This is an interesting question and I'll offer my own
view as food for thought as a recent listner and first time caller
to this Biomx group....

The question of muscle tearing can be thought of as mechanical failure of
biologic tissue. The muscle is perhaps best modeled as a viscoelastic
material. The question becomes, how does the mechanical structure
of a muscle fiber change with respect to the number or work cycles.
As the biochemical factors change, the rate of cross bridge binding
and unbinding is believed to be constant when comparing rep # 1 to rep # 10.
However, the material properties of muscle may show a decreased
time dependence with increasing rep, thus the higher number the rep,
the less 'give' or 'slack' in theory, a unit of muscle is able to perform
as a result of either
1. locked cross bridges (cramping, rigor mortis)
2. connective tissue deterioration during the set (composed of repititions)
3. a hole host of contributing factors relating to structure-function
and physiology during contraction.

this is a very interesting question and can be approached
experimentally, via material testing
or analytically through optimization design/analysis.
or case studies of those that have torn their pecs ( which I believe
was written in one of the muscle magazines a year ago or so)

The importance of time dependent changes addressed the principles of
rate of reactions, recruiting of fiber bundles, initial conditions
of the muscle (remembering the importance of prestretching before
exercising), metabolism of the individual,athletic condition of the
individual, dietary input of the individual, rate of lifting,
the amount of weight being lifted,and failure mechanics of composite
materials should be considered to address this question fully.
However, the model becomes complicated rather quickly and the answer
could be found by following up some the force studies of myofibers, I
believe by Faulkner,Brooks, and/Dennis. The drawback to single fiber
studies is the effects of recruiting and rate of loading a whole muscle
cannot be determined.

The short story is the muscle unit has greater mechanical integrity
at the beginning of the set, but can produce greater force.

For the last rep, the muscle unit has less mechanical integrity, but
produces less force (or does it, if you're lifting the same weight ten

Based on this, I would say for the last rep you are producing the same
amount of force, at a slower rate of loading, recruiting fiber
bundles with greater frequency, but of shorter duration, causing microdamage
within and between myofibers (muscular and connective tissue respectively
but not exclusively) Hence, the last rep would be the one to do the damage.

Anecdotely, I've never seen an athlete tear a muscle or sprain soft tissue
within the first five minutes of a game, providing they have been
educated to stretch and warm up appropriately and they weren't slide tackled
from behind. But have noticed most of these injuries occur during the
second half or later periods. Even if this is a question of coordination,
the forces resulting from uncoordination are acting on a fatigues system.
The same issues apply to the biochemical and structural effects
of fatigue on the musculoskeletal system.

Alexander Kazerooni, MS
Biomedical Engineering & Kinesiology
University of Michigan, Ann Arbor