Here's how to do a perfect push-up: From a face-down position, place your hands slightly wider than shoulder-width apart. Place your toes or knees on the floor, and try to create a perfect diagonal with your body, from the shoulders to the knees or feet. Keep the glutes [rear-end muscles] and abdominals engaged. Then lower and lift your body by bending and straightening your elbows, keeping your torso stable throughout.
The popular belief is that two training methods are needed to be physically fit: working with weight for muscle strength, and aerobics for cardiovascular fitness. This is untrue. One of the biggest jobs of the cardiopulmonary system (heart and lungs) is to service the muscles. If the cardiopulmonary system were a retail store, the muscular system would be its biggest customer. When your muscular system works harder, the cardiopulmonary system works harder; it's not the other way around. So, working your muscles hard will force the cardiopulmonary system to work hard. Muscular work of sufficient intensity requires the cardiopulmonary system to work hard to meet muscular demands, so one activity takes care of both muscular and cardiopulmonary fitness. And that activity is strength training. Think about it, you can't exercise the cardiopulmonary system without exercising the muscular system! So, although the fitness industry remains blind to the above facts, strength training will provide you with every exercise-related health benefit you could possibly want. Doing "cardio work" is a waste of time and physiological resources, and can actually be counterproductive.
Inappropriate exercise can do more harm than good, with the definition of “inappropriate” varying according to the individual. For many activities, especially running and cycling, there are significant injuries that occur with poorly regimented exercise schedules. Injuries from accidents also remain a major concern,[85] whereas the effects of increased exposure to air pollution seem only a minor concern.[86][87]
Another very important brain area that mediates, and in turn is affected by the stress response, is the hippocampus.27 The consequences of impaired regulation of cortisol secretion are manifold, ranging from effects in peripheral tissues (eg, osteoporosis) to changes in the central nervous system.28 Most of the effects seen in chronic stress situations can be explained by the occupation of the two glucocorticoid receptors in the brain. In normal situations, the mineralocorticoid receptor will be occupied, whereas the glucocorticoid receptor has lower affinity for the natural ligand corticosterone (cortisol) than the mineralocorticoid receptor and is extensively activated only after stress and at the peaks of the circadian rhythm. One of the main functions of glucocorticoid receptors is to normalise brain activity some hours after an organism has been exposed to a stressful event and to promote consolidation of the event for future use.25 28 To this purpose, corticosteroids feed back in precisely those circuits that are initially activated by the stressor and are enriched in glucocorticoid receptors: limbic forebrain neurons and the paraventricular nucleus of the hypothalamus.
The military press is similar to the shoulder press but is performed while standing with the feet together. (It is named "military" because of the similarity in appearance to the "at attention" position used in most militaries) Unlike the seated shoulder press, the military press involves the majority of the muscles of the core as stabilizers to keep the body rigid and upright, and is thus a more effective compound exercise.
Cross-training means mixing in different workouts and training methods rather than focusing on just one type of workout. Not only does this help create a well-balanced fitness plan, but it can help you reach specific goals, too. For example, if you’re getting ready to run a race, you’ll want to cross-train with strength and yoga workouts, which will complement your running and help improve your performance and decrease the chance of injury by building muscle and increasing flexibility. “If you only include one form of training, you may be holding yourself back from the results you deserve,” says Lefkowith.
Figure 1 Squat. Checkley9 Checkley E. A natural method of physical training. New York (NY): William C. Bryant & Co.; 1890. [Google Scholar] and Randell, reproduced with kind permission of Wellcome Library. Demonstrated by Barbara Mortimer Thomas.26 Wellcome Library [Internet]. Rodway H. Training for childbirth - and after (1940). 2015 Sep 24 [cited 2015 Oct 3]. Available from: [Google Scholar]
I purchased Insanity a few years back and only lasted for 3-4 workouts. The workouts were too difficult for me and although I enjoyed it, I just failed to commit to it. Fast forward to July of this year..... I am not even sure why I attempted it again, but I am glad I did. First things first; this program is physically challenging to the point that I would recommend anyone going into it be cleared by their doctor BEFORE starting. Seeing a physician after you've injured yourself is the equivalent of closing the barn after the horses got out! There, it's been said. Now on to my review of this product.

