Most summer training plans build a young athlete’s conditioning, agility, and strength. What they often leave out is the one demand football camp makes on day one: an all-out sprint. When a player has not run at full speed since spring, and the first time they do is in full pads and heat, the hamstring is being asked to perform its hardest job with the least preparation.
This article explains what a hamstring strain is, how it is graded, why it occurs, and what recovery actually looks like — along with the risk factors that matter most before camp, including California’s rules on heat and gradual increases in training load.
Quick Answers, Before the Details
What is a hamstring strain? A tear in one of the three muscles that run down the back of the thigh, most often where the muscle fibers narrow into the tendon.
How is it graded? Grade 1 (mild), Grade 2 (partial tear), or Grade 3 (complete tear), based on how much muscle tissue is damaged.
What usually causes it? Sprinting at or near top speed — especially when the hamstring has not been trained recently to handle that demand.
When is it an emergency? A loud “pop,” sudden inability to walk or keep playing, or significant swelling. In a teenager, these same signs can also mean a growth-plate injury rather than a routine muscle strain, so they warrant same-week evaluation.
Does nutrition or hydration matter? Yes. Under-fueling and dehydration are linked to a meaningfully higher injury rate in adolescent athletes. Both are straightforward, fixable pieces of a prevention plan.
What Is a Hamstring Strain?
The hamstrings are three muscles that run down the back of the thigh — the biceps femoris, semitendinosus, and semimembranosus. Together they bend the knee and extend the hip, two key actions that power a sprinting stride.
A strain is a tear in the muscle. It can range from a few overstretched fibers to a complete rupture. Most hamstring strains occur in the biceps femoris, usually where the muscle narrows into its tendon — the strong, rope-like tissue that anchors muscle to bone.
Not every hamstring strain is simply a more severe version of the same injury. The exact location of the tear also matters: within the muscle belly, at the point where muscle meets tendon (the musculotendinous junction), or inside the tendon itself. Location influences both recovery time and the risk of reinjury.
One important distinction: this article focuses on acute hamstring strains — sudden tears that happen during a sprint, kick, or stretch. A different condition, proximal hamstring tendinopathy, causes deep lower-buttock pain that builds gradually over weeks or months rather than starting with a sudden pop. It requires a different treatment approach and is covered separately.
How Are Hamstring Strains Graded?
Most people are told they have a Grade 1, Grade 2, or Grade 3 strain:
- Grade 1 — a mild overstretch with only microscopic tears in the muscle fibers. Soreness and tightness are common, with little loss of strength. Athletes often return to light activity within one to two weeks.
- Grade 2 — a partial tear. Pain, weakness, and often swelling or bruising are noticeable. Recovery is typically measured in several weeks.
- Grade 3 — a complete tear, sometimes with the ends of the muscle pulling apart. This is the injury most likely to produce a “pop,” a sudden collapse, and a recovery measured in months rather than weeks.
That three-level system is useful for a quick conversation, but it is not very precise. Over the past decade, researchers have developed more detailed grading systems based on MRI findings. The most widely used is the British Athletics Muscle Injury Classification. It evaluates both the exact location of the tear and whether the tendon itself is involved — not just how much tissue is damaged.
This distinction matters in practice. Two athletes who are both labeled “Grade 2” can have very different recovery timelines if one injury extends into the tendon and the other does not. That is one reason imaging and a careful hands-on evaluation matter more than the grade number alone once an injury appears more than mild.
How Do Hamstring Strains Happen?
Most hamstring strains that are not caused by a direct blow occur during sprinting — not during stretching or an easy jog. A smaller number happen during an extreme stretch, such as a high kick.
Here’s the mechanical reason. Just before the foot hits the ground in a sprint, the hamstring must stay under heavy load while it is actively lengthening — similar to slowly lowering a heavy suitcase rather than simply holding it steady. This is called an eccentric contraction. It is the moment the muscle is working hardest and stretching the most at the same time. That combination of high load and high length is why most hamstring strains cluster around maximum-effort sprinting rather than jogging or a gradual warm-up.
