The Endurance Paradox
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The Endurance Paradox

Bone Health for the Endurance Athlete

Thomas J Whipple, Robert B Eckhardt

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eBook - ePub

The Endurance Paradox

Bone Health for the Endurance Athlete

Thomas J Whipple, Robert B Eckhardt

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About This Book

The endurance athlete faces a paradoxā€”you're going farther and faster, you're feeling stronger, but your bones are getting weaker. New, compelling evidence shows that the very activities that expand our mental and physical abilities may be reducing the durability of our skeletons. In this book, Thomas Whipple, a leading orthopaedic clinical specialist, and Robert Eckhardt, a scientist specializing in the musculoskeletal system, team up to explain how athletes at any level can maintain the delicate balance between endurance exercise and optimum bone health over a lifetime. Translating important scientific advances into accessible language, they explain the muscle-bone connection, and cover training strategies and exercises, nutrition, calcium, stress fractures, rehabilitation, running mechanics, footwear, posture, and pharmaceuticals. An essential guide and ideal text for exercise physiologists, endurance athletes, fitness enthusiasts, and coaches.

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Publisher
Routledge
Year
2016
ISBN
9781315418636

Chapter One
Strength Training

The Muscle-Bone Connection

It may seem paradoxical that the cardiovascular and pulmonary systems respond favorably to progressively larger exercise volumes while your muscles and bones actually may become weaker. Even though you may experience improved fatigue resistance and therefore feel ā€œstrongerā€ as you add meters or miles to your weekly training log, the forces acting on bones remain relatively consistent and donā€™t increase substantially in magnitude. From a mechanical perspective, the forces acting on the skeleton during endurance exercise are relatively small when compared to those generated during other types of sports.
The largest forces encountered by our bones are those associated with vigorous muscle contractions and with the ground reaction forces associated with landing. Intriguingly, studies of weight lifters and gymnasts reveal that these groups have significantly higher levels of muscle strength and therefore bone mass than do endurance athletes. Consider the forces on the skeleton that are associated with barbell squatting while bearing 800 pounds, or dismounting from the uneven bars! In comparison to strength and power athletes, endurance specialists score well below average on tests of force production (strength) or explosive muscular activity (power). As one representative example, a 1967 study by Costill that evaluated explosive leg strength found that untrained individuals could jump 20.9 in while elite marathon runners could jump only 13.5 in.
The low levels of muscular strength and power exhibited by most endurance athletes appear to be an adaptation to training rather than an innate trait. This can be appreciated better with the story of Lou Castagnola, a two-hour, 17-minute marathoner. In 1967 when in top shape, Louā€™s VO2 max was world class at 72.4 ml/kg x min while his vertical jump was only 11.5 in. Following the 1968 U.S. Olympic trials he stopped training and led a sedentary life. Three years later he was retested and it was discovered that his VO2 max had declined to 47.6 ml/kg x min, but his vertical jump actually had increased to 20.3 in. Although Louā€™s bone mass was not tested, it may be reasonable to assume that when in top form his bone mass was reduced, since vertical jump height has been found to be correlated with bone mass at the hip (Strong and Tucker, 2003). In the clinic, we often encounter middle-aged recreational endurance athletes who have a difficult time performing a six-inch vertical jump.
The reason for the decline in muscular strength and power that is associated with endurance exercise may be related to a variety of factors including loss of skeletal muscle, reduced levels of anabolic hormones, alteration in muscle fiber type, or energy deficit. However, the most important factor may be better understood after reading the following description of how the neuromuscular system operates.
Exercise scientists often study and discuss the neuromuscular system by compartmentalizing it into central (nervous system) and peripheral (muscular) components. The nervous system (NS) is of paramount importance in the development of muscular strength and, for our purposes, forces that ultimately act on bones. In basic terms, the NS is linked to the peripheral muscles by afferent motor nerves that innervate specific muscle fibers. This arrangement is defined as a motor unit (MU). Historically, MUs have been classified as being of the small and slow (type I) or large and fast (type IIb) variety, based upon a muscle cellā€™s diameter and contraction speed. Additionally, a hybrid (type IIa) muscle fiber type with intermediate characteristics often is referred to in the scientific literature. However, it now is generally recognized by exercise scientists that muscle fibers really do not exist in discrete forms. Instead, muscle fibers at the sub cellular level exist on a continuum based upon several factors including expression of filament proteins, metabolic potential, and calcium handling characteristics. All muscles do contain both the basic slow and fast motor units but the percentage of these is unique to a particular muscle and largely genetically determined. Endurance athletes generally have a higher percentage of the slow motor units that innervate small muscle fibers, which are ideally suited for prolonged use at relatively slow velocities. Conversely, fast units have poor fatigue resistance but are capable of exerting more powerful actions.
Increases in muscle force production are achieved by different CNS mediated mechanisms. The size principle of muscle recruitment states that small motor units are recruited first during low intensity exercise or force requirements. Then, as force production demands are increased, and as exercise intensity is raised, larger motor units are progressively recruited. Motor units are never partially activated; they are either operational or at rest.
