at 4.5 km h−1 (A); running at 8 km h−1 (B); running at 12 km h−1 (C); and running at 15 km h−1 (D). For each of the 4 figures the following recordings are shown from bottom to top: the soleus EMG (SOL) and the tibialis anterior EMG (TA) (linear envelopes averaged from 12 sweeps); the ankle angle; the amplitude of the maximal M wave (Mmax) of the soleus muscle; and finally the amplitude of the soleus H reflex expressed as percentage Mmax and corrected for variations of Mmax during the gait cycle. For the H reflex and Mmax, each cross represents one stimulation given every 2 s. The stance phase starts on the left at 0 ms and ends at the vertical dashed line. Note that the scales for the soleus and anterior tibial EMG differ from A to D.

Figure 3. H reflex modulations

In the top tracings, the modulation of the soleus H reflex is shown as the average from 7 subjects during walking at 4.5 km h−1 (+), running at 8 km h−1 (), running at 12 km h−1 (^) and running at 15 km h−1 (×). In the left column the results were obtained by the method of the present study, in which the maximal M wave (Mmax) was measured in every sweep (Method 1). In the right column the same data were obtained by the method described by Capaday & Stein (1987) (Method 2 (C & S)). For each of the two methods the 4 lower tracings show individual data for each subject and for each velocity investigated. Each of the 7 subjects is represented by a specific symbol. All data are expressed as a percentage of Mmax. For Method 1, Mmax was measured in every sweep while for Method 2, the Mmax measured in the standing position was used for normalization. For both methods, only M waves between 15 and 35 % of Mmax were accepted. The gait cycle was divided into 16 time slices and normalized with heel strike at 0 and 100 %.

Figure 4. Peak amplitude of the soleus H reflex during walking (4.5 km h−1) and during 3 running speeds

Hatched columns represent values obtained by the method of the present study where the maximal M wave (Mmax) was measured in every sweep. Open columns represent values obtained by the method of Capaday & Stein (1987) where variations of Mmax during the gait cycle are not accounted for. The bars are mean values of 7 subjects and the error bars are +s.e.m.* Statistically significantly different from walking at 4.5 km h−1 (P < 0.05).

Figure 5. Recordings of the maximal M wave

The upper panels show the relation between stimulus intensity and the recorded M wave for two subjects. It can be seen that the maximal M wave was achieved at a stimulation intensity of about 20 mA and stayed relatively constant up to the maximum capacity of the stimulator. ×, sweeps with a single stimulation; ^, sweeps with a double stimulation. In the latter condition the intensity of the second stimulus was set to 42 mA for both subjects. The lower panels show reference excitability curves of the same two subjects with (^) and without (×) the second but supramaximal stimulus applied to every sweep. It can be seen that the ‘double stimulus’ condition did not affect the amplitude of the H reflex.

Figure 6. Two subjects with a large difference in variation of Mmax shown during running at 8 km h−1

The subject on the left showed a rather small variation in Mmax during the gait cycle, while the subject on the right showed large variations. The H reflex is expressed as the percentage of the maximal M wave (Mmax) measured in every sweep (×) and during standing (•) (Capaday & Stein, 1987). For the subject shown on the right this variation resulted in a clearly lower H reflex amplitude during the stance phase of running when using the method of Capaday & Stein (1987). The discrepancy could not be demonstrated for the subject shown on the left. The reason for the disparity is likely to be the use of an excessively high stimulus intensity during the stance phase for the subject shown on the right (see text for further explanation).