Jump up ^ Carroll ME, Smethells JR (February 2016). "Sex Differences in Behavioral Dyscontrol: Role in Drug Addiction and Novel Treatments". Front. Psychiatry. 6: 175. doi:10.3389/fpsyt.2015.00175. PMC 4745113. PMID 26903885. There is accelerating evidence that physical exercise is a useful treatment for preventing and reducing drug addiction ... In some individuals, exercise has its own rewarding effects, and a behavioral economic interaction may occur, such that physical and social rewards of exercise can substitute for the rewarding effects of drug abuse. ... The value of this form of treatment for drug addiction in laboratory animals and humans is that exercise, if it can substitute for the rewarding effects of drugs, could be self-maintained over an extended period of time. Work to date in [laboratory animals and humans] regarding exercise as a treatment for drug addiction supports this hypothesis. ... However, a RTC study was recently reported by Rawson et al. (226), whereby they used 8 weeks of exercise as a post-residential treatment for METH addiction, showed a significant reduction in use (confirmed by urine screens) in participants who had been using meth 18 days or less a month. ... Animal and human research on physical exercise as a treatment for stimulant addiction indicates that this is one of the most promising treatments on the horizon. [emphasis added]

Let’s just call this the accelerated beginner’s guide to bodybuilding. In this plan, your first month of training will be demanding, but not so demanding as to cause injury (or worse yet, burnout), and progressive in the sense that each week you’ll graduate to different exercises, higher volume, more intensity or all of the above. After four weeks you’ll not only be ready for the next challenge but you’ll have built a significant amount of quality muscle. In other words, one month from now you’ll look significantly better with your shirt off than you look now. (How’s that for results?)
It features 12 different 30-second exercises, with five seconds of rest in between. It’s great for beginners and athletes, syncs with your iPhone Health App to take your other daily movement into account, and the workout library has 22 presets that you can customize to create thousands of variations. You can swipe right or left during the exercises to see how much time you have left, watch the instructor, or listen to music from your iTunes.
One new exercise is added to each bodypart routine to provide even more angles from which to train your target muscles to promote complete development. You’ll hit each muscle group with two exercises of 3­–4 sets each: four sets for large bodyparts (chest, back, shoulders, quads, hamstrings) and three sets for smaller bodyparts (biceps, triceps, abs, calves). The result is 16 total sets for the week for large bodyparts and 12 sets total for smaller ones—again, working in the 8–15-rep range—which is a substantial increase in volume from Week 1.
Before anyone’s crowned Cap’n Crunch, remember form is key. Lie on your back with the knees bent and feet flat on the floor. With hands behind the head, place the chin down slightly and peel the head and shoulders off the mat while engaging the core. Continue curling up until the upper back is off the mat. Hold briefly, then lower the torso back toward the mat slowly.
Two moves is better than one, right? You may want to do this move on a mat or a towel for padding. Start in a high plank position with core tight. Lower onto both forearms at the same time, maintaining a tight core and level hips. Now push back up onto hands at the same time to return to starting position. Finish by drawing right knee into chest, then left knee into chest, doing a mountain climber.
DC-based personal trainer Chris Perrin says to do hip bridges if you want to get your derriere round and toned. “Lie on the floor. With bent legs, place both feet flat on the floor. Both arms down, hands on each side of your hips,” he explains. “Raise your low back off the floor by pushing through the heels of the feet. Once your hips are at their highest point—without straining your back—pause and return your lower back to the floor.” And squeeze those glutes the entire time!
Ken Hutchins' SuperSlow technical manual represents the first major advancement in exercise science since Arthur Jones' Nautilus Bulletins were published back in the early 1970's. Unlike most of the books that have been written on the subject over the past few decades, which are based on assumption and faulty reasoning, Mr. Hutchin's SuperSlow manual presents an exercise protocol based on solid reasoning, and principles logically derived from the classical sciences of biology and mechanical physics, and for the first time provides a proper definition of the word: exercise. I very strongly recommend this book to everyone with an interest in exercise, especially physicians, therapists and exercise instructors, who are looking for a safer, a more time-efficient, and a more productive method of exercise for themselves, their patients or clients. SuperSlow is not just better than other exercise protocols, it is so far superior to every other activity ever devised for the purpose of physical conditioning that no meaningful comparison is even possible. This is the future of exercise.