But not for the first time in August.
Why the First Sprint of Camp Is the Wrong Time for a First Sprint
This is where the title of this article becomes a training principle rather than a slogan. Consider an athlete who spends the summer lifting, conditioning, and doing agility drills — but never actually sprints at full speed. Camp then becomes their first real exposure to the exact demand that causes most hamstring strains. That exposure often arrives in full pads, in the heat, during the first week of two-a-days.
Research on hamstring injury prevention increasingly points to sprinting itself as an underused part of preparation — performed intentionally and built up gradually in the weeks before camp. It is not a replacement for strength work. It is a partner to it.
A long-running study of collegiate sprinters offers a useful illustration. As their injury-prevention approach evolved — from strength training alone, to strength plus agility work, to strength combined with agility and flexibility training — hamstring injury rates dropped substantially at each stage. No single exercise produced the reduction by itself. It was the layering of several qualities, including sprint work, that made the difference.
This is not only a high-school problem. The same pattern appears every summer at the NFL level, where more than half of all hamstring injuries occur during training camp — for many of the same underlying reasons: months away from real sprinting, followed by a sudden jump back into it.
What Raises the Risk of a Hamstring Strain?
A Previous Hamstring Injury Is the Biggest Warning Sign
Of everything researchers have studied, nothing predicts a future hamstring strain more strongly than a past one. A history of any hamstring strain roughly triples the risk of a new injury, and a strain within the past year raises that risk nearly five-fold. Getting older is the other consistently strong factor.
For a teenage athlete heading into camp, the most important question is simple: Has this hamstring been hurt before, and was it ever fully rehabbed?
A history of hamstring strain is linked to roughly 3 times the risk of a new hamstring injury. A strain within the past year raises that risk almost 5-fold (Green et al., 2020 — a large research review combining data from 71,324 athletes and 8,319 hamstring injuries).
Tight Hamstrings and Muscle Strength Balance
Tight hamstrings and a strength imbalance between the hamstrings and the quadriceps (the muscles on the front of the thigh) have long been taught as classic risk factors. They are not wrong to consider, but the research on that specific strength balance — often called the hamstring-to-quadriceps or “H:Q” ratio — is more mixed than conventional advice suggests. A 2022 review found that this ratio, on its own, is not a reliable predictor of who gets hurt.
What the research does show clearly is that strength training and hamstring-focused exercise reliably improve that balance. They also reduce tightness when it is a genuine issue. Both remain worth training, even though no single number on a strength test should be treated as a crystal ball.
How the Training Program Is Built
How a program is structured matters more than any single exercise inside it. Inadequate warm-up, accumulating fatigue without enough recovery, and increasing training load too quickly are all modifiable factors linked to higher hamstring injury risk. Fatigue in particular has been shown to weaken both the hamstrings and the quadriceps and to shift that strength balance in ways that likely raise risk.
In practice, this means favoring a program that builds gradually, protects recovery time between hard sprint sessions, and does not stack a first full-speed sprint on top of an already tired, under-slept, under-hydrated week of two-a-days.
Can Strength Training Prevent a Hamstring Strain? The Nordic Curl, Explained
The Nordic hamstring curl is the single most studied exercise for preventing hamstring strains. To perform it, an athlete kneels with their ankles held down by a partner or a pad, then slowly lowers their torso forward toward the ground, using the hamstrings to control the descent as long as possible. That controlled lowering is an eccentric contraction — the same type of muscle action, performed at a slower speed, that a sprinting hamstring must produce at full speed.
The overall research is encouraging: programs that include the Nordic curl have been linked to roughly a 50% reduction in hamstring injury rates in large combined studies. A more recent, stricter re-analysis of the same trial data, however, found the protective effect was smaller and not fully conclusive when only the highest-quality studies were included. In short, the Nordic curl is a legitimate tool with a clear mechanical rationale, but how it is taught and how consistently it is performed likely matter as much as simply adding it to a program.
