Mechanomyographic assessment of contractile properties within seven segments of the human deltoid muscle.

The aim of this study was to determine, by a non-invasive whole muscle mechanomyographic technique (wMMG), how muscle segment contractile properties varied within the segments of the multifunctional deltoid muscle, and how such variations in contractile properties may reflect the muscle segment's function and fibre type composition. We hypothesised that muscle segment contractile properties, consistent with slower twitch muscle fibre populations, would be associated with the deltoid's prime mover abductor muscle segment (middle head), rather than the prime mover flexor and extensor muscle segments (anterior and posterior heads). Eighteen healthy and athletic University students (nine males and nine females; mean age 20-24 years) volunteered for this study. Each subject's right upper limb was secured with the forearm flexed to 30 degrees and the shoulder in 45 degrees of abduction. The wMMG laser sensor was positioned perpendicular to the middle of each muscle segment, to record the involuntary lateral displacement of the muscle belly following a maximal, single twitch, percutaneous neuromuscular stimulation (PNS) [180 V (max.); 80 mA (max.); 50 mus]. Ten trials were recorded from each of the seven deltoid segments for a total of 70 trials per subject. From each segment, eight variables were analysed from the recorded wMMG waveforms; maximal displacement (D (max)); delay time (T (d)); contraction time (T (c)); sustain time (T (s)); relaxation time (T (r)) and half relaxation time ((1/2)T (r)), average rate of contraction (ARC) and the average rate of relaxation (ARR). The results indicated that the contractile properties of the seven segments of the deltoid muscle showed significant (P < 0.05) variation in a medial to lateral direction. Medially the strap-like segments of the anterior (S1, S2) and posterior heads (S4-S7), with larger moment arms for shoulder flexion and extension respectively, had the fastest contractile properties. In contrast the multipennate segment 3, with the largest moment arm for shoulder abduction, had the slowest contractile properties (P < 0.05). Muscle segment contractile properties were matched to the biomechanical and architectural characteristics of the individual muscle segments.