"Look for something with a lot of short segments," says Neporent. "This way, you can do a 10-minute set and you're not committed to a longer routine." When you're ready, you can add the segments together and the workout will still flow. DVD technology has made it easier for video exercisers to do short segments or put several together, depending on their ability
This exercise is similar in movement to the back squat; however, the bar sits in the front rack position across the collarbones and shoulders of the athlete. HOW TO DO IT: As you drive back up, it is imperative that you raise your elbows to the sky to keep the bar in the correct position. The core should be tight to prevent the back from rounding. If you have mobility issues in the front rack position, you can cross your forearms in front of your body, parallel to the ground. MUSCLES USED: Glutes, quads, hamstrings, calves and core.
In the 1950s postwar period, American capitalism prospered and families began moving to the suburbs. This led to an increase in automobile sales, as driving became a more viable transportation option than walking or taking public transportation, which took a small toll on public health. At the same time, families increasingly owned televisions and stay-at-home mothers spent much of their time at home during the day. As such, stay-at-home mothers became television's primary audience during the day, and created a market for televised workouts.

14.  If you don't feel like working out, don't! Listen to your body. Just because it's your "scheduled day" doesn't mean you must work out. If your body is taking a little longer to recover than usual, so be it. It won't hurt you to lay off for an additional day or two; you will not lose anything you've gained. If your body isn't ready, it's better to skip days than to work out anyway. Every seven days is only a target (some people have found every eight or nine days reap the same benefits).
We recently developed a high intensity one leg dynamic exercise (OLDE) protocol to measure muscle endurance and investigate the central and peripheral mechanisms of muscle fatigue. The aims of the present study were to establish the reliability of this novel protocol and describe the isokinetic muscle fatigue induced by high intensity OLDE and its recovery. Eight subjects performed the OLDE protocol (time to exhaustion test of the right leg at 85% of peak power output) three times over a week period. Isokinetic maximal voluntary contraction torque at 60 (MVC60), 100 (MVC100) and 140 (MVC140) deg/s was measured pre-exercise, shortly after exhaustion (13 ± 4 s), 20 s (P20) and 40 s (P40) post-exercise. Electromyographic (EMG) signal was analyzed via the root mean square (RMS) for all three superficial knee extensors. Mean time to exhaustion was 5.96 ± 1.40 min, coefficient of variation was 8.42 ± 6.24%, typical error of measurement was 0.30 min and intraclass correlation was 0.795. MVC torque decreased shortly after exhaustion for all angular velocities (all P < 0.001). MVC60 and MVC100 recovered between P20 (P < 0.05) and exhaustion and then plateaued. MVC140 recovered only at P40 (P < 0.05). High intensity OLDE did not alter maximal EMG RMS of the three superficial knee extensors during MVC. The results of this study demonstrate that this novel high intensity OLDE protocol could be reliably used to measure muscle endurance, and that muscle fatigue induced by high intensity OLDE should be examined within ~ 30 s following exhaustion.
* Respect the body's design when exercising. We are capable of doing many things the body was not designed to do. And this is also true when it comes to physical activity. We're designed to walk, climb, and sprint. We walked a lot back in the good old days when we foraged in paradise, we climbed to get the sweetest fruit, and we occasionally sprinted to escape danger. Take a cue from little kids: they love to climb, when they run they sprint (try to get them to do distance running), and they can walk just fine. When we do the activities we're designed to do, that's when we'll be in great shape... we won't be under-active, and we won't be over-active, and both are bad for the body, for different sets of reasons.
Sensitivity of ACTH and PRL for the detection of OTS was four out of four and five out of five, respectively (table 2; cutoff, 200% at the second exercise test) and for the detection of NFO was four out of five and three out of three, respectively. Sensitivity of cortisol (cutoff, 200% at the second test) and GH (cutoff, 1000%) for the detection of OTS was four out of five and two out of five, and for the detection of NFO, one out of five and two out of four, respectively (table 2).
Studies show that with fibromyalgia, the initial rise in oxidative stress will begin to decrease as you continue your workout; however, with CFS, prolonged exercise can increase the oxidative stress and the associated pain. This is where you might feel malaise after exercise as well. You can see why it is necessary to start slow and work up with consistency. Having severe M.E. myself, I know it can be done and it does take persistence. 