Programs that include the Nordic hamstring curl have shown up to roughly a 50% drop in hamstring injury rates in large combined studies (van Dyk et al., 2019, covering 8,459 athletes). A stricter 2021 re-analysis using only the highest-quality trials found the effect was favorable but not fully proven on its own — a reminder that technique and consistency matter as much as simply doing the exercise.
A few practical pointers for doing it well:
- Slow and controlled beats fast and sloppy. The entire point is resisting the fall, not collapsing into it.
- A little goes a long way. Research comparing higher- and lower-volume Nordic curl programs found similar strength gains from both — useful for a summer schedule that is already full.
- It mainly trains the knee, not the hip. Sprinting lengthens the hamstring at both the hip and the knee, while the Nordic curl primarily challenges it at the knee. Some researchers suggest pairing it with a hip-focused exercise such as a Romanian deadlift to address both ends of the muscle.
- Keep it up. Strength gains fade within a few weeks of stopping, so a program that ends in June will not still be protecting an athlete in August.
None of this means eccentric exercise by itself prevents hamstring strains. No single exercise, used in isolation, is a guaranteed fix, and the research does not support treating any one of them that way. What the evidence does support is combining eccentric strength work with real sprinting, flexibility training, and a progressive training plan matched to the athlete in front of you.
What Are California’s Heat Rules for Football Camp?
Two-a-day practices in August stack three risk factors on top of each other at once: an athlete who may not be sprint-ready, unfamiliar heat exposure, and a sudden jump in training load — often all in the same week. Football has the highest rate of heat-related illness of any high school sport, and national data show that risk is roughly ten times higher during the preseason than during the regular season.
Both national safety guidelines and California’s own rules exist specifically to prevent teams from front-loading that risk all at once. National guidelines from athletic trainers call for practice intensity and length to increase gradually over the first one to two weeks. States that require schools to follow these guidelines see measurably fewer cases of heat illness than states that do not.
California’s Interscholastic Federation (CIF), the governing body for high school sports in the state, applies that same principle directly to football with a required 14-day acclimatization period. Athletes ease into full training over two weeks, and protective gear is added in stages rather than all at once from day one.
This timeline matters for hamstrings as well as heat safety. A body that is overheated, under-conditioned for sprinting, and ramping up training load all in the same week has very little margin for error. Following the acclimatization schedule is not only a heat-illness rule — it also helps prevent the exact pile-up of risk factors that camp otherwise creates in week one.
Does Nutrition and Hydration Affect Hamstring Injury Risk?
Muscle and tendon tissue rebuilds itself between practices — but only if the body has enough fuel and fluid to support that rebuilding. Two-a-day camp practices in August place high demands on a teenage athlete: more calories burned, more fluid lost to sweat, and less recovery time between sessions.
When food and fluid intake fall short of that demand, researchers call it low energy availability — the body is not taking in enough energy to cover both training and the normal processes of growth and tissue repair. It is more common than most families expect. One large review of more than 6,000 athletes found that nearly 45% showed signs of low energy availability. A separate study that followed adolescent female athletes through a nine-week season found that those who screened at risk beforehand were injured more than three times as often as their teammates. The large majority of those injuries were ordinary muscle and joint problems, not unusual diagnoses.
The likely mechanism is straightforward. When the body is short on fuel, it prioritizes survival functions over maintenance ones. Rebuilding muscle and tendon tissue is a maintenance job, so it is one of the first processes to be shortchanged. The result can be tissue that is less prepared to handle the sudden loads of hard sprinting. This pattern is common enough in growing athletes that we have written a full guide to Relative Energy Deficiency in Sport (RED-S), covering the warning signs and what treatment looks like.
Among adolescent female athletes screened before a nine-week season, those who scored at high risk for low energy availability were injured at more than three times the rate of their lower-risk teammates — and 93% of those injuries were ordinary strains and sprains (Smulligan et al., 2025, Physical Therapy in Sport).
