I was a physiologist by training before I was a physiotherapist. I spend more time now managing patients with connective tissue injuries than I do looking down a microscope, but I haven’t lost sight of the influence that these types of experiments can have. While I may no longer be at the “bench” (at least for now), my clinical practices are influenced from levels of evidence that don’t necessarily get that much attention in clinical circles. In physiotherapy, much of the knowledge we use daily has been informed and influenced by basic physiological experiments from an array of models from single cells through to transgenic animal models.
With that in mind, I wanted to focus on some great, basic physiological work that has recently influenced my approach to managing connective tissue injuries in the clinic.
There’s more to a program than just exercise selection
There are great post-operative ACL programs out there, many freely available online (like Fowler Kennedy’s). The majority consist of a list of exercises, separated by time (sometimes by functional measures (criterion) or patient-reported outcomes), with little consideration to the specifics of exercise prescription itself. While it’s easy to chalk this up to an oversight, it’s more likely that room is left for the clinician to exercise their own judgement in determining the exercise parameters.
Exercise physiologists have long studied how the manipulation of specific training parameters (frequency, intensity, etc….) influence the adaptations of muscle to exercise. Considering muscle, we know that the training conditions required to maximize muscle strength are not the same as muscular endurance, or even muscle size. But what if the specificity of training parameters extends beyond functional or morphological adaptations within muscle? Can we manipulate our training program to optimize the adaptations in other tissues, like tendons and ligaments?
Work from Paxton et al (1) on engineered ligaments investigated just that. Engineered ligaments aren’t perfect, there are some differences between a ligament created in a cell culture dish and the one in your knee. These differences in cellularity and types of collagen ultimately result in a weaker ligament, yet one that can produce collagen at a greater rate than an in vivo ligament (2). Engineered ligaments are nevertheless a useful proxy as they allow for quick, specific experimentation not possible with human studies.
Now Paxton et al (1) exposed these engineered ligaments to differing mechanical protocols, various frequencies, intensities and durations of stretch while measuring the activation of ERK1/2, a signaling protein known to be important to stretch-induced collagen synthesis. After a certain point, activation of ERK1/2 was maximized, failing to increase with more stimulation. It appeared that stretching the ligaments with an amplitude of 2.5% strain for 10 mins was sufficient, and anything above that did not result in any appreciable benefit. This cycle could be repeated every six hours, any sooner and the cells failed to respond.
After refining their exercise parameters on the ERK1/2 data, the authors found that intermittent stretching for ten minutes every six hours over seven days increased collagen synthesis and tensile strength of the engineered ligaments over continuously stretching the ligaments over the same length of time.
From a clinical perspective, I encourage my patients with a connective tissue injury (like an ACL reconstruction) to distribute their exercises throughout the day, that three bouts of 10 mins distributed over the day with six hours of rest may be beneficial as opposed to one 30 min session. The exercises should target the affected joint, and while the in vitro data suggests a small ROM may be sufficient, I consider the relationship of joint ROM to ligament loading on a case-specific basis when selecting exercises.
Don’t let training fall to the wayside
Ligament injuries, particularly when reconstructive surgery is required, often result in reduced physical activity. This is hardly an acute issue either, deficits in physical activity can persist long after the initial injury and rehabilitative period. I have no plan of extolling the systemic benefits of exercise, but rather wonder if the post-exercise hormonal environment can impact connective tissue growth (i.e. ACL graft healing) through a systemic mechanism, or simply, when exercises are completed by areas not directly related to the injured area. Recent work from Stu Phillips’ group (3-8) have questioned the role various hormones in the adaptations of muscle to exercise, and lead others (9)to hypothesize that these may play a greater role in connective tissue remodeling in response to exercise.
West et al (9) had participants complete a strength training session previously validated to increase growth hormone production (five sets of leg press, three supersets of knee extension/flexion with one minute rest). Serum was isolated following blood taken pre- and post-exercise, and engineered ligaments were exposed to determine if a hormone rich, post-exercise serum accelerated ligament growth.
Ligaments grown in the presence of exercise serum had greater collagen content. Mechanical properties of the engineered ligaments had increased maximal tensile load, although data on ultimate tensile strength, while favoring exercise-treatment, did not reach statistical significance (10%, p=0.15). Changes in collagen content did not correlate with differing GH levels within the serum, and exposure to increasing concentrations of recombinant growth hormone did not alter engineered ligament collagen content or mechanical properties.
This preliminary evidence isn’t sufficient to suggest that general exercise, when provided in a connective tissue rehabilitation program will improve graft or ligament healing directly. I do think it provides additional justification to address training in general when dealing with patients who have had a connective tissue injury. A key component of my initial sessions with anyone with a ligament injury, particularly involved reconstructions with prolonged rehabilitation timelines, is to address prior exercise/training patterns and create practical solutions to train around the injury.
Nutritional interventions to support ligament/graft healing and growth
At this point, it seems that everyone and their dog has completed a study investigating the effects of protein or some nutritional supplement on muscle, but far fewer have turned their attention to connective tissue. In a recent review, Baar et al. (2) notes:
Compared with muscle, the science of nutritional interventions that can improve soft-tissue function in humans is in its infancy.
Fortunately, that’s changing. Early experiments from Paxton et al (10) demonstrated engineered ligaments withstood greater tensile stresses, were stiffer, and had greater collagen content than ligaments grown with control media.
Now it’s a bit of a stretch to jump from cell culture dish to human supplementation, but thankfully the work is in progress. Shaw et al (11) tested the effects of gelatin supplementation, which is rich in collagen-compatible amino acids (proline, etc…), on engineered ligaments, as well as in vivo markers of collagen synthesis.
