Why one-sided carrying and rocking loads your shoulder
Nicola Tik
Shoulder discomfort is one of the more common physical complaints of new parenthood, and it often builds so gradually that many people do not connect it to their carrying and settling routine until it is already well established. This article explains what is happening in the shoulder during one-sided carrying and rocking, and why the shoulder on the side that does most of the holding tends to feel it most.
The shoulder is a remarkably mobile joint, but that mobility comes at the cost of inherent stability. Unlike the hip, which sits in a deep socket, the shoulder joint is shallow and relies heavily on the surrounding muscles, particularly the rotator cuff, to hold it in place and manage load. When the shoulder is asked to sustain a fixed position under load for extended periods, those stabilising muscles work continuously without the recovery that movement provides.
Carrying a baby on one side places the shoulder of that arm under exactly this kind of sustained demand. The rotator cuff muscles are working constantly to stabilise the joint against the weight of the baby, the muscles of the upper arm and shoulder blade are holding the arm in a fixed supporting position, and the upper trapezius, the broad muscle running from the neck to the shoulder, is under continuous tension to maintain the whole position. Across a full day of carrying, the cumulative demand on these structures is considerable.
Most people naturally gravitate towards carrying on one side, typically the non-dominant arm, which keeps the dominant hand free for other tasks. Whatever the reason, the effect is the same. One shoulder absorbs the majority of the carrying load day after day, while the other rarely shares it equally.
The shoulder that carries most does not simply get stronger from the repeated demand. Without adequate recovery between periods of sustained load, the stabilising muscles gradually fatigue, the tendons and soft tissue of the shoulder accumulate the kind of strain that produces aching and stiffness, and the joint becomes more sensitive to demands that would not normally trouble it. Many people notice the shoulder on their dominant holding side feeling heavier, less comfortable, and slower to recover from effort before they have recognised the pattern behind it.
Rocking and settling a baby introduces a different kind of shoulder load from sustained carrying. Rather than a static hold, rocking involves repetitive rhythmic movement through the shoulder, elbow, and wrist, sustained for as long as the settling takes. The shoulder muscles are not holding a fixed position but they are working continuously through a small repeated range, which places a different kind of demand on the tendons and soft tissue of the shoulder.
This repetitive low-level movement is particularly demanding on the tendons of the rotator cuff because it keeps them in continuous activity without the variation in load that broader movement would provide. Across many settling sessions through the day and night, the tendons accumulate a pattern of repetitive strain that contributes to the aching and tenderness many new parents notice across the front and top of the shoulder after periods of sustained settling.
The shoulder manages the early weeks of new parent activity reasonably well in most cases. The difficulty is that the load builds steadily as the baby grows heavier and the cumulative hours of carrying and settling increase. The shoulder that felt fine in the first few weeks may begin to feel noticeably different by the sixth or eighth week, not because anything has suddenly changed, but because the accumulated load has gradually reached a threshold the joint and surrounding structures can no longer quietly absorb.
Disrupted sleep compounds this further. The muscles and tendons of the shoulder repair during sleep, and when sleep is repeatedly broken, that repair is incomplete. The shoulder starts each day carrying forward the fatigue and strain of the day before, which means it reaches its threshold more quickly under the same level of demand.