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Wimps might have the last laugh on gym-obsessed hunks - gram-for-gram, bodybuilders' muscles are weaker than those of people who never touch weights, research has shown.

Scientists found that bodybuilders had extraordinarily large muscle fibres capable of generating forces far greater than normal for an average human.

Yet tests showed that a gram of their muscle was less powerful than the same amount of tissue from someone who did not weight train.

Lead scientist Professor Hans Degens, from Manchester Metropolitan University, said: ''Most of us are impressed by the enormous muscle bulk of bodybuilders and think that these people must be extremely strong, like the 'Incredible Hulk'.

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''The surprising thing, however, was that a gram of muscle from bodybuilders produced less force than that from non-bodybuilders, and it thus seems that the 'muscle quality' is less in bodybuilders.

''It appears that excessive muscle growth may have detrimental effects on the quality of the muscle, and one may well be better off with normal-sized muscles than with metabolically expensive large muscles.''

Bodybuilders are only strong because they have so much muscle mass - enough to compensate for the weakness of each gram, the research suggests.

''We had no indication that the proteins generating force - muscle motor proteins - work less in bodybuilders, but it could be that they have fewer motor proteins per gram muscle,'' Prof Degens added.

''It would be interesting to see what aspect in the training of bodybuilders causes this decrease in muscle quality.''

The scientists, whose findings are reported in the journal Experimental Physiology, took small muscle samples from the thighs of 12 male bodybuilders, six power athletes such as sprinters, and 14 men who were physically active but did not weight train.

Individual muscle fibres were isolated and tested to see how fast and powerfully they contracted. The results of these experiments allowed the team to make an assessment of muscle quality.

In contrast to bodybuilders, power athletes appeared to have an improved level of muscle quality, the researchers found.

''The training method seems to have an impact on muscle quality, which is of great importance for trainers and coaches interested in improving either performance or appearance of athletes,'' said Prof Degens.