The effect is achieved through attaching weighted pieces to the body (or to other garments) which leave the hands free to grasp objects.
It is normally done in the form of small weights, attached to increase endurance when performed in long repetitive events, such as running, swimming, punching, kicking or jumping.
Research demonstrates that using an overload during sprinting or speed drills requires lower-body muscles to generate more force against the ground which could lead to gains in strength and power and ultimately faster acceleration during running.
A study has shown that using a weighted vest can increase the metabolic costs, relative exercise intensity, and loading of the skeletal system during walking.
Simpler to put on and remove than weighted vests, it simulates how humans carry things, like young or injured comrades or camping supplies, rather than the storage of body fat.
Limitations in flexibility forward or backward or to the side in arms may or may not be present depending on the pack as they vary greatly.
A standard form of military and fire fighter training is not only be able to carry a backpack, but to march and run with one loaded down with a concrete "marble".
A disadvantage is that the tightness required to stop movements could possibly increase the damages in those with carpal tunnel syndrome.
Their advantage, beyond a slight increase in distance for leverage, is the lack of strain on the wrist from having weight isolated there, and better simulating sparring gloves.
They are generally not very heavy, and since they limit the grip anyway, unless being used to practice specific striking fists, light dumbbells are more common.
The location of the mass more readily duplicates the natural fat-storage mechanism of the human body and being closer to the core.
In leg raise exercises, it allows more activation of the hip flexors (and abdominals) without putting more strain on the quadriceps muscles for extension, making it good for sports-specific training on movements like knees and jumping.
Thigh weights are not ideal for movements involving quadriceps use, as they require tightness which may limit the use of the muscle and blood flow to the legs.
Further from the core, fixated above the ankle around the lower shin and Achilles tendon, due to leverage much less weight is needed to increase the forces on the body.
As they are attached to a region with a far smaller diameter than the thigh, there is not room for much weight without greatly changing the effective width of the lower leg.
An advantage over thigh weights is that they are not attached to any major muscle or fat storage region, so tightness is not a factor and it can be used in almost any exercise.
If an individual loses the weight without being trained to adapt to the transition, he may overexert himself without checking at the end of the movement and overextend a tendon.
This is more of a risk when people fully extend their limbs in such movements and do not come to a controlled stop at the end, limiting muscle flexion.
It is a major advantage in training rotational hip stabilizers, to work on turnout for martial arts and dance postures.