The two primary results of this study showed: 1) The Sol, MG, and LG H-reflexes during prone lying and standing for three active ankle positions were similar in some respects. The Sol, MG, and LG H-reflex maximum amplitudes were inhibited during standing compared to lying with the ankle in the neutral position, inhibited during dorsiflexion compared to the neutral ankle position during lying and standing, and facilitated during plantarflexion compared to the neutral ankle position while standing; and 2) The Sol, MG, and LG muscle activities differed in terms of reflex recruitment (peak-to-peak amplitudes and thresholds of stimulation).
Similarities between the Sol, MG, and LG H-reflex recordings
The similarities between the muscles (Sol, MG, and LG) H-reflex average maximum amplitudes, particularly during standing, may be explained by their role as antigravity postural muscles in which the suppression of these muscles would prevent body instability (swaying) and retain balance necessary for upright posture. Similarities may also be explained by how these muscles perform during locomotion .
The suppression of the Sol, MG, and LG H-reflex maximum amplitudes during standing upright may be due to presynaptic inhibition, in which somatosensory afferents from the foot sole and/or stretch receptors from the soleus muscle may have been the source of presynaptic inputs. This was previously reported as a possible mechanism for this unintuitive result of greater H-reflex amplitudes during prone lying compared with active standing [7, 8, 10–14]. Reciprocal inhibition may also have contributed to the recorded suppression of the Sol, MG, and LG H-reflex maximum amplitudes. This was more likely during active ankle dorsiflexion while lying or standing, when the contraction of antagonistic muscles (i.e., anterior tibialis) may have evoked reciprocal Ia inhibition on the triceps surae muscles , resulting in the observed average Sol, MG, and LG H-reflex maximum amplitude suppression. Another inhibitory mechanism which may have contributed to the recorded suppression of the Sol, MG, and LG H-reflex maximum amplitudes is vestibular inhibition . In this study, participants' H-reflexes were measured after stabilizing standing posture to avoid the vestibular effect on the H-reflex measurements due to dynamic postural changes. Therefore vestibular inhibition is an unlikely explanation for these results, but may have contributed. These mechanisms cannot be distinguished in this study, as it is possible all three inhibitory mechanisms (i.e., presynaptic, reciprocal, and vestibular influences) may have contributed to the H-reflex suppression among the Sol, MG, and LG with varying amounts of inhibition.
The facilitation of the Sol, MG, and LG H-reflexes during standing on metatarsal (maximum plantarflexion) may be explained by the increase in the excitability drive to the alpha motoneurons of the Sol, MG, and LG muscles as subjects contracted these muscles against gravity on the body's mass . By comparison, in the present study there were no changes for the Sol, MG and LG H-reflex maximum amplitudes during prone lying with ankle in plantarflexion. The likely explanation for this is the greatly reduced requirement of excitability drive to the alpha motoneurons of the Sol, MG, and LG muscles as they were only displacing the foot segment against gravity rather than the entire body mass.
The inhibition of the Sol and MG H-reflexes during ankle dorsiflexion and facilitation during ankle plantarflexion reported in this study were in agreement with a previously reported study . Pinniger and colleagues  reported Sol and MG H-reflexes were inhibited during passive lengthening and facilitated during passive shortening actions. The current study extends those observations for the Sol and MG inhibition and facilitation during active lengthening and shortening, respectively. The inhibition and facilitation of the Sol and gastrocnemius H-reflexes under similar conditions of static postures and ankle positions reported in this study, however, were in conflict with previously reported results . Moritani and colleagues  found differences between the Sol and MG H-reflex amplitudes (i.e., facilitation of the gastrocnemius and inhibition of the Sol) during the dynamic functional performance during hopping. Differences between the Moritani, et al. study  and the present findings may be explained in part by the differences between the dynamic and static functional movements during H-reflex measurements. In this study, participants were either statically lying or standing while maintaining the three ankle positions.
Although the major goal of this study was to establish the normal activation patterns of the muscles around the ankle joint under varied recordings condition, the study findings may be of clinical relevance. It provides a reference standard for comparison of patients with L5 nerve root impingement, as existing H-reflex procedures do not allow valid examination of the L5 nerve root. However, the validation of the clinical diagnosis of L5 and S1 radiculopathies requires patients with proven radiculopathies at different levels and it is the focus of ongoing research. Nevertheless, testing patients with proven radiculopathies using the H-reflex has been shown to be more useful during standing as compared to lying. It increases the sensitivity of the H-reflex for detecting subtle changes . Thus, future studies of patients with L5/S1 radiculopathies may find it beneficial to use this method of examining H-reflexes for differentiating L5 and S1 impingements.