Alternating hot, cold stimulation relaxes stiff shoulders

Alternating heat and cold stimulation (HC), as delivered by a wearable thermo-device, lowers objective muscle hardness, leading to improvements in subjective measures, such as muscle stiffness and fatigue, according to a recent study.

“In addition, improvements in muscle hardness in the HC condition were found to be associated with the degree of skin cooling during the intervention,” the researchers said.

Twenty young healthy men (mean age 20.3 years) participated in the study and were asked to complete a 30-minute typing task four times over four sessions each at least 1 week apart. During each session, after the typing task, participants were subjected to a randomly selected temperature stimulation condition (HC, heat, cold, or none), delivered by a wearable device attached to the bilateral upper trapezius muscles. After each task, objective and subjective outcomes were assessed with respect to the dominant hand.

Across all four sessions, the researchers documented no significant increase in trapezius muscle hardness after the 30-minute typing task, in contrast to both their expectations and existing literature. [Appl Ergon 2001;32:473-478]

Under the HC condition, however, mean muscle hardness increased from 1.41 N at baseline to 1.43 N after typing, followed by a significant decrease to 1.37 N after using the thermo-device (p<0.05). In contrast, postintervention muscle hardness after applying heat (1.44 vs 1.46 N), cold (1.44 vs 1.44 N), or no (1.43 vs 1.42 N) thermal stimulation was not significantly different as compared with after the typing task. [J Physiol Anthropol 2022;41:1]

Notably, skin temperature and its changes during the intervention were also associated with improvements in muscle hardness under the HC condition. Data from 19 participants showed that from an average baseline reading of 31.1°C, mean skin temperature reached as high as 41.7°C and as low as 28.5°C during the intervention.

Correlation analysis revealed that muscle hardness improvement was significantly affected by the degree of skin cooling, particularly in terms of the maximum cooling (r, 0.634; p<0.01) and the cumulative cooling change (r, –0.548; p<0.05) in skin temperature. No such effects were reported for heating.

In turn, the HC-associated reduction in muscle hardness translated to significant improvements in subjective measures relative to no thermal stimulation, including greater refreshed feelings and lower muscle stiffness and fatigue. The heat condition also elicited similar responses from participants, while cold stimulation did not.

“Our results showed that the reduction in muscle hardness during HC was associated with the degree of skin temperature cooling rather than heating. This result indicates the involvement of physiological effects expected with cold stimulation,” such as the control of inflammation, oedema, spasticity, and pain, the researchers said.

“Further investigations on the ratio and intensity of cooling should be conducted in the future to establish the optimal HC protocol, while elucidating the muscle stiffness mechanism or fatigue improvement,” they added.