Strong is not always fast.
We see it constantly in return to play discussions. An athlete comes back from a hamstring strain, ticks every box on paper — full range, symmetrical strength on the dynamometer, no pain on stretch or contraction — and yet something still feels “off” to them, or to us, when they get back onto the track or the field.
For a long time the answer to this was to keep testing peak strength. If the numbers matched left to right, the tissue was considered ready. But peak strength is a measurement of how much force a muscle can eventually produce, given enough time to reach it. And time is exactly what the hamstring doesn’t have during a sprint.
In the late swing phase of a sprint stride, as the leg whips forward and the hamstring has to slam the brakes on the shin before foot strike, that entire deceleration happens in around a tenth of a second. There simply isn’t time for the muscle to reach its ceiling. What matters in that moment isn’t how strong the hamstring eventually becomes, but how quickly it can get there — its rate of force development.
This is where a lot of previously strained hamstrings quietly fall short. Athletes with a history of strain can produce completely normal peak force, and still show a markedly slower rise to that force — in some cases over 30 percent slower on the injured side, and interestingly, often slower on the uninjured side too. That second finding tends to surprise people, but it points to something beyond the local tissue: a more systemic drop in how quickly the nervous system can recruit that muscle group after injury, not just a scar tissue problem.
It’s why we’ve shifted a lot of our later stage rehab testing towards early phase force output rather than eventual peak output. Practically, that means looking at how much force is produced in the first 50 to 100 milliseconds of a contraction, not just the highest number an athlete can eventually reach on the dial. It also means testing at longer muscle lengths, closer to where the hamstring is actually working hardest during sprinting, rather than the mid-range positions that are easiest to test in.
None of this is as simple as it sounds. Early rate of force development is a genuinely noisy measure to collect. Split it into different time windows or muscle lengths and you can get quite different numbers between sessions from the same athlete, even when nothing has actually changed in the tissue. We get far more reliable, trackable numbers when we standardise the time window we’re measuring within and express early force as a percentage of that individual’s own peak, rather than chasing an absolute number in isolation.
The upshot for anyone returning from a hamstring strain is this: passing a strength test tells you the muscle can produce force. It doesn’t tell you whether it can produce that force fast enough to do the job it’s actually needed for. Chasing the second question is a large part of what separates athletes who return and stay fit, from those who return and quietly break down again a few weeks later.