I did the original P90X and gained a lot of upper body strength. The one issue with the original P90X is that, after weeks and weeks of doing the videos, you start to realize how much Tony talks and how much time you're wasting -- standing there with your weights -- waiting for him to start the exercise. What I love about the + videos is that he cuts most of that out. It's straight to the exercise and he barrels through. It's a little disorienting in the beginning, as you don't know how to do the exercises at first, but it's so nice knowing that I can be finished with the workout in 30 minutes, instead of 60 for most of the regular P90X DVDS.

... Differences in the duration of each stage and the load increments can alter the cardiorespiratory and metabolic response, and therefore the measurement ( Bentley et al., 2007;Julio et al., 2017). As suggested by pioneering studies (Buchfuhrer et al., 1983;Lukaski et al., 1989), recent investigations ( Midgley et al., 2007) and reviews ( Julio et al., 2017), traditional longer GXTs (i.e., 20-30 min) to determine LT including increments each 3-5 min would prevent the athlete from achieving their MAS due to accumulative fatigue, dehydration, muscle acidosis, and cardiovascular drift. This is critical because MAS is a pertinent and widespread criterion to set training intensities for endurance disciplines (Billat and Koralsztein, 1996;Jones and Carter, 2000). ...
Jump up ^ Rao AK, Chou A, Bursley B, Smulofsky J, Jezequel J (January 2014). "Systematic review of the effects of exercise on activities of daily living in people with Alzheimer's disease". Am J Occup Ther. 68 (1): 50–56. doi:10.5014/ajot.2014.009035. PMC 5360200. PMID 24367955. Alzheimer’s disease (AD) is a progressive neurological disorder characterized by loss in cognitive function, abnormal behavior, and decreased ability to perform basic activities of daily living [(ADLs)] ... All studies included people with AD who completed an exercise program consisting of aerobic, strength, or balance training or any combination of the three. The length of the exercise programs varied from 12 weeks to 12 months. ... Six studies involving 446 participants tested the effect of exercise on ADL performance ... exercise had a large and significant effect on ADL performance (z = 4.07, p < .0001; average effect size = 0.80). ... These positive effects were apparent with programs ranging in length from 12 wk (Santana-Sosa et al., 2008; Teri et al., 2003) and intermediate length of 16 wk (Roach et al., 2011; Vreugdenhil et al., 2012) to 6 mo (Venturelli et al., 2011) and 12 mo (Rolland et al., 2007). Furthermore, the positive effects of a 3-mo intervention lasted 24 mo (Teri et al., 2003). ... No adverse effects of exercise on ADL performance were noted. ... The study with the largest effect size implemented a walking and aerobic program of only 30 min four times a week (Venturelli et al., 2011).
Between August 2012 and June 2013, all men and women born between years 1936 to 1942 (aged 70–77 years), with a permanent address in the municipality of Trondheim, Norway, were invited to participate in a randomized controlled trial, the Generation 100 study. The primary aim of Generation 100 is to determine the effect of five years of exercise training on morbidity and mortality. The Generation 100 study protocol and study sample characteristics have been published previously [19].
Also important to know is how to determine how much weight you should use. Start with a light weight and perform a set. Continue adding weight until you can do the desired number of reps with good form, which includes moving slowly enough that you're using muscle—and not momentum—to lift the weight. The last rep should be difficult, but not impossible, and you should be able to keep good form while doing it.
Jump up ^ Int Panis, L; De Geus, Bas; Vandenbulcke, GréGory; Willems, Hanny; Degraeuwe, Bart; Bleux, Nico; Mishra, Vinit; Thomas, Isabelle; Meeusen, Romain (2010). "Exposure to particulate matter in traffic: A comparison of cyclists and car passengers". Atmospheric Environment. 44 (19): 2263–2270. Bibcode:2010AtmEn..44.2263I. doi:10.1016/j.atmosenv.2010.04.028.

