Does Less Force on an Exercise Ball Mean You Can Lift Less?

Suppose you are choosing between a bench and an exercise ball for a pressing exercise. Then you come across a headline about “force loss” on the ball, with a percentage that sounds large enough to make the choice for you. Before it does, take a moment to picture the test behind the number. Was someone trying to push as hard as possible without moving, or lifting a bar with a set weight? Which of those situations did you have in mind?

The two force-loss figures came from different pressing tasks. In one test, ten resistance-trained men pushed for five seconds against floor-fixed handgrips. Their maximum force was 59.6% lower with Swiss-ball support than with bench support. The bench supported the head, shoulders and buttocks; the ball supported only the shoulders, with feet grounded. In a separate moving-barbell test, twenty male collegiate judo athletes produced about 6% lower computed peak external force on the ball than on the bench. Both percentages describe a ball-versus-bench comparison, but only the first describes maximum stationary force.^1,2

The moving press used the same absolute load on both supports: 50% of each athlete's flat-bench one-repetition maximum, the most they could lift once on that bench. Athletes attempted each repetition at maximal upward speed, and software calculated peak output from bar acceleration. This was neither a measurement of direct muscle force nor a test of the maximum weight they could lift on the ball. Keeping the load fixed also did not establish that it represented the same relative effort on each support.²

Muscle-electrical recordings provide a separate kind of information. Surface electromyography, or EMG, sampled five muscles on the right side in the experiment that measured maximum stationary force. No statistically significant support-surface differences were detected in those recordings. A nonsignificant difference does not establish equivalent electrical activity, equal effort or a measured amount of stabilizing work. The experiment also included moving dumbbell conditions for EMG, but those did not supply an equivalent maximum moving-force measurement.¹

The gap between roughly 60% and 6% therefore cannot show that movement itself prevented most of the force loss. The experiments differed in participants, load, equipment, support and force measurement; neither isolated contraction type while holding those other differences constant. Each result establishes an acute output difference for its own task. Neither experiment measured perceived difficulty or established long-term training benefits, and the contrast between them cannot supply those missing conclusions.^1,2

Before using either percentage to judge an exercise, decide which question you want it to answer. Are you comparing maximum force, performance with a particular load, or the value of a training method? Check the result against that question. You can assess a specific force measurement without making a broader decision about whether unstable pressing is a good or bad exercise.