In the case of free weights, that force is gravity pulling downward. This means if we are to exercises the desired muscle groups in this movement, we must orientate our body supine on a bench so our muscles contract under the force of gravity acting upon the weight.
However, with machines, we can alter the direction of force. Gravity still acts on the weight but the machine alters direction of force via use of lever(s) or cable pulley(s). This means on lever chest press machine, for example, we would need to be upright with the hands behind the lever handles to work these same muscles. In any case, the alignment of target muscles must be positioned against the resistive forces, whichever direction they happen to be orientated.
Other subtle variations to the body's alignment or its appendages can make the difference between targeting the muscles or working an entirely different set of muscles. In other cases with less than ideal alignment, the target muscle may still be exercised somewhat, but likely not as nearly as effective as a more ideal positioning would allow.
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