DISPERSION OF SHOULDER HELICAL AXES DURING UPPER LIMB MOVEMENTS AFTER MUSCLE FATIGUE

P. Adamo¹, F. Temporiti^1,2, S. Trombin¹, F. Natali¹, C. Cescon³, M. Barbero³, R. Gatti^1,2

Background: Shoulder complex is characterized by joint rotation center displacement during upper limb movements and capsular, ligament and muscular structures are essential to provide stability. Shoulder complex stability can be quantified through the analysis of helical axes (HAs) dispersion, which has been reported to be influenced by neuromuscular control. Fatigue represents a variable able to influence neuromuscular control, altering muscular activation timing, kinematics and proprioception during upper limb movements.

Purpose: The aim of the study was to describe changes in shoulder complex HAs dispersion after muscle fatigue during upper limb movements in young healthy subjects.

Methods: Thirty healthy right-handed volunteers (age 23.2±2.6 years) were asked to perform two cycles of 15 shoulder flexion and rotation movements with both upper limbs during two different recording sessions, spaced by 6 hours of rest. Subsequently to each session, participants were fatigued at right side using the same task performed in isometric condition (50% MVC) for three times. After a rest of 10 seconds, they repeated the task with right and with left upper limbs. Retro-reflective markers were placed on participants’ arms and trunk and kinematics was recorded by an optoelectronic system (SMART DX, BTS, Italy) before and after fatigue. HAs dispersion was computed using the Mean Distance (MD - cm) between HAs and the barycenter of the distribution and the Mean Angle (MA - degrees) between HAs and mean axis.  

Results: Increased MD (from 1.18±0.23 to 1.34±0.05; p=0.001) and MA (from 6.6±1.84 to 7.07±1.82; p=0.019) were found in right side after fatigue during flexion. Moreover, increased MD (from 0.58±0.17 to 0.67±0.15 - p=0.002 for right side and from 0.7±0.16 to 0.78±0.21 - p=0.038 for left side) and MA (from 0.62±0.99 to 2.98±0.71 - p=0.047 for right side and from 2.89±0.49 to 3.17±0.62 - p=0.019 for left side) were found in both limbs after fatigue during rotation.  

Conclusion(s): Fatigue seems to alter neuromuscular control leading to increased HAs dispersion during upper limb movements. This phenomenon might derive from compensatory strategies of scapular muscles aimed at replacing the stabilizer function of the fatigued rotator cuff muscles. Interestingly, this phenomenon occurred on the right side during flexion and on both sides during rotation due to the contralateral synergic activity of external rotator muscles during fatigue sessions.

Implications: HAs parameters can be a non-invasive approach for the assessment of shoulder complex stability or risk of injury in subjects undergoing fatigue. Moreover, in order to prevent musculoskeletal disorders at the level of shoulder complex, this data may be considered when exercise involving upper arms are proposed as rehabilitative or sportive activities.

Funding, acknowledgements: The authors did not receive any funding.

Keywords: Shoulder complex, Helical axes, Muscle fatigue

Topic: Musculoskeletal: upper limb

Did this work require ethics approval? Yes

Institution: Humanitas Clinical and Research Center - IRCCS

Committee: Internal Ethical Committee of the Humanitas Clinical and Research Center

Ethics number: CLF19/05