The energy expenditure of different activities depends on various factors such as the intensity, duration, and the muscle groups involved. In general, rapid movements of the hands, like feeding yourself quickly, may require more energy in a short burst compared to slowly positioning your back while sitting down. Rapid hand movements involve the contraction of muscles and may expend more energy in a short period, but it is a brief activity.
On the other hand, maintaining a certain posture or positioning your back involves isometric contractions, where muscles are engaged to hold a static position. This can also consume energy, but it's usually over a longer duration and may not be as intense as quick, dynamic movements.
It's essential to note that the actual energy expenditure can vary from person to person based on factors like fitness level, body composition, and overall health. Additionally, individual perceptions of effort may influence how strenuous an activity feels. If you're interested in specific calorie expenditure, you may want to look into tools that calculate calories burned during different activities, but keep in mind that these are estimates and can vary.