The other primary mechanism that increases force production is rate coding. Rate coding refers to the frequency at which a motor unit is activated. In general, the rate of activation of a motor unit is positively related to force production. Therefore, muscle force requirements during progressively more challenging exercise are met by the central nervous systemā€™s activation of progressively larger motor units at progressively faster firing rates. Maximal muscle force thus is achieved when both slow and fast motor units are recruited simultaneously and their rate of activation is high (fused). The ability to engage progressively larger motor units at faster rates is subject to training. Well-trained athletes are capable of activating a much higher percentage of their motor units than novices or the untrained. Major improvements in an individualā€™s ability to generate more force can be achieved by training these nervous system pathways. This type of improvement therefore is often referred to as neuromuscular strength development, and stands in contrast to the muscular adaptations that occur in the skeletal muscles themselves, the most common being hypertrophy or enlargement.
Improvements in neuromuscular activation are achieved by exercise techniques that induce large force requirements or fast velocities or combinations of both. Please note: We did not mention efforts that induce fatigue. Fatigue in muscles can be appreciated as a reduction in force production or a decrease in velocity. Most individuals can appreciate the fatigue that results when performing a set of pull-ups or push-ups to failure. The early repetitions are executed in smooth and near effortless manner. As fatigue starts to develop, the level of effort increases and the movement speed decreases, until eventually, all movement stops and further exercise is impossible. From a neurological perspective, fatigue can be described as a reduction in both the number of motor units recruited as well as a reduction in speed at which they are operating. Therefore, if the training objective is to recruit the greatest number of motor units possible, and thereby exert greater forces on muscles and bones, exercise methods that lead to fatigue are counterproductive.
The physical characteristic that researchers often cite as being related to a muscleā€™s strength is itā€™s cross-sectional area (CSA) and for most individuals, especially novice athletes, there is generally a positive relationship between CSA and force production. However, it is important to realize that this correlation does not necessarily exist for either highly trained or untrained subjects. In fact, most people are familiar with individuals who have very impressive strength yet very small diameter muscles.
Muscle fibers, also known as muscle cells, are divided into smaller functional units known as sarcomeres. Sarcomeres contain the filament proteins actin and myosin that are responsible for muscle contraction and relaxation. At the level of the muscle, strength training has the potential to induce both hyperplasia (increase in muscle fiber number) and hypertrophy (enlargement) with hypertrophy being the more predominant consideration.
Two types of muscle fiber hypertrophy exist: sarcoplasmic and myofibrillar. Sarcoplasmic hypertrophy is characterized by an increase in muscle fiber size in the absence of an increase in contractile proteins, while myofibrillar hypertrophy occurs through increase in the volume of contractile proteins. The difference may be appreciated by the consideration of a body builder who has large muscle diameter but relatively poor strength, as compared to an Olympic style weight lifter who possesses smaller muscle mass but superior strength. Obviously, the endurance athlete considers muscle bulk that does not support improved force production capability a liability. As far as the skeleton is concerned, gains in muscle size without concomitant increases in strength are of no functional value.
The overall aim of the bone remodeling process is to achieve a skeleton that is just strong enough to withstand the largest forces that are habitually encountered by each individual athlete. Therefore, if muscular strength and power are decreased as an endurance athlete engages in exclusively low intensity endurance exercise, bone mass also will be reduced. Accordingly, for most endurance athletes, a primary cause for the progressive loss of muscular strength and power is related to the lack of recruitment of the fast motor units in training. Stated another way, you become good at what you practice. If you habitually engage in running, biking, or swimming, especially at low intensity levels and to the point of fatigue, the larger motor units never are recruited.
Although many of the specifics of bone remodeling remain undefined, bone strain appears to be a primary governor of the process. Strain is the term used to define the very small bending deformations that occur in bone in response to loading or stress. More specifically, if exercise-induced stresses are increased to the point at which bone strain is elevated above a certain threshold, bone formation processes are mechanically triggered. The magnitudes and frequencies of the mechanical stresses are far more important in regulating bone remodeling than the number of repetitions. To appreciate this point, a study done on rats is helpful. The experimental animals were exposed to only 10 repetitions of a high impact load at different weekly frequencies (1W, 3W, 5W, 7W) for eight weeks. Bone strength was not improved statistically in the rats exercising only one time per week yet increased by an impressive 40% in the rats performing daily exercise over those that were exercising either three or five times per week (Umemura, et al., 2008). We infer that if exercise stress levels do not induce adequate strain or are performed frequently enough, no positive remodeling will occur, regardless of how many repetitions are performed. Such is the case with greater volumes of endurance exercise; since stress levels remain constant, there is no significant increase in bone strain. Running 60 minutes versus 30 minutes does not induce greater strains on bone, only more repetitions. In a study by J.D. MacDougall and Colleagues (1992) it was reported that running up to 20 miles per week tended to increase bone mass of the lower leg but that running further (up to 75 miles/week) resulted in no further increases, but rather instead, a tendency toward reduced bone mass.
From the standpoint of long-term skeletal health, the endurance athlete needs to offset the losses of muscular and bone strength that appear as a consequence of the highly repetitious but low force demands that are characteristic of running, swimming, and cycling. To accomplish this objective, a program of exercise is needed to consistently recruit the larger motor units and thus maintain (or increase!) elevated strain levels. Interestingly enough, an individual may possess a greater amount of muscular ā€œstrengthā€ (as defined by the amount of force that can be produced for a single repetition) prior to initiating a novel endurance-training regime than they will after getting started. In the case of an experienced athlete, strength and power often deteriorate when training volumes are increased. Unfortunately, many endurance athletes are at a loss to know what type of exercise or loading routines will reduce the likelihood of bone loss while augmenting their endurance performance.