In their first experiment, blood was extracted from participants at rest and one hour following supplementation with placebo (maltodextrin), 5 g, or 15 g of gelatin. Engineered ligaments were grown in the presence of either the pre-supplementation serum or following consumption of the placebo, or gelatin. The results demonstrated that ligaments grown in the serum extracted one hour after supplementation, a time-point whereby blood levels of proline and lysine peaked, improved ligament collagen content. Despite increased collagen, the engineered ligaments had similar functional profiles. While the cross-sectional area was comparable across all conditions, placebo, 5-g, and 15-g gelatin supplemented serum increased against pre-supplementation for maximal tensile loads, stiffness (modulus) and ultimate tensile strength.
Next, they had participants complete a 72-hr protocol, ingesting 15 g of gelatin with vitamin C one hour before exercise. Over the 72-hour period they exercised every six hours, completing six minutes of rope-skipping. They evaluated serum levels of marker of collagen synthesis (amino terminal propeptide of type I collagen), abbreviated PINP. While PINP concentrations increased in placebo and 5-g gelatin supplemented groups, the greatest increase was seen in the 15-g gelatin supplemented group. We can’t be sure where this collagen was being made, but this is at least some initial evidence that there may be a beneficial effect of pre-exercise gelatin supplementation on collagen synthesis, when provided in combination with exercise.
These early experiments hint that exercise and nutritional supplementation, when timed and dosed in a specific manner may result in greater collagen synthesis within ligamentous tissue; however, we can’t be certain how this extrapolates outside the limitations of the in vitro models used.
While I’m reluctant to broadly apply these recommendations across all types of connective tissue injuries, they’ve had the greatest impact on how I approach ligamentous sprains and reconstructions. I’ve started to recommend short bouts of repetitive exercises involving the affected joints for approximately 10 min, separated by six hours. I consider roughly three of these sessions distributed throughout the day a win, as I don’t expect clients/patients to wake up at night to do this.
It’s worth devoting time, whether in your initial assessment or over the course of a few sessions to include physical activity/training counselling. For athletes on established programs, this may mean liaising with their strength and conditioning staff. For others, simple instructions on using local gym facilities, or even short, machine based circuits based on their confidence could be sufficient.
Finally, nutritional supplementation with gelatin (15g) and vitamin C (50 mg) may increase collagen synthesis and enhance the adaptations that your clients/patients work hard for in the clinic/gym. Make sure you know the limits of your scope, and remember the limitations of the evidence I’ve presented above.
There are of course limitations when applying results from basic physiological experiments in the clinic. And while I’m a staunch supporter of the evidence-based movement, what we do clinically can be influenced by so much more than RCTs, metas, and systematic reviews. We don’t necessarily have to wait for RCTs, meta and systematic reviews, but when applying basic physiological findings we have to remain aware that ultimately, we should hold them to the scrutiny of different experimental designs and higher levels of evidence.
- Paxton JZ, Hagerty P, Andrick JJ, Baar K. Optimizing an intermittent stretch paradigm using ERK1/2 phosphorylation results in increased collagen synthesis in engineered ligaments. Tissue Eng Part A. 2012 Feb;18(3-4):277–84.
- Baar K. Minimizing Injury and Maximizing Return to Play: Lessons from Engineered Ligaments. Sports Med. 2017 Mar 22;92:95.
- Wilkinson SB, Tarnopolsky MA, Grant EJ, Correia CE, Phillips SM. Hypertrophy with unilateral resistance exercise occurs without increases in endogenous anabolic hormone concentration. 2006 Dec;98(6):546–55.
- Schroeder ET, Villanueva M, West DDW, Phillips SM. Are acute post-resistance exercise increases in testosterone, growth hormone, and IGF-1 necessary to stimulate skeletal muscle anabolism and hypertrophy? Med Sci Sports Exerc. 2013 Nov;45(11):2044–51.
- West DWD, Burd NA, Tang JE, Moore DR, Staples AW, Holwerda AM, et al. Elevations in ostensibly anabolic hormones with resistance exercise enhance neither training-induced muscle hypertrophy nor strength of the elbow flexors. J Appl Physiol. 2010 Jan;108(1):60–7.
- West DWD, Phillips SM. Associations of exercise-induced hormone profiles and gains in strength and hypertrophy in a large cohort after weight training. 2012 Jul;112(7):2693–702.
- West DWD, Kujbida GW, Moore DR, Atherton PJ, Burd NA, Padzik JP, et al. Resistance exercise-induced increases in putative anabolic hormones do not enhance muscle protein synthesis or intracellular signalling in young men. J Physiol (Lond). 2009 Nov 1;587(Pt 21):5239–47.
- Morton RW, Oikawa SY, Wavell CG, Mazara N, McGlory C, Quadrilatero J, et al. Neither load nor systemic hormones determine resistance training-mediated hypertrophy or strength gains in resistance-trained young men. J Appl Physiol. American Physiological Society; 2016 May 12;121(1):jap.00154.2016–138.
- West DWD, Lee Barthel A, McIntyre T, Shamim B, Lee CA, Baar K. The exercise‐induced biochemical milieu enhances collagen content and tensile strength of engineered ligaments. J Physiol (Lond). 2015 Oct 15;593(20):4665–75.
- Paxton JZ, Grover LM, Baar K. Engineering an in vitro model of a functional ligament from bone to bone. Tissue Eng Part A. 2010 Nov;16(11):3515–25.
- Shaw G, Lee Barthel A, Ross ML, Wang B, Baar K. Vitamin C-enriched gelatin supplementation before intermittent activity augments collagen synthesis. Am J Clin Nutr. 2017 Jan;105(1):136–43.