Why it is difficult to do one side farmer carry i.e I am carrying heavy weight only on one side. I can carry 60 pound each on my both hands in farmer carry. But one sided is hard enough. Is one side also is any kind of excercise , read somewhere that it puts unnecessary strain on spine. But in everyday life we will carry weight on one side only.
One sided or Unilateral Farmer Carries are actually a fantastic exercise.
Since carrying the weight on one side causes a lateral flexion (side-bending) and rotational forces. To prevent yourself from literally falling over, these forces must be neutralized -- by utilizing smaller stabilizing musculature.
In contrast, during a Standard Farmers Carry you have weights in both hands, rather activating key stabilizers the weights you're carrying simply offset. The anti-rotational / lateral flexion component has been removed.
One thing to note. “Unnecessary” stress on the on the spine (core) is created by movement done in an unnatural, non-functional position.
For example, sit-ups cause dangerously high compression and shearing forces to your spine -- about 730 pounds (3300 N) which well exceed safe levels established by the CDC.
Larger muscles, with high force production (global musculature) must compensate by improperly activating to counteract these forces.*
Done over time, your literally programming your nervous system to recruit (activate) these muscles at the wrong time -- such as smaller movements when those large muscles aren't needed.
Unilateral/imbalanced movements are more difficult than bilateral/balanced movements precisely because (and to the extent) of the imbalance, which you must overcome by exerting asymmetric force.
Bilateral/balanced movements such as barbell squats and deadlifts are better for strength development than unilateral/imbalanced movements because the latter, while difficult, significantly reduce the amount of weight that you can handle over a long effective (and safe) range of motion.
Squats, deadlifts, overhead presses, and bench presses stimulate essentially all of the muscles (large and small) that are worth training for most people.