Strength Training Considerations

  1. Exercise selection
  2. Parameters (frequency, intensity, volume)
  3. Getting started
  4. Progression
  5. Integration

Exercise Selection

The primary goal of this chapter is to provide a framework for the endurance athlete to develop an exercise routine that will achieve the following objectives:
  1. Decrease the likelihood of bone loss
  2. Maximize general muscular strength
  3. Minimize muscle hypertrophy
  4. Improve endurance performance
Secondary benefits may include improved force production in sport specific movement patterns as well as reduce the potential for injury. In order to minimize excessive energy expenditure and maximize results, appropriate exercises and training parameters must be identified.
It is also important to consider that strength exercises create mainly local effects on bones (hormonal changes also occur but these are somewhat less well defined). In other words, when a muscle contracts and exerts a strain on a specific bone, only that particular bone (or part of that bone) will be influenced. Accordingly, if the primary goal of strength training is skeletal health, exercise selection must target body regions that are prone to bone loss. These locations include the spine, hip, and forearm. Furthermore, we believe that exercises should activate muscles in a manner that is consistent with function. Examples of resistance exercises that meet both criteria include:
  • ā–  Dead lift
  • ā–  Squat (front, back, or overhead)
  • ā–  Bent row
  • ā–  Lunge
  • ā–  Olympic lifts (snatch, clean and jerk)
Our favorite exercises for improving total body strength are the dead lift, or what has been historically termed ā€œthe health lift,ā€ and one of the squat varieties. There now are many fine instructional videos for your review. One excellent source is the Crossfit web site (www.crossfit.com). In addition, we suggest the web site of Dan John (www.danjohn.org). Dan is a track and field coach as well as an accomplished throwing and strength athlete (American Record holder, National Weightlifting and Throwing Champion, Highland Games competitor), lecturer, author, and academic. Although many strength and conditioning experts describe strengthening programs that require a multitude of exercises, we are in agreement with Dan when he states, in his book Never Let Go (2009, p. 18):
  1. The body is one piece
  2. There are three kinds of strength training:
    1. Putting weight overhead
    2. Picking it off the ground
    3. Carrying it for time or distance
  3. All training is complementary
An extremely effective strengthening program may require performing as few as one or two exercises. For interest and variety, however, you may want to consider learning more of the basic strength exercises listed above.

Resistance Exercise Training Parameters

Examination of the cross-sectional and longitudinal research studies indicates that strength training generally results in improved bone mass and/or attenuation of age-related bone loss. However, the effect on bone can be variable and many scientific interventions have resulted in little or no effect. In fact, some studies have shown that resistance exercise actually can result in accelerated bone loss! Therefore, a thorough appreciation of what actually constitutes a successful versus u...

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