Inhibition of soleus Hoffmann reflex by ankle-foot orthosis application in healthy volunteers.

BACKGROUND: An ankle-foot orthosis is suggested to actively assist dorsiflexion of the foot by induction of a positive proprioceptive impact to ankle dorsiflexor muscles. However, an objective proof is missing. OBJECTIVES: To assess the proprioceptive impact of an ankle-foot orthosis application by Hoffmann reflex recordings of the soleus muscle under static conditions. It was hypothesized that the use of an ankle-foot orthosis facilitated dorsiflexor motor function and thus a decreased the soleus Hoffmann reflex. STUDY DESIGN: Experimental study in healthy volunteers, pre-post test design. METHODS: In all, 20 healthy volunteers were enrolled in order to assess the proprioceptive impact of orthosis application. The Hoffmann reflex was recorded before, during, and after orthosis application. RESULTS: Under orthosis application, the Hoffmann reflex significantly decreased as compared to before (p < 0.05) and after application (p < 0.05). CONCLUSIONS: Findings indicate an inhibition of plantarflexors probably induced by facilitation of ankle dorsiflexors under static conditions. At first glance, it seems that foot orthoses primarily have a stabilizing effect on ankle joints in terms of simple mechanical bandages. However, the present results suggest an additional active impact on proprioceptive control. CLINICAL RELEVANCE: The putative neuromodulatory effect on motor control may support the application of such ankle-foot orthoses in, for example, drop foot. Furthermore, the objective assessment of a neurophysiological mode of action of orthoses by Hoffmann reflex recordings might be an appropriate primary outcome parameter in clinical trials.

Effect of cutaneous blood flow on absorption of insulin: a methodological study in healthy male volunteers.

Absorption of drugs from subcutaneous tissue depends on several factors, including tissue perfusion at the administration site. Tissue perfusion can be manipulated by e.g. application of local heat. This may subsequently alter the rate or amount of absorption of drugs from a subcutaneous depot. The aim of the present study was to investigate if increased tissue perfusion after controlled local heating can change the absorption of subcutaneously administered short-acting insulin (Actrapid®, 100IU/ml). Thirteen healthy Caucasian males participated in two randomized experimental sessions; one session with locally applied controlled heat at the injection site, and a control session without local heat application. Tissue perfusion (blood flow) was monitored with Laser Doppler Imaging, and blood samples were taken to assess the levels of glucose and insulin. Local heat application at the site of insulin injection significantly enhanced tissue perfusion by approximately 145%. However, no correlation was found between insulin absorption and tissue perfusion. Based on our findings, it was concluded that tissue perfusion is not the rate-limiting factor in the absorption of high-concentration short-acting insulin from a subcutaneous depot. It is suggested that dissociation of insulin hexamers into dimers and monomers is a major rate limiting factor to the absorption.