Effects of age and leg length upon central loop of the Gastrocnemius-soleus H-reflex latency

Background central loop of the gastrocnemius-soleus H-reflex latency (Tc) that looks promising in the diagnosis of S1 radiculopathy; has been investigated in a few studies and only two of them have focused on the constitutional factors affecting it. Although leg length has been shown to contribute to the Tc, the role of age is controversial. More confusing, none of the previously performed studies have used strict criteria to rule out subclinical neuropathy, so the results could be misleading. This study has been performed to determine the influence of leg length and age on Tc among a carefully selected group of healthy volunteers. Methods after screening forty six volunteers by taking history, physical examination and a brief electrophysiologic study; forty of them were selected to enroll into the study. Tc was obtained in all the study subjects and leg length and age were recorded for correlational analyses. Results this group was consisted of 26 males (65%) and 14 females (35%) with the age range of 19–65 years (Mean ± SD: 37 ± 10.7) and leg length range of 29.5–43 centimeters (36.4 ± 3.4). Mean ± SD for Tc was 6.78 ± 0.3. We found a significant correlation between Tc and leg length (p value= 0.003, r = 0.49 and confidence interval 95% = 0.59–0.88), no significant correlation was found between age and Tc (p value= 0.48, r = 0.11), also we obtained the regression equation as: Tc = 0.04L + 5.28 Conclusions in contrast to leg length, age was not correlated with Tc. Future studies are required to delineate other contributing factors to Tc.


Background
The H-Reflex evaluates S 1 radiculopathy [1]. The meas-ured latency, however, is neither specific nor sensitive for S 1 spinal nerve disease, as it traverses a long pathway. Pease et al [2] were the first who described the central loop of the gastrocnemius-soleus H-Reflex latency (Central S 1 loop latency or T c ) and suggested it might be promising in the diagnosis of S1 radiculopathy [1,2].
Unfortunately, T c has been the subject of few studies, and as far as we know, only 5 articles [2][3][4][5][6] have been published on this issue so far. Among them, two have specifically evaluated the constitutional factors contributing to T c . Leg length has been shown to have a significant effect on T c . It is controversial whether age entails a similar effect. Wang et al [5] found a direct correlation between age and Tc. This observation was not confirmed by Ghavanini et al in an independent study [6].
The current study has been performed to determine the influence of leg length and age on T c .

Methods
We enrolled 46 volunteers to this study after obtaining informed consent. Following a standard history taking, all of them underwent physical examination and a brief electrophysiologic evaluation [7] to rule out asymptomatic polyneuropathy, including determination of: right peroneal nerve conduction velocity (PNCV), distal motor latency of right deep peroneal nerve (PDML) and standard gastrocnemius-soleus H reflex latency (T p ). We defined our exclusion criteria as: history of sacral radiculopathy or diabetes mellitus or any other disease with potential to cause neuropathy, any abnormality in neurological or musculoskeletal physical examination, or any of the following findings: PNCV less than 40 m/s, PDML more than 5 ms or prolonged T p (according to Braddom and Johnson's study [8].
Since we were supposed to rule out subclinical peripheral neuropathy and one component of the related electrodiagnostic study was measuring the distal motor latency for the deep peroneal nerve, the temperature at the dorsum of the foot was kept almost at 32°Celsius.
The leg length of each person was measured as the distance from middle of the midpopliteal crease to the point at the most proximal part of the medial malleolus, in centimeters.
Subject's age to the nearest year was also recorded.
For obtaining T c , we used DANTEC 2000 c equipment, the sensitivity, sweep, and filter were set at: 0.2-1 mv/div , 5 ms/ div , and 2-10,000 Hz respectively. The technique was the same as described in the Literature [1,2]. Briefly: the volunteers lied prone on the examining table with the feet off the edge of the plinth. The E 1 was placed at the middle of the line connecting midpoint of popliteal crease to the point at the most proximal part of the medial malleolus, and the E 2 over the Achilles tendon (both were surface electrodes). The ground electrode was posed proximal to E 1 and a disc electrode (anode) was placed on the anterior superior iliac spine. Then we inserted a monopolar 70 mm needle (cathode) at a point 1 cm medial to the posterior superior iliac spine, perpendicular to the frontal plane, and retracted it just a little after reaching the sacrum. Stimulus duration of 1 ms at 0.5 HZ was then applied while increasing current intensity to obtain both H and M waves simultaneously. M wave is the earlier wave and H is the later one. The interpeak latency was measured in milliseconds (ms) and recorded as T c . This measurement was only performed on the right lower extremity.
Descriptive statistics were applied to depict Mean ± SD of age, leg length and Tc. The independent effect of leg length and age on T c was assessed by multiple regression model. The analyses were performed using SPSS 10.0 software. Kolmogrov-Smirnov test was used for evaluating the normal distribution of the variables.

Results
From 46 subjects who volunteered to participate in this study; five cases were excluded after history taking and physical examination (two because of history of sacral radiculopathy, two because of diabetes mellitus and one because of asymmetry in ankle reflexes) and one case after electrophysiologic evaluation; thus we completed the study with 40 subjects. Subjects' characteristics are shown in table 1.
There was no correlation between T c and age (p value = 0.48, r = 0.11) We also found this regression equation: T c = 0.04L + 5.28 (L is leg length in centimeters, T c is represented in milliseconds.)

Discussion
In this study we found a significant correlation between leg length and Tc, but we were unable to show such a relation between age and T c .
Pease et al were the first, studied T c [2,4], and reported Mean ± SD of 7 ± 0.3 ms which is very close to our results (Tc= 6.78 ± 0.3). They didn't specifically consider the leg length, age or any other potential confounding variables to T c .
Zhu et al [3] evaluated 60 persons and reported Mean T c : 6.8 ms and its SD: 0.33 ms, again close to our results. They also reported that T c and person's height were correlated but didn't study any correlation between age and T c .
Wang et al [5] evaluated 40 persons and found this regression equation: T c = 0.02A + 0.003H + 0.92 (H: Height and A: Age), and stated that age is a contributing factor on T c .
Another research was performed by Ghavanini et al [6], in which 39 subjects were evaluated. The reported T c ± SD was 6.9 ± 0.4; two regression equations were also suggested: T c = 0.097Tp + 4.045 and Tc = 0.051L + 4.92 (L=leg length in centimeters); results are close to ours, and age was not found to affect T c .
A summary of the above data plus detailed demographic data are provided in the table 2.

Limitations
In this study we focused on age and leg length as potential contributing factors on the T c . we didn't control, rand-omize or observe other possible confounding (contributing) factors with potential to affect this parameter.
We observed a significant correlation between leg length and T c (P value = 0.003, r= 0.49, CI 95% = 0.59-0.88), that is compatible to a previous published work [3] (r = 0.54, p value less than 0.01).
Had we found any association between T c and age, the question might have been raised that subclinical neuropathy of old age could have been contributive; obviously, this is not the case in our study.
Although F-wave has been used to evaluate the possibility of proximal neuropathy; it was not measured in this study. Alternatively, we measured H-reflex latency to exclude proximal neuropathy [11].
It should be emphasized that noninvasive methodologies for the diagnosis of subclinical S1 radiculopathy are now available [12]. It is also acceptable to stimulate the S1 spinal nerve at the S1 foramen by magnet, instead of deep tissue needling; nevertheless, we used more popular techniques for this study.

Conclusions
We found that between age and leg length, only the latter can affect T c . It may be reasonable to consider leg length for calculating T c and to "narrow" the normal limits.
Further studies with larger sample sizes are required for detecting other contributing factors and standardizing T c according to leg length.

Competing interests
None declared.