Degenhardt B [Internet]. Once upon a time: the evolution of Pilates mat work. 2012 Nov 21 [cited 2015 Aug 30]. Available from:  Pilates swiftly gained prominence with dancers and celebrities, which led to his notoriety as a New York City exercise personality. In 1932, Pilates taught Contrology exercises at the mythological Jacob’s Pillow Dance Festival as he did every summer until 1951.42,43 Degenhardt B [Internet]. Once upon a time: the evolution of Pilates mat work. 2012 Nov 21 [cited 2015 Aug 30]. Available from:

Jump up ^ Pratali L, Mastorci F, Vitiello N, Sironi A, Gastaldelli A, Gemignani A (November 2014). "Motor Activity in Aging: An Integrated Approach for Better Quality of Life". Int. Sch. Res. Notices. 2014: 257248. doi:10.1155/2014/257248. PMC 4897547. PMID 27351018. Research investigating the effects of exercise on older adults has primarily focused on brain structural and functional changes with relation to cognitive improvement. In particular, several cross-sectional and intervention studies have shown a positive association between physical activity and cognition in older persons [86] and an inverse correlation with cognitive decline and dementia [87]. Older adults enrolled in a 6-month aerobic fitness intervention increased brain volume in both gray matter (anterior cingulate cortex, supplementary motor area, posterior middle frontal gyrus, and left superior temporal lobe) and white matter (anterior third of corpus callosum) [88]. In addition, Colcombe and colleagues showed that older adults with higher cardiovascular fitness levels are better at activating attentional resources, including decreased activation of the anterior cingulated cortex. One of the possible mechanisms by which physical activity may benefit cognition is that physical activity maintains brain plasticity, increases brain volume, stimulates neurogenesis and synaptogenesis, and increases neurotrophic factors in different areas of the brain, possibly providing reserve against later cognitive decline and dementia [89, 90].
We have included step by step instructional guides for over 500 different resistance training exercises. This database covers a wide variety of different exercises including free weights, CrossFit, kettlebells, machines, bodyweight, medicine ball, elastic bands, exercise ball, Pilates and stretching movements. Choose from a list for a specific muscle group or select by exercise type to pick the best exercises for your workout. Each instructional page will show you how to properly perform a resistance training exercise with detailed photos and exercise advice for each movement. It’s like having your very own personal trainer. These exercise guides will help set you on the right track so you can get in the best shape of your life!
The hypothalamus is under the control of several “higher” brain centres and several neurotransmitters 13 known to play a major role in various neuroendocrine and behavioural functions, for example, activation of the HPA axis, feeding and locomotion.14 Therefore, the typical HPA axisrelated hormones cortisol, adrenocorticotrophic hormone (ACTH), prolactin (PRL) and human growth hormone (GH) were targeted in the present study.
The barre method uses your own body weight for resistance and focuses on small, deliberate movements that focus on specific groups of muscles, specifically muscles that aren’t used in other workouts. Muscles are worked to the point of fatigue, and then stretched for relief. Proper form, body alignment and posture is stressed, which in turn leads to an overall strengthening of core muscles and the appearance of an aligned, lean body.
Hessel J [Internet]. Joseph H. Pilates biography. 2015 Jul 8 [cited 2015 Aug 30]. Available from:  He claimed that ‘sports are wonderful … but of little value for correcting whats wrong with you … corrective exercise is the only way to build a beautiful, strong, youthful body.’44 Readers Digest [Internet]. Ray MB. Cutting a fine figure (1934). 2012 Dec 13 [cited 2015 Aug 30]. Available from: [Google Scholar] He explained in his 1939 book ‘Your Health’: ‘Practically all human ailments are directly traceable to wrong habits which can only be corrected through the immediate adoption of right (natural, normal) habits’ … ‘the present day efforts of our so-called health departments are in vain so far as physical health is concerned’ … ‘this condition will prevail until such time as marks the recognition of a standard formation of sound and sane physical culture, based upon the natural laws of life.’45 Pilates J. Your health. Nevada: Presentation Dynamics; 1934. [Google Scholar] In his 1945 book ‘Return to Life Through Contrology,’ Pilates clearly supported Checkley, Müller, and Alexander stating that ‘Contrology is not a system of haphazard exercises designed to produce only bulging muscles’ … ‘Rather, it was conceived to limber and stretch muscles and ligaments so that your body will be as supple as that of a cat and not muscular like the body of a brewery-truck horse.’46 Pilates J, Miller WJ. Return to life through Contrology. Nevada: Presentation Dynamics; 1945. [Google Scholar] In this book, Pilates presented a list of 34 full-body equipment-free exercises, which if performed regularly at home, were promised to ‘give you suppleness, natural grace, and skill that will be unmistakably reflected in the way you walk, in the way you play, and in the way you work’ … ‘You will find your body development approaching the ideal, accompanied by renewed mental vigor and spiritual enhancement’. Pilates famously stated: ‘Physical fitness is the first requisite of happiness.’46 Pilates J, Miller WJ. Return to life through Contrology. Nevada: Presentation Dynamics; 1945. [Google Scholar] As with Müller,15 Müller JP. My system. London: Link House; 1904. [Google Scholar] Randell,25,26 Randell M. Training for childbirth from a mother's point of view. 4th ed. London: J. & A. Churchill Ltd.; 1949.