Dehydration adds a second layer of risk. It reduces blood flow to muscle, disrupts the electrolyte balance that nerves use to control muscle contraction, and slows the muscle repair that follows training — all of which can leave a hamstring more vulnerable during the heaviest sprint days of camp. We have covered the research on hydration, muscle function, and recovery in more depth here.
None of this means an athlete needs to eat more of everything or obsessively track calories. It means that camp performance and injury resistance depend on showing up fed and hydrated, not just conditioned. At Performance Sport Care, a hamstring evaluation always includes a few questions about training load, sleep, and how the athlete has been eating and drinking that week — not just the muscle itself. Fueling and hydration are rarely the whole story behind a strain, but they are an easy, fixable part of it that is worth ruling in or out early.
How Long Does Recovery Take?
Once a strain has occurred, a few principles consistently hold true:
- The grade points to a rough timeline, but does not lock it in. Mild (Grade 1) strains often allow return to light training within one to two weeks. More serious partial tears typically take several weeks to a few months. Complete tears — especially those involving the tendon — can take considerably longer and may warrant a surgical opinion.
- Careful, early movement tends to outperform total rest. A progressive rehab plan that gradually reloads the muscle usually produces a faster and more reliable return to sport than simply resting and waiting.
- Preventing a repeat injury is part of the plan, not an afterthought. Rehab programs that add agility drills and core-stability work to standard strengthening have been shown to lower re-injury rates — even in cases where they do not necessarily shorten the initial recovery time. That distinction matters for families deciding when “good enough to play” also means “good enough to stay healthy.”
- Return to sprinting should be based on how the athlete performs, not the calendar. Because the injury itself occurs at high sprint speeds, a rehab plan that never tests progressive sprinting before clearing an athlete for camp is skipping the exact demand that matters most.
What Should Parents Watch For?
Most hamstring strains are uncomfortable but straightforward. A smaller number of situations deserve a same-week evaluation instead of a “wait and see” approach:
- Sudden, sharp pain in the back of the thigh during a sprint, jump, or kick
- A “pop” the athlete hears or feels at the moment of injury
- Immediate inability to keep running or playing
- A limp that lasts more than a day or two
- Visible swelling or bruising in the back of the thigh or lower buttock
These warning signs matter even more for teenage athletes. Teenagers are still growing, and the hamstring tendons attach to the pelvis at a growth plate called the ischial apophysis — the still-soft area of bone at the “sit bone” that has not yet fully hardened and fused.
In an adult, a hard sprint or sudden stretch typically tears muscle or tendon. In a growing teenager, the same force can pull a small piece of bone away from the pelvis at that growth plate. This is an avulsion fracture — a bone injury, not a muscle strain — even though it can feel and look nearly identical at first.
The story is the same in both cases: sudden pain, a pop, and collapsing during a sprint. Because the early symptoms overlap so closely, it is easy to miss an avulsion on the first exam. Delayed diagnosis has been linked in research to longer recovery and persistent pain. Symptoms alone cannot reliably tell you whether a growing teenager has a muscle strain or a bone avulsion.
That is why imaging is essential. Any teenager who shows the warning signs above should receive definitive imaging — preferably an MRI, which can both identify a bone avulsion and grade a muscle strain in a single study — rather than starting a routine “hamstring strain” recovery plan based on symptoms alone.
If an avulsion fracture is confirmed, healing progress can usually be followed with plain X-rays in the weeks that follow. This is faster and less expensive than repeating MRIs once the diagnosis is established.
Where Does an Evaluation Fit In?
A pattern shows up often enough to be worth naming. An athlete strains a hamstring mid-summer, rests until it “feels fine,” and returns to full-speed sprinting the same week camp intensity ramps up — without ever testing that hamstring at sprint speed in a controlled setting first. Two or three weeks later, the same muscle is hurt again, usually a little worse than the first time. The research on repeat injuries explains why this pattern is so common. It is rarely bad luck. More often, it is a return-to-sprint plan that moved faster than the muscle was actually ready for.
