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.
