SAN DIEGO – Is it okay for pregnant women to attend CrossFit classes? Are patients who run for fun at increased risk for osteoarthritis? Does stretching before exercise provide any benefits to athletes?
Experts discussed these topics during a session titled “Mythbusters in sports medicine” at the annual meeting of the American Medical Society for Sports Medicine.
Does exercise negatively impact pregnancy?
The idea that strenuous exercise during pregnancy can harm a baby’s health is a myth, according to Elizabeth A. Joy, MD.
“Women having healthy, uncomplicated pregnancies should be encouraged to be physically active throughout pregnancy, with a goal of achieving 150 minutes per week of moderate-intensity activity,” she said. “Fit pregnant women who were habitually performing high-intensity exercise before pregnancy can continue to do so during pregnancy, assuming an otherwise healthy, uncomplicated pregnancy.”
Results from more than 600 studies in the medical literature indicate that exercise during pregnancy is safe for moms and babies, noted Dr. Joy, medical director for community health and food and nutrition at Intermountain Healthcare in Salt Lake City, Utah.
In fact, the 2008 Physical Activity Guidelines for Americans state that pregnant women who habitually engage in vigorous-intensity aerobic activity or who are highly active “can continue physical activity during pregnancy and the postpartum period, provided that they remain healthy and discuss with their health care provider how and when activity should be adjusted over time.”
Such advice wasn’t always supported by the medical profession. In fact, 1985 guidelines from the American College of Obstetricians and Gynecologists recommended that women limit exercise to no more than 15 minutes at a time during pregnancy and keep their maternal heart rate less than 140 beats per minute. ACOG also discouraged previously sedentary women from beginning an exercise program during pregnancy.
“Sadly, women are still getting this advice,” said Dr. Joy, who is also president of the American College of Sports Medicine. According to the 2005-2010 National Ambulatory Medical Care Survey, only 18% of pregnant women reported receiving counseling to be physically active during their pregnancies (Matern Child Health J. 2014 Sep;18[7]:1610-18). “That is just unacceptable,” she said.
In a prospective study of the association between vigorous physical activity during pregnancy and length of gestation and birth weight, researchers evaluated 1,647 births among primiparous women (Matern Child Health J. 2012 Jul;16[5]:1031-44).
Dr. Elizabeth A. Joy
They conducted telephone interviews with the women between 7-20 weeks gestation and assigned metabolic equivalent of task values to self-reported levels of physical activity. Of the 1,647 births, 7% were preterm.
Slightly more than one-third of the women (35%) performed first-trimester vigorous physical activity. The average total vigorous activity reported was 76 minutes per week, 38% of which was vigorous recreational activity.
Women who performed first-trimester vigorous recreational physical activity tended to have lower odds of preterm birth. They also tended to have lighter-weight babies, but this did not reach statistical significance (P = .08). The authors concluded that first-trimester vigorous physical activity “does not appear to be detrimental to the timing of birth or birth weight.”
In a separate analysis, researchers evaluated acute fetal responses to individually prescribed exercise according to existing physical activity guidelines in active and inactive pregnant women (Obstet Gynecol. 2012 Mar;119[3]:603-10).
Of the 45 study participants, 15 were classified as nonexercisers, 15 were regularly active, and 15 were highly active. The women underwent treadmill assessment between 28 weeks and 33 weeks, while fetal assessment included umbilical artery Doppler, fetal heart tracing and rate, and biophysical profile at rest and immediately post exercise.
The researchers observed no differences between the groups in mode of delivery, birth weight, and Apgar scores. During vigorous-intensity exercise, all umbilical artery indices showed decreases post exercise.
“Although statistically significant, this decrease is likely not clinically significant,” the researchers wrote. “We did not identify any adverse acute fetal responses to current exercise recommendations.” They went on to conclude that the potential health benefits of exercise “are too great for [physicians] to miss the opportunity to effectively counsel pregnant women about this important heath-enhancing behavior.”
In a more recent randomized study, Swedish researchers evaluated the efficacy of moderate-to-vigorous resistance exercise in 92 pregnant women (Acta Obstet Gynecol Scand. 2015 Jan; 94[1]:35-42).
The intervention group received supervised resistance exercise twice per week at moderate-to-vigorous intensity between 14-25 weeks of their pregnancy, while the control group received a generalized home exercise program. Outcome measures included health-related quality of life, physical strength, pain, weight, blood pressure, functional status, activity level, and perinatal data.
The researchers found no significant differences between the two groups and concluded that “supervised regular, moderate-to-vigorous resistance exercise performed twice per week does not adversely impact childbirth outcome, pain, or blood pressure.”