This is one of the more common reasons families bring a teenage athlete to Performance Sport Care in Walnut Creek. It is often not the first strain, but the second one — or a summer of nagging tightness that never quite resolved before camp started. Getting an accurate diagnosis first, including ruling out a growth-plate injury when the history calls for it, is what determines whether a return-to-sprint plan will actually hold up once camp intensity arrives.
No single technique is being offered as a cure-all — the research does not support that for any provider. What the evidence does support is combining the right approaches in the right order for the athlete in front of you. That may include manual therapy where it is indicated, targeted eccentric and sprint-specific strengthening, and a training plan that respects both the tissue and California’s heat rules. That is exactly what an initial evaluation is for — ideally scheduled before the first day of camp rather than after the first setback.
Ready Before Camp Starts?
A hamstring strain, recurring tightness, or an incomplete recovery from a past injury is worth evaluating before camp begins. An assessment now creates time to build a progressive return-to-sprint plan — rather than reacting after the first setback of the season.
If your athlete is heading into football camp with any of these concerns, an evaluation at Performance Sport Care in Walnut Creek can provide clear answers and a practical plan matched to the demands of preseason.
Request an AppointmentFrequently Asked Questions
What is a hamstring strain?
A hamstring strain is a tear in one of the three muscles down the back of the thigh — the biceps femoris, semitendinosus, or semimembranosus — or in their tendons. It most often occurs near where the muscle narrows into its tendon during a sprint.
How are hamstring strains graded?
The traditional system uses Grade 1 (a mild overstretch with minimal fiber tearing), Grade 2 (a partial tear with some strength loss), and Grade 3 (a complete tear). Newer MRI-based systems, such as the British Athletics Muscle Injury Classification, add more detail by also grading the exact location and whether the tendon is involved — which affects recovery time and re-injury risk more than the grade number alone.
Can Nordic hamstring curls prevent a hamstring strain?
Programs that include the Nordic hamstring curl are linked to meaningfully lower hamstring injury rates in combined research, mainly by building the kind of strength the hamstrings need while sprinting. The evidence is favorable but not absolute on its own, so it works best as one part of a broader plan that also includes real sprinting, flexibility, and gradual training progress — the kind of combination a sports chiropractor like Dr. Cancel would build around the individual athlete rather than hand out as a one-size-fits-all fix.
Why is a previous hamstring injury the biggest risk factor for a new one?
Research following tens of thousands of athletes has found that any past hamstring strain roughly triples the risk of a new one. A strain within the past year raises that risk almost five-fold — more than any other studied factor, including flexibility or strength balance. Incomplete rehab and a return-to-sprint plan that moves faster than the muscle is ready for are common, fixable reasons why.
What is California’s heat acclimatization rule for football camp?
California Interscholastic Federation, or CIF, guidance calls for a 14-day acclimatization period for football. Protective gear is added in stages — helmet only for the first several days, then helmet and shoulder pads, then full pads — while practice intensity and length increase gradually. This lines up with national heat-illness-prevention guidelines.
When should a young athlete’s thigh pain be checked out right away?
Sudden sharp pain in the back of the thigh, a felt or heard “pop,” immediate inability to keep playing, a limp that lasts more than a day or two, or visible swelling and bruising all call for a prompt evaluation. In growing teenagers, these same signs can point to a growth-plate injury at the hamstring’s attachment point instead of a routine muscle strain, especially after a sudden sprint, jump, or stretch. Those symptoms alone do not tell the two apart — an MRI is what distinguishes a muscle strain from a bone avulsion.
Can poor nutrition or dehydration raise the risk of a hamstring strain?
Yes, alongside the other risk factors above. Research on adolescent athletes shows that low energy availability — not taking in enough food to cover both training and normal growth — is linked to more than triple the injury rate during a season, most of it ordinary muscle and joint injuries. Dehydration separately impairs blood flow, nerve signaling, and post-training muscle repair. Neither replaces an evaluation for a strain that has already happened, but both are easy, fixable pieces of a prevention plan.
