Exercise and Resistance to Infection

• Sedentary individuals are prone to infection

• Moderate exercise attenuates susceptibility to infection

• Excessive exercise may increase risk of infection above both moderate and sedentary rates

J-shaped Model of Resistance to Infection

Why?

• Moderate exercise – improves circulation – promotes protein synthesis– stimulates blood cell production

Evidence for Moderate Exercise

• 45 min, 5 times/week of brisk walking had 1/2 as many days with URTI compared to sedentary group

• elderly who walked 40 min, 5 times /week had less than 1/2 the incidence of URTI compared to sedentary (21% vs. 50%)

Excessive exercise

• stimulates cortisol– promotes protein breakdown– inhibits anabolism– inhibits immunological function

Evidence for Excessive Exercise

• 6 times as many runners got URTI compared to non-participating runners

• runners training 96 km/week had 2 x URTI as those doing 32 km/week

• races of 5 - 21 km do not increase the risk of URTI in the week following competition

Link Between Muscle and Systemic Physiology (or Why is Specificity

Important?)• Training study

– trained one leg for 13 sessions – (15 min @workload which elicited HR = 170)– at the beginning of the study & end of each

week, NE ,E , LA, VE were tested at same initial workload

– training effect was observed• all variables were lower at submaximal workload

Was the Training Effect Systemic or Muscular

• after 13 sessions the untrained leg was trained for 5 sessions at same workload as trained leg

• if training effect was Systemic, there should be transfer of training effect while exercising with the non-trained leg

• non-trained leg responded as if there were no training effect what-so-ever

One Legged Training Effect

Peripheral Adaptations

• decreases in – NE– E– LA– VE– HR

• these adaptations are specific to the muscles trained

How are these peripheral responses and adaptations

manifested?

Peripheral Feedback

• Group III and Group IV nerve fibers– respond to tension, temperature and chemical

changes in muscles– increase firing actions in proportion to changes

in metabolic rate

• example of feedback mechanism

Peripheral Control Mechanisms

Central Command

• higher brain centers initiate command to perform task– physiological responses are adjusted to meet

expected demand (sympatheti, Q, V)

• if more motor units are recruited to develop tension, greater physiological adjustments are instigated to meet expected metabolic demands

• example of feedforward mechanism

Central Control of Motor Unit Recruitment

How are these controls related to adaptations observed after endurance

training?

Prior to Training

• To perform a fixed sub-maximal workload motorunits must be recruited

• more “mitochondria poor” muscle fibers must be recruited to perform the task– greater central drive– greater peripheral disruption of physiological

homeostasis (H+, adenosine, lactate)

After Training

• more mitochondria in muscle fibers

• fewer motor units needed to perform same oxidative work

• reduced central command

• reduced peripheral disruption of homeostasis (H+, adenosine, lactate)

Physiological Effects of Strength Training

Terms

• Muscular Strength - maximum force a muscle or group can generate (1- RM)

• Muscular Endurance - ability to perform repeated contractions against sub-maximal load

• Law of Initial Values - applies to strength training as well as endurance training and VO2max

Principles of Strength Training

• Overload• Specificity• Reversal

– all apply in similar respects to endurance training

• low reps/hi load (2 - 8 reps) build strength• hi reps/low load (15 - 20 + reps) build

muscular endurance

Training Studies

• most training studies are of short duration (8 - 20 weeks)

• in these studies, most strength gains are a result of neural adaptation– learning – coordination– ability to recruit prime movers

• in long-term, strength gains result of increases in size of prime movers

Neural and Muscular Contributions to Strength

Neural Adaptations

• in contrast to endurance training, transfer of adaptation does occur– transfer of motor unit recruitment is

responsible, not hypertrophy

• improved synchronization of motor unit firing

• improved ability to recruit motor units

Muscular Enlargement

• Type I vs. Type II– type II produce more force than I– type II increase in size more than I– high rep/low weight results in smaller size

increases than low rep/ high weight• may also result in more slow twitch (muscular

endurance)

Strength vs. Endurance

• strength training does not result in increases in capillary density (decreases due to muscle enlargement)

• training hi reps can alleviate this to an extent (size vs. capillaries)

• mitochondrial density also reduced ( size vs. mito)

Hypertrophy vs. Hyperplasia

• hypertrophy - increase in muscle size w/o increase in number of fibers

• hyperplasia - increase in number of fibers

• strength training results in increases in size due to hypertrophy

• generally believed hyperplasia does not occur in humans

Simultaneous Strength and Endurance Training.

Generally….• Performing strength and endurance training

simultaneously results in more favorable capillary density and mitochondrial adaptations than strength training alone

• endurance training may attenuate strength gains to a certain extent

• strength training may enhance endurance training (ie. TTE @ 80 % VO2max)

• monotony

Evidence

• 10 weeks of combined strength and endurance training resulted in – similar VO2max gains compared to endurance

only– similar strength gains until week 9 when they

leveled off compared to strength only

• 10 weeks of 3 x week strength training added to endurance program after endurance adaptations had leveled off

– 30 % gain in strength– no hypertrophy– ~ 20 % improvement in time to exhaustion at

80% VO2max

Take Home Message

• endurance training can attenuate or entirely compensate for the “negative” responses to strength training without detracting from the strength gains to a great extent

– capillary density– mitochondrial density

• strength training may enhance endurance training by – increasing the amount of contractile protein

available to perform oxidative work and ..– compensating for training monotony