Despite all that’s known about exercise during pregnancy, a few practice and research gaps remain.
For one, Dr. Joy said, the relationship between performing physically demanding work during pregnancy in combination with moderate-to-vigorous exercise remains largely unknown.
“Even within health care, you have residents, nurses, and others working in hospitals,” she said. “That’s demanding work, but we don’t know whether or not moderate-to-vigorous exercise in combination with that kind of work is safe. Also, although women tend to thermoregulate better during pregnancy, we still don’t fully understand the impact of elevated core body temperature, which may occur with regularly performed vigorous-intensity exercise over the course of pregnancy.”
Does running cause knee OA?
During another talk at the meeting, William O. Roberts, MD, characterized the notion that running causes knee osteoarthritis as largely a myth for recreational runners. However, elite runners and athletes who participate in other sports may face an increased risk of developing the condition.
Well-established risk factors for knee OA include post–joint injury proteases and cytokines and injury load stress on articular cartilage. “Other risk factors include overweight and obesity, a family history of OA, exercise, heavy work that involved squatting and kneeling, and being female,” said Dr. Roberts, professor of family medicine and community health at the University of Minnesota, Minneapolis.
He discussed three articles on the topic drawn from medical literature. One was a retrospective cross-sectional analysis of 2,637 Osteoarthritis Initiative participants, 45-79 years of age, who had knee-specific pain or knee x-ray data 4 years into the 10-year–long study (Arthritis Care Res. 2017 Feb;69[2]:183-91).
More than half of the participants (56%) were female, their mean body mass index was 28.5 kg/m2, only 20% reported more than 2,000 bouts of running during their lifetime, and about 5% had run competitively.
Adjusted odds ratios of pain, radiographic OA, and symptomatic OA for those prior runners and current runners, compared with those who never ran, were 0.82 and 0.76 (P for trend = .02), 0.98 and 0.91 (P for trend = .05), and 0.88 and 0.71 (P for trend = .03), respectively.
Dr. William O. Roberts
The authors concluded that running does not appear to be detrimental to the knees, and the strength of recommendation taxonomy was rated as 2B.
In a separate analysis, researchers performed a systematic review and meta-analysis of 11 cohort (6 retrospective) and 4 case-control studies related to running and knee arthritis (Am J Sports Med. 2016 May;45[6]:1447-57). The mean ages of subjects at outcome assessment ranged from 27 years to 69 years, and the sample size ranged from 15 to 1,279 participants. The four case control studies assessed exposure by mailed questionnaire or by personal interview.
The meta-analysis suggests that runners have a 50% reduced odds of requiring a total knee replacement because of OA.
“It contradicts some previous studies, and there were confounders,” Dr. Roberts said of the analysis. “The one that I noticed is that people would delay surgery to keep running. That’s what I find in my practice.”
The researchers were unable to link running to knee OA development. Moderate- to low-quality evidence suggests a positive association with OA diagnosis but a negative association with requirement for a total knee replacement.
Based on published evidence, they concluded there is no clear advice to give regarding the potential effect of running on musculoskeletal health and rated the strength of evidence as 1A.
A third study Dr. Roberts discussed investigated the association between specific sports participation and knee OA (J Athl Train. 2015 Jan. 9. doi: 10.4085/1062-6050-50.2.08). After locating nearly 18,000 articles on the topic, the researchers limited their meta-analysis to 17 published studies.
They found that the overall risk of knee OA prevalence in sports participants was 7.7%, compared with 7.3% among nonexposed controls (odds ratio, 0.9). However, risks for knee OA were elevated among those who participated in the following sports: soccer (OR, 3.5), elite long-distance running (OR, 3.3), competitive weightlifting (OR, 6.9), and wrestling (OR, 3.8). The researchers concluded that athletes who participate in those sports “should be targeted for risk-reduction strategies.”
“So, does running cause knee OA? It depends,” said Dr. Roberts, who is also medical director of Twin Cities in Motion. “There is a potential risk to high-volume, high-intensity, and long-distance runners, but there does not appear to be a risk in fitness or recreational runners. Of course, you can’t erase your genetics.”
He called for more research on the topic, including prospective longitudinal outcomes studies, those that study the role of genetics/epigenetics in runners and nonrunners who develop knee OA and those focused on the knee joint “chemical environment,” referring to recent work that suggests that running appears to decrease knee intra-articular proinflammatory cytokine concentration (Eur J Appl Physiol. 2016 Dec;116[11-12]:2305-14).
