Eight weeks of sprinting lengthened hamstring fascicles by over 2 cm
In short
Fourteen participants (10 female, 4 male, mean age 21.6) completed 8 weeks of speed training built on maximal-velocity running, resisted sprinting, and acceleration drills, with lower-body lifting limited to a low volume of back squats to maintain baseline strength. Biceps femoris long head fascicle length, measured by 2-D ultrasound, increased by 2.23 cm on the left and 2.44 cm on the right (p < 0.0001), and vastus lateralis also lengthened significantly (right: 1.48 cm, 95% CI 0.95–2.01), with large effect sizes (g = 1.40–2.03). Meanwhile 40-metre sprint, jump, and maximal strength outcomes showed no significant change, and only flying 10-m improved, by a small effect (g = -0.27).
Eight weeks of running changed muscle architecture. Biceps femoris long head fascicle length grew 2.23 cm on the left and 2.44 cm on the right (p < 0.0001). Over the same period, 40-metre sprint times, jumps, and maximal strength did not significantly change.
Why does fascicle length matter?
Fascicle length describes how long the muscle fibre bundles run in series, and it is a key determinant of sprint performance. Lengthening has mostly been observed after eccentric strength training or after programs that combine sprinting with resistance work. Whether sprint training on its own could do it, independent of concurrent sport training, was unclear.
What happened across eight weeks without lifting?
The sample was 14 participants (10 female, 4 male; 21.6 ± 3.3 years; 68.0 ± 11.8 kg). Sessions consisted of maximal-velocity and resisted sprinting plus technical acceleration drills, with lower-body lifting restricted to a low volume of back squats purely to maintain baseline strength. No other lower-body training was permitted, so the observed changes could be attributed to the sprint work.
Measured by 2-D ultrasound, biceps femoris long head (BFlh) fascicle length increased 2.23 cm on the left (95% CI 1.75–2.70) and 2.44 cm on the right (95% CI 1.99–2.90). Vastus lateralis lengthened significantly on both sides, with the right at 1.48 cm (95% CI 0.95–2.01). Effect sizes were large across every muscle measured (g = 1.40–2.03).
Why did performance stay flat?
This is the most useful part of the result. There were no significant main effects for time on 40-metre sprint, jump, or strength outcomes. The only movement in the right direction was a small effect for flying 10-m (g = -0.27, 95% CI -0.46 to -0.09). The architecture shifted substantially inside eight weeks; the performance numbers had not caught up.
It is also a question of what counts as a training effect — a change in the tissue or a change on the clock. As with the observations that tendons lag behind strength and that strength gain outruns muscle mass, tissue, structure, and performance move on different timelines.
What a lifter should take from it
Your Muscle Index only reads squat, bench, and deadlift 1RMs, so eight weeks of sprinting leaves almost no trace on it. What the data do say is that training without a barbell still remodels muscle architecture. A sprint block does not leave the lower body unstimulated — and equally, there is no basis for expecting it to move your numbers in eight weeks.
Read alongside the finding that speed work raises force before velocity, plus stretch training and muscle length and eccentric overload adding sarcomeres, the picture is that there is more than one route to a longer fascicle.
This was a single-group design with 14 participants and no control group. Fascicle length was estimated from 2-D ultrasound, and eight weeks may simply be too short for performance changes to appear.
Frequently asked questions
What is fascicle length and why does it matter?
It is a measure of muscle architecture describing how long the fibre bundles run in series. It is considered a key determinant of sprint performance, so lengthening is treated as a hallmark adaptation to speed training.
Can running alone change muscle architecture?
In this study it did. After 8 weeks of sprint-focused training, with lower-body lifting limited to a low volume of back squats for strength maintenance, biceps femoris long head fascicle length increased by 2.23 cm on the left and 2.44 cm on the right (p < 0.0001).
Did sprint performance improve too?
No. There were no significant changes in 40-metre sprint, jump, or maximal strength. Only flying 10-m showed improvement, and only as a small effect (g = -0.27).
Why would structure change without performance changing?
Tissue adaptation and performance improvement appear to run on different timelines. Eight weeks was enough to produce architectural change, but likely too short for that change to convert into faster times.
Is this a reason for lifters to sprint?
Not as a way to raise big-three 1RMs — there is no evidence of that here. It does suggest that a sprint block still remodels lower-body muscle, so the legs are not going unstimulated during one.
Source: PubMed