Where is Performance Sport Care located?
Performance Sport Care is located at 1776 Ygnacio Valley Road, Suite 106, Walnut Creek, CA 94598. Call (925) 945-1155. We serve Walnut Creek, Lafayette, Danville, Concord, Pleasant Hill, and greater Contra Costa County.
Selected References
Grading and mechanism: British Athletics Muscle Injury Classification — imaging-based grading proposal (Pollock et al., 2014, British Journal of Sports Medicine).
Grading and mechanism: Narrative review of hamstring injury timing during high-speed running, late swing phase mechanics (Kenneally-Dabrowski et al., 2019, prospective/biomechanical review, Scandinavian Journal of Medicine & Science in Sports).
Eccentric training / Nordic curl (isolated modality): Systematic review and meta-analysis of 8,459 athletes on Nordic hamstring exercise programs and injury rate (van Dyk et al., 2019, British Journal of Sports Medicine).
Eccentric training / Nordic curl (isolated modality, methodologic reappraisal): Re-analysis restricted to randomized controlled trials, finding the protective effect favorable but not statistically conclusive alone (Impellizzeri et al., 2021, Journal of Clinical Epidemiology).
Combined/multimodal prevention: 24-season descriptive study of collegiate sprinters comparing strength-only, strength-plus-agility, and strength-plus-agility-plus-flexibility prevention phases (Sugiura et al., 2017, Orthopaedic Journal of Sports Medicine).
Combined/multimodal prevention, risk factor management: Systematic review and meta-analysis of 108 randomized controlled trials on hamstring training and risk factor management (Rudisill et al., 2022, The American Journal of Sports Medicine).
Risk factors, index and recurrent injury: Systematic review and meta-analysis of 71,324 athletes and 8,319 hamstring injuries recalibrating risk factor associations (Green et al., 2020, British Journal of Sports Medicine).
Risk factors, strength ratio (isolated modality/measure): Systematic and critical review of the hamstring-to-quadriceps torque ratio as an injury predictor (Kellis et al., 2022, Journal of Sport and Health Science).
Heat acclimatization guidance: National Athletic Trainers’ Association Inter-Association Task Force preseason heat-acclimatization guidelines for secondary school athletics (Casa et al., 2009, Journal of Athletic Training).
Heat acclimatization guidance, policy effectiveness: Quasi-experimental interrupted time-series study of state-mandated heat-acclimatization guidelines and exertional heat illness rates (Kerr et al., 2019, Environmental Health Perspectives).
Adolescent avulsion fracture: Case series and diagnostic review of ischial tuberosity avulsion fractures in adolescents, an easily missed diagnosis (Gidwani et al., 2004, BMJ).
Adolescent hamstring injury epidemiology: Three-season cohort study of 1,157 young athletes describing hamstring and avulsion injury patterns by age (Valle et al., 2018, Scandinavian Journal of Medicine & Science in Sports).
Nutrition / energy availability prevalence: Systematic review and meta-analysis of 6,118 athletes across 46 studies on low energy availability prevalence (Gallant et al., 2024, Sports Medicine).
Nutrition / energy availability and injury, adolescent-specific: Prospective cohort study of adolescent female athletes over a nine-week season linking pre-season low-energy-availability screening to in-season injury rate (Smulligan et al., 2025, Physical Therapy in Sport).
Hydration and muscle function/recovery: See our companion article on hydration, cortisol, and muscle recovery for the full underlying research base.
This article is educational and not a substitute for personal advice from a licensed health care provider. Exercises or recommendations that are appropriate for one person may not be appropriate for another, particularly if a history of pain or injury exists.
If you have persistent or worsening pain, pain that radiates into an extremity, pain that is accompanied by fever, or any other unusual symptoms please consult a qualified provider for an individual evaluation or dial 911 in case of emergency.
