Plantarflexor stretch training increases reciprocal inhibition measured during voluntary dorsiflexion

Abstract

Agonist-mediated reciprocal inhibition (RI) in distal skeletal muscles is an important neurophysiological phenomenon leading to improved movement coordination and efficiency. It has been shown to be reduced in aged and clinical populations, so the development of interventions augmenting RI is an important research goal. We examined the efficacy of using chronic passive muscle stretching to augment RI. The influence of 3 wk of plantarflexor stretching (4 × 30 s, two times/day) on RI of soleus and gastrocnemius initiated by tonic, voluntary dorsiflexion contractions [20% of maximum voluntary contraction (MVC)] was examined in 11 healthy men who performed stretch training and in nine nontraining controls. Hoffmann's reflexes (H-reflexes) were elicited by tibial nerve stimulation during both weak isometric (2% MVC) plantarflexions and dorsiflexion contractions at 20% MVC. Changes were examined at three joint angles, normalized to each subject's range of motion (ROM; plantarflexed = 10 ± 0°, neutral = -3.3 ± 2.9°, dorsiflexed = -16.5 ± 5.6°). No changes were detected in controls. A 20% increase in ROM in the stretch subjects was associated with a significant decrease in maximum H-reflex (H(max)): maximum evoked potential (M(max)), measured during 2% plantarflexion at the plantarflexed and neutral angles in soleus and at the plantarflexed angle in gastrocnemius (P < 0.05-0.01). By contrast, decreases in H(max):M(max) during 20% dorsiflexion contract were also seen at each angle in soleus and at the dorsiflexed angle in gastrocnemius. However, a greater decrease in H(max):M(max) measured during voluntary dorsiflexion rather than during plantarflexion, which indicates a specific change in RI, was detected only at the dorsiflexed angle (-30.7 ± 9.4% and -35.8 ± 6.8% for soleus and gastrocnemius, respectively). These results demonstrate the efficacy of soleus-gastrocnemius stretch training in increasing agonist-mediated RI from tibialis anterior onto soleus-gastrocnemius in young, healthy individuals at dorsiflexed, but not plantarflexed, joint angles.

Range of motion, neuromechanical, and architectural adaptations to plantar flexor stretch training in humans

“…The subject was slightly reclined (hip angle at 75°; 0°ϭ full extension) to minimize the tension at the back of the knee, which can prevent maximal dorsiflexion (12,15). The foot was firmly strapped to the ankle dorsi/plantar flexion footplate of the dynamometer, with the lateral malleolus aligned with the dynamometer's axis of rotation.…”

“…The stimulation protocol used was similar to that described previously (11,12). A 3.2-cm anode (fabric gel adhesive electrode, ValuTrode CF3200; Nidd Valley Medical, Harrogate, UK) was placed on the patella and a 1-cm-diameter cathode was placed over the tibial nerve in the popliteal fossa.…”

“…Square-wave electrical stimulation pulses of 1-ms duration were delivered via a constant current stimulator (DS7A; Digitimer, Hertfordshire, UK) at intervals of ϳ10 s once the subject had produced a stable, voluntary plantar flexion moment equal to 2% of the MVC measured at the specific joint angle. This procedure was used to improve measurement reliability (12,40). The stimulus intensity was increased gradually until a plateau in the M-wave amplitude (Ϯ3% variability) was clearly observed (M max), allowing quantification of the full H/M recruitment curve using 18 -30 stimuli (see example, Fig.…”

“…2), and a reference electrode was placed on the lateral malleolus. This electrode configuration has been used previously (12,22,45) and was found to provide larger H-and M-wave amplitudes and greater test-retest reliability in pilot testing. Markings were applied with a permanent marker (and reapplied if necessary) to ensure identical electrode placements after the 3-wk intervention period.…”

“…Additionally, both rotation and elongation of gastrocnemius medialis (GM) muscle fascicles and elongation of the whole muscle were measured using ultrasound imaging methods, and elongation of the Achilles tendon was estimated from changes in GM length and ankle angle. ROM and H-reflex data from some of the present subjects have been incorporated into a previously published analysis detailing changes in spinal reciprocal inhibition after stretch training (12).…”