“It’s okay to run for fitness, because the health benefits far outweigh the risk of knee OA,” Dr. Roberts said. “If you run hard and long, it could be a problem. We probably should be screening for neuromuscular control to reduce anterior cruciate ligament disruption.”
Does it help to stretch before exercise?
Stretching before engaging in exercise is a common practice often recommended by coaches and clinicians – but it appears to have no role in preventing injuries during exercise itself.
Several decades ago, investigators subscribed to muscle spasm theory, which held that unaccustomed exercise caused muscle spasms.
“The thought was that muscle spasms impeded blood flow to the muscle, causing ischemic pain and further spasm,” Valerie E. Cothran, MD, said during a presentation at the meeting. “Stretching the muscle was thought to restore blood flow to the muscle and interrupt the pain-spasm-pain cycle. This theory has been discredited for 40 years, but the practice of stretching before exercise persists.”
According to Dr. Cothran, of the department of family and community medicine at the University of Maryland, Baltimore, a limited number of randomized, controlled trials exist on the topic – and many are fraught with limitations, such as the evaluation of multiple stretching methods and variable types of sports activities and the inclusion of multiple cointerventions.
One systematic review evaluated 361 randomized, controlled trials and cohort studies of interventions that included stretching and that appeared in the medical literature from 1966 to 2002 (Med Sci Sports Exerc. 2004 Mar;36[3]:371-8). Studies with no controls were excluded from the analysis, as were those in which stretching could not be assessed independently or those that did not include people engaged in sports or fitness activities.
The researchers determined that stretching was not significantly associated with a reduction in total injuries (OR, 0.93). “There is not sufficient evidence to endorse or discontinue routine stretching before or after exercise to prevent injury among competitive or recreational athletes,” they concluded.
Dr. Valerie E. Cothran
The following year, Lawrence Hart, MBBch, of McMaster University, Hamilton, Ont., assessed the same set of data but eliminated some of the confounding factors of the previous analysis, including studies that had limited statistical power (Clin J Sport Med. 2005 Mar;15[2]:113). The final meta-analysis included six studies.
Dr. Hart found that neither stretching of specific leg-muscle groups or multiple muscle groups led to a reduction in total injuries, such as shin splints, tibial stress reaction, or sprains/strains (OR, 0.93). In addition, reduction in injuries was not significantly greater for stretching of specific muscles or multiple muscle groups (OR, 0.80, and OR, 0.96, respectively). “Limited evidence showed stretching had no effects on injuries,” he concluded.
A more recent systematic review analyzed the efficacy of static stretching as part of a warm-up for the prevention of exercise-related injury (Res Sports Med. 2008;16[3]:213-31). The researchers reviewed 364 studies published after 1990 but before 2008, and they included seven in the final analysis: four randomized, controlled trials and three controlled trials.
All four randomized, controlled trials concluded that static stretching was ineffective in reducing the incidence of exercise-related injury, and only one of the three controlled trials concluded that static stretching reduced the incidence of exercise-related injury. In addition, three of the seven studies reported significant reductions in musculotendinous and ligament injuries following a static stretching protocol.
“There is moderate to strong evidence that routine application of static stretching does not reduce overall injury rates,” the researchers concluded. “There is preliminary evidence, however, that static stretching may reduce musculotendinous injuries.”
The final study Dr. Cothran discussed was a systematic review of two randomized, controlled trials and two prospective cohort studies on the effect of stretching in sports injury prevention that appeared in the literature between 1998 and 2008 (J Comm Health Sci. 2008;3[1]:51-8).
One cohort study found that stretching reduced the incidence of exercise-related injuries, while two randomized, controlled trials and one cohort study found that stretching did not produce a practical reduction on the occurrence of injuries. The researchers concluded that stretching exercises “do not give a practical, useful reduction in the risk of injuries.”
Some studies have demonstrated that explosive athletic performance such as sprinting may be compromised by acute stretching, noted Dr. Cothran, who is also program director of the primary care sports medicine fellowship at the University of Maryland, Baltimore. Current practice and research gaps include few recent randomized, controlled trials; few studies isolating stretching alone; and few that compare the different forms of stretching, such as dynamic and static stretching, she added.
“There is moderate to strong evidence that routine stretching before exercise will not reduce injury rates,” she concluded. “There is evidence that stretching before exercise may negatively affect performance. Flexibility training can be beneficial but should take place at alternative times and not before exercise.”
Dr. Joy disclosed that she receives funding from Savvysherpa and Dexcom for a project on the prevention of gestational diabetes. Dr. Roberts and Dr. Cothran reported having no financial disclosures.