Middle-aged adults cocontract with arm ADductors during arm ABduction, while young adults do not. Adaptations to preserve pain-free function?

Middle-aged individuals cocontract with adductor muscles during abduction. This may be crucial for counteracting deltoid forces, depressing the humerus and ensuring free passage of subacromial tissues underneath the acromion during abduction. We questioned whether adductor co-contraction is always present, or develops during ageing, in which case it may explain the age-related character of common shoulder conditions such as Subacromial Pain Syndrome. In a cross-sectional analysis with electromyography (EMG), activation patterns of the latissimus dorsi, teres major, pectoralis major and deltoid muscle were assessed during isometric force tasks in 60 asymptomatic individuals between 21 and 60 years old. Cocontraction was expressed as the degree of antagonistic activation relative to the same muscle's degree of agonistic activation, resulting in an activation ratio between -1 and 1, where lower values indicate more cocontraction. Using linear regression analyses, we found age-related decreases in the activation ratio of the latissimus dorsi (regression estimate: -0.004, 95% CI: -0.007 to 0.0, p-value: 0.042) and teres major (regression estimate: -0.013, 95% CI: -0.019 to -0.008, p-value: <0.001). In contrast to young individuals, middle-aged individuals showed a high degree of adductor cocontraction during abduction. This may indicate that during ageing, alterations in activation patterns are required for preserving pain-free shoulder function.

Deltoid muscle morphometry as an index of impaired skeletal muscularity in neonatal intensive care.

We hypothesised that extremely premature infants would have decreased muscle mass at term-corrected age compared to term-born infants and that the degree of reduced muscle mass acquisition would correlate with the duration of invasive mechanical ventilation. The MRI brain scans of infants admitted in the neonatal unit at King's College Hospital between 1 January 2010 and 1 June 2016 were retrospectively reviewed. The coronal cross-sectional area of the left deltoid muscle (DCSA) was measured in 17 infants born < 28 weeks of gestation and in 20 infants born at term. The prematurely born infants had a median (IQR) gestation age of 25 weeks (24-27) and the term infants 40 weeks (38-41). The duration of invasive mechanical ventilation for the prematurely born infants was 39 days (14-62) and that for the term infants 4 days (2-5), p < 0.001. DCSA was smaller in prematurely born infants (median 189, IQR 176-223 mm2) compared to term-born infants (median 302, IQR 236-389 mm2), p < 0.001. DCSA was related to gestation age (r = 0.545, p = 0.001), weight z-score at MRI (r = 0.658, p < 0.001) and days of invasive mechanical ventilation (r = - 0.583, p < 0.001). In conclusion, extremely premature infants studied at term had a lower muscle mass compared to term-born infants. CONCLUSION: Our results suggest that prolonged mechanical ventilation in infants admitted in neonatal intensive care is associated with reduced skeletal muscle mass acquisition. What is Known: • Prolonged mechanical ventilation in adult intensive care patients has been associated with skeletal muscle dysfunction and atrophy. • The cross-sectional area of the deltoid muscle has been used to evaluate muscle atrophy in infants with a previous branchial plexus birth injury. What is New: • Premature infants studied at term exhibit lower cross-sectional area of the deltoid muscle than their term counterparts. • Prolonged mechanical ventilation could be associated with skeletal muscle impairment.