Since almost any exercise requires some strength and some level of aerobic fitness, I recommend a training program that is a mix of both strength training and conditioning; similar to what I do but on a smaller scale (to start). The Stronglifts 5x5 workout that I mentioned above is an excellent place to start and requires only three days a week. Assuming you can exercise five days a week, you can do some aerobic/anaerobic work on Tuesdays and Thursdays. If five days a week is too much, start with a three-day strength workout. If you also want to do conditioning, you can choose to perform it after your strength routine for 10–30 minutes, or just shift your focus after a few weeks of strength training and do conditioning only for a week or two.

This Chinese martial art that combines movement and relaxation is good for both body and mind. In fact, it's been called "meditation in motion." Tai chi is made up of a series of graceful movements, one transitioning smoothly into the next. Because the classes are offered at various levels, tai chi is accessible — and valuable — for people of all ages and fitness levels. "It's particularly good for older people because balance is an important component of fitness, and balance is something we lose as we get older," Dr. Lee says.
Wildman S. Kafka's Calisthenics. Slate [Internet]. 2011 Jan 21 [cited 2015 Aug 30]. Available from:  He was conferred a knighthood by the King of Denmark in 1919 and his work was granted patronage by the Prince of Wales in 1925.15,16 Müller JP. My system. London: Link House; 1904.
The deadlift is performed by squatting down and lifting a weight off the floor with the hand until standing up straight again. Grips can be face down or opposing with one hand down and one hand up, to prevent dropping. Face up should not be used because this puts excess stress on the inner arms. This is a compound exercise that also involves the glutes, lower back, lats, trapezius (neck) and, to a lesser extent, the hamstringcacas and the calves. Lifting belts are often used to help support the lower back. The deadlift has two common variants, the Romanian deadlift and the straight-leg-deadlift. Each target the lower back, glutes and the hamstrings differently.
Most gyms assault their members with a cacophony of distractions – thumping music, blaring televisions, and grunting patrons. We are careful to maintain a clean and distraction-free facility. There is no music and there are no mirrors or televisions. The temperature is kept at 68 degrees. The sessions are one-on-one with a focus on privacy. Instructors are dressed professionally at all times and closely monitor and record every aspect of their client's performance.
For active types, nagging injuries nag a little louder; hard workouts deplete you a bit more. For serious recreational athletes, performance begins to drop, even if you maintain your training regimen. Whatever your sport of choice — be it distance running, competitive cycling, or pick-up basketball — you can expect your performance to plateau and recovery to take a bit longer.
The exercise videos became a series, and Fonda eventually released 23 workout videos. Fonda's exercise videos were such a success because they were the first media of their time to send the message that it was socially acceptable for women to participate in exercise culture and become muscular. Fonda's videos also helped to spark the aerobic exercise trend of the 1980s, not to mention the period's iconic fashion trend: a neon exercise outfit with a leotard and leg warmers. The aerobic exercise craze became so widespread that a Muppets parody tape titled Miss Piggy's Aerobique Exercise Workout Album was released in 1982. The album cover featured Miss Piggy in a Fonda-esque exercise outfit, complete with leg warmers and a leotard, in a pose imitating that which was famously modeled by Jane Fonda on the cover of her first tape.
Frequency, intensity, type, location and social setting (alone vs. together with others) of exercise were assessed using exercise logs from 618 older adults (aged 70–77 years) randomized to MCT or HIIT. All participants completed exercise logs after each exercise session they performed during one year. Pearson Chi-square tests were run to assess the association between intensity, type, location and social setting of exercise with training group.
Why do we exercise? We all know it's good for our health, but have you ever thought about it?  Do you exercise the way you do because you've heard that's the way it should be done? Is it possible that the current way of working out could be good for some parts of our body, but bad for others... are we doing more harm than good? Are we spending more